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Communication and Networks Assignment

The attached file has all instructions. The deadline is the first of February. Also there are every topic’s presentation. Need 100%.

Diplomain Information Technology

Communications and Networks

Instruction for CA3 Individual Assignment

January 2021 Semester

Continual Assessment 3 Individual Assignment (40%)

Task:

The objective of this assignment is to allow students to apply the concepts taught in
class and gain some knowledge on the designing networks. When doing research
beyond lesson materials, students are encouraged to check the validity of the
sources before using it in the assignment.

This assignment will make use of what you have learnt in:

• Lesson 9 Wired Media • Lesson 13 Modulation

• Lesson 10 Wireless Media • Lesson 14 Multiplexing

• Lesson 11 Error Handling • Lesson 15 Interconnection
Technologies

• Lesson 12 Transmission Modes • Lesson 16 Packet and Circuit
Switching

Read the following article on the Digital Solutions about Safe Reopening:
Article (1): https://www.imda.gov.sg/programme-listing/smes-go-digital/Digital-
Solutions-For-Safe-Reopening

DigiSec is an IT consultancy firm that helps education institutions digitalise their
processes and enables remote working. What distinguishes DigiSec from other IT
consultancy firms is that they believe in a combination of cloud and LAN
technologies to enable efficient data communication.

DigiSec has a list of remote working and collaboration tools that institutions can use
to expand their workforce overseas. This is also important for large multi-national
institutions to save on commute cost for their employees.

DigiSec is planning to setup a branch in Singapore and they are aware of the current
state of the virus pandemic that has forced many education institutions to conduct
classes virtually.

To qualify working for DigiSec, you are required to showcase some of your
knowledge in networking. Your task is to write a report to explain some networking
concepts.

Task Instructions
1. Make sure your report is related to DigiSec and Article (1).

2. Please make use of definitions and concepts covered in this module. You are

encouraged to use external sources to further support the concepts but not

replace the definitions.

3. You are required to read the Report Instructions. Failure to do so will result in

deduction of up to 20% of the total marks.

https://www.imda.gov.sg/programme-listing/smes-go-digital/Digital-Solutions-For-Safe-Reopening

Question 1
(i) Write a summary, in at least 300 words, for Article (1). In your summary,

describe, with examples, what is online collaboration, virtual meetings, e-
signatures, workforce management and visitor management.

(ii) Many education institutions in Singapore are moving into virtual classroom
solutions. Explain and justify, in at least 100 words, which category of tools
should education institutions be looking into in Article (1).

(iii) Distinguish, in at least 200 words, between serial and parallel transmissions.

Do include diagrams, where possible, to illustrate the differences.

(iv) DigiSec is planning to introduce software and tools to private education

institutions to support remote learning. Recommend and justify, in at least 200
words, whether serial or parallel transmission is more suitable for:

a. High speed transmission
b. Long distance transmission at low cost

Question 2

(i) DigiSec is tasked by one of the education institutions to set up a telephone
system within their campus. Describe, in at least 100 words, the purpose of
modulators and demodulators in such scenario.

(ii) Explain with examples, in at least 200 words, FOUR (4) different types of
multiplexing techniques.

(iii) Distinguish, in at least 100 words, the difference between modulation and

multiplexing.

(iv) Propose and justify in your own words, in at least 200 words, a network
topology and wired/wireless media that DigiSec can recommend to their
clients. Your proposal should include network topology diagram that
supports the following:

1) 100x computers/laptops
2) 10x printer
3) 10x scanner
4) 1x Email server
5) 1x Domain Name Server

Question 3

(i) One of the error checking techniques that DigiSec recommends to their clients
is Hamming Code. Describe and show how the following byte can be coded
using an even Hamming Code.

1010 1001

(ii) Suppose one of the bits is corrupted and the bits received were as followed:
1100 0110 0101

a. Describe and show how a one-bit error can be detected and then
corrected.

b. Describe and give the correct byte that was transmitted.

Report Instructions

Format:

1. The report should have a cover page, content page, write-up on the task,

references, and optional appendices.

2. The cover page should include:

i. Institution name and institution logo of this programme

ii. Module name, term, and year

iii. Date of submission

iv. Student’s name and student ID numbers

3. Each question should start on a separate page and must be answered in paragraph

forms. Bulleted points and tables may only be used to support the answer.

4. The references and citations should be presented in Harvard or APA format and

should have at least THREE (3) references.

Report Word Limit

1,500 to 2,000 words

Report Font and Spacing

Font: Calibri, Black

Font size: 12 and 1 ½ or double spacing.

Penalty Marks for Late Submission

By one day: 20% to be deducted from total marks.

More than one day: submission will NOT be graded.

Plagiarism and Collusion

The submitted report must show evidence that this is students’ own work. ZERO

MARKS will be awarded if there are no workings, reasonable explanations, or

citations. Please be reminded that plagiarism and collusion is a serious offence, and

all cases will be referred to the administration. Grades will be withheld if the submission

is suspected of plagiarism or collusion till investigations are completed.

Important Deadlines
CA3 Written Report Deadline: 15 February 2021, 11.59a.m. Submit the softcopy

of your report via D2L/eGlobal.

Marking Rubric

The marking rubric for the written report can be found in the appendix of this

assignment. Please understand the marking rubric carefully and allocate your time

accordingly.

Lecturer Contact

You should contact your lecturer via your SIM email whenever you have any issue

about your project. You may send your email to: wbkoh001@mymail.sim.edu.sg.

mailto:wbkoh001@mymail.sim.edu.sg

Appendix

S/N Criteria Excellent Very good Good Acceptable Weak

1 Understanding
of the

Assignment
(5%)

Student has interesting,
logical, reasonable, and

non-obvious interpretation
of the assignment

Student has logical,
reasonable, and non-

obvious interpretation of
the assignment

Student has logical
and reasonable

interpretation of the
assignment

Student has logical
interpretation of
the assignment

Student has weak
interpretation of
the assignment

2 Question 1
(40%)

Student has a clear
understanding of the

question requirements
and can describe these

requirements effectively

and appropriately

Student has a clear
understanding of the
question requirements
and can describe these

requirements
appropriately.

Student has
sufficient

understanding of the
question

requirements and
can describe these

requirements
appropriately.

Student has limited
understanding of

the question
requirements and
is able to describe
these requirements

to some extent.

Student has limited
understanding of

the question
requirements but is
unable to describe

these
requirements.

3 Question 2
(30%)

Student has provided
appropriate solutions with

detailed explanation

Student has provided
appropriate solutions

with detailed
explanation to some

extent

Student has provided
appropriate solutions

with sufficient
explanation

Student has
provided solutions

but one of the
solutions is

inappropriate

Student
misunderstood the
question or has not

provided any
solution

4 Question 3
(20%)

Student has provided in-
depth description of their

workings

Student has described
in some detail about

workings

Student has
described the

workings
Student has
described the

workings to some
extent

Student has briefly
described the

workings
performed

Referencing
(5%)

Student has used more
than sufficient referencing
and practiced appropriate
referencing and citations

style

Student has used
sufficient referencing

and practiced
appropriate referencing

and citations style

Student has used
some referencing

and practiced
citations style

Student has used
limited referencing

Student has used
less than

satisfactory
amount of
referencing

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 20: TCP and UDP Transport Service

Lesson 20 Learning Outcomes
Distinguish between UDP and TCP transport service
Identify the components in a UDP datagram
Explain how TCP handles congestions
Explain TCP three-way handshake
Identify the components in a TCP segment

Lesson 20 Outline
UDP Transport Service
TCP Transport Service

End-to-End Communication
IP cannot distinguish among multiple application programs running on a given host
Source and destination field identifies a host
Does not contain additional bits to identify an application
Treats a computer as endpoint of communication
Transport-layer protocols are end-to-end protocols
Allows an application to be an endpoint of communication

Transport Protocols
Two main transport protocols
User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)
UDP is easy to understand
End-to-end: distinguish among multiple applications running on a given computer
Connectionless: does not requires connection to be establish before communicating
Message-oriented: sends and receives individual messages

UDP as Thin Protocol
UDP is less complex:
Best-effort Delivery: same best-effort delivery semantics as IP
Arbitrary Interaction: allows an application to send/receive to/from many other applications
Operating System Independent: provides means of identifying application programs that does not depend on identifiers used by the local OS
Thus, it is sometimes characterised as thin protocol

Message-Oriented
Each requests for UDP to send data places the data in a single message for transmission
Does not divide a message into multiple packets
Does not combine messages for delivery
On the positive side, applications preserve data boundaries as each message remains the same as transmitted
On the negative side, each UDP message must fit into a single IP datagram

Best-effort Delivery
Best-effort delivery means messages can be:
lost, duplicated and delivered out-of-order
Consequences for applications:
Must either be immune to the problems or take steps to detect and correct problems

UDP Application Limitations
Audio applications can tolerate packet errors
If sender places small amount of audio in each message, loss of a packet produces small gap
Gap will be heard as a pop or click
Online shopping application cannot use UDP
Packet errors can have serious consequences
Duplication of a message can result to duplicate orders or double charging

Endpoint Identification
UDP defines protocol port numbers that are independent of the OS
UDP provides mapping between protocol port numbers and program identifiers that OS uses
All computers running UDP recognise standard protocol port numbers, independent of the OS

UDP Datagram
Each UDP message is called a user datagram and consists of two parts:
Short header: specifies sending and receiving application
Payload: data being sent
Source: Douglas, C (2016) Computer Networks and Internets

Checksum & Encapsulation
UDP header contains 16-bit optional field named UDP checksum (like Internet checksum)
To verify headers
Each UDP datagram is encapsulated in an IP datagram for transmission across the Internet

Source: Douglas, C (2016) Computer Networks and Internets

Practice 20.1
What are the THREE (3) guarantees of message delivery of UDP that make it a best-effort protocol?
As a best-effort protocol, what are the applications that are suitable to use UDP and what are those that are unsuitable?

Lesson 20 Outline
UDP Transport Service
TCP Transport Service
Flow & Congestion Control
Three-way Handshake
TCP Segment Components

Transmission Control Protocol
A programmer assumes that data will arrive correctly, and OS guarantees that data will be delivered reliably
Transmission Control Protocol (TCP) provides reliable transport service
No lost, no duplication, on time and ordered delivery

TCP Characteristics
Connection Orientation: an application must first request a connection to a destination
Point-to-Point: each TCP connection has exactly two endpoints
End-to-End: distinguish among multiple applications running on a given computer
Complete Reliability: guarantees that the data sent across a connection will be delivered completely and in order
Full duplex: allows data to flow in either direction

TCP Services
Stream Interface: an application sends a continuous sequence of octets
Does not group data into records or messages
Reliable Connection Startup: allows two applications to reliably start communication
Graceful Connection Shutdown: ensures that both sides have agreed to shut down the connection

Virtual Connections
Connections in TCP are virtual connections as they are achieved in software
Two machine exchange messages to achieve the illusion of a connection
Uses IP to carry messages which each TCP message is treated as data
TCP software is needed at each end of a virtual connection but not on intermediate routers

TCP Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Problems to Consider
Ensuring reliability: reliable transport service
End System Reboot: either of the two end systems might crash and reboot
Heterogeneous End Systems: a fast sender can overrun a slow receiver
Congestion in the Internet: intermediate switches and routers can become overrun by traffic

Handling Order & Duplicates
To handle duplicates and order, sender attach a sequence number to each packet
Receiver examines the sequence number to determine how the packet should be handled
Either use it to arrange packets in order
Or if the packet has already been delivered, discards the duplicated copy

Retransmission for Lost Packets
To handle packet loss, transport protocols use positive acknowledgement (ACK)
Receiver sends ACK message that reports successful reception
Sender starts a timer whenever it sends a packet
If ACK arrives before timer expires, cancels the timer
If timer expires before ACK, sender retransmit and starts timer again

Retransmission Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Stop-and-go Flow Control
Flow control techniques are employed to handle a fast computer from sending so much data to overrun a slower receiver
Simplest flow control is stop-and-go
Sender waits after transmitting each packet
When receiver is ready, it sends a control message (usually ACK)
But result in extremely low throughput

Sliding Window Flow Control
Sliding window is another flow control technique
Window size: maximum amount of data that can be sent before an acknowledgement arrives
Source: Douglas, C (2016) Computer Networks and Internets

Flow Control Comparison
Source: Douglas, C (2016) Computer Networks and Internets
Stop-and-go
Sliding Window

Congestion
Congestion occurs when a network is unable to allow further packets from flowing through
Resulting in delay
Can cause intermediate network device to run out of memory and begin discarding packets
Retransmission for lost packets will sends more packets into the network
Network can become unusable which can result to congestion collapse

Congestion Control
To control congestion:
Network device sends a special message to the source of packets when congestion occurs
Receiver use increased delay or packet loss as estimate of congestion and inform sender
Congestions is mostly result of packet burst
Sudden increased in transmission or retransmission

Adaptive Retransmission
Rather than a fixed retransmission timeout, TCP monitors current delay on each connection
Adaptive retransmission: modify retransmission timeout according to network conditions
TCP estimates round-trip delay for each active connection by measuring time it receive a response
If delay is constant, adjusts timeout slightly longer than average round-trip delay
If delay varies, adjusts to a value greater/lower than average to accommodate peaks

Adaptive Retransmission Illustration
Source: Douglas, C (2016) Computer Networks and Internets
Congested Network
Less Congested Network

Lesson 20 Outline
UDP Transport Service
TCP Transport Service
Flow & Congestion Control
Three-way Handshake
TCP Segment Components

TCP 3-way Handshake
Source: https://www.youtube.com/watch?v=n-2YRCMX6Kc

Three-Way Handshake
To establish or terminate connections, TCP uses a technique call 3-way handshake
Handshake ensures that TCP will not open or close a connection until both ends have agreed
During the 3-way handshake to start a connection, each side sends a message that specifies
Initial buffer size for flow control and sequence number

Handshake Messages
Synchronization (SYN) segment: control message used in a 3-way handshake to create a connection
Finish (FIN) segment: control messages used in a 3-way handshake to close a connection
TCP also requires each end to generate a random 32-bit sequence number that becomes the initial sequence

Connection Establishment
Source: Douglas, C (2016) Computer Networks and Internets

Reconnection & Termination
If application re-establish a new TCP connection after a computer reboots, TCP will choose a new random number
Probability of selecting a random value that matches the value used previously is low
To close a connection, TCP uses FIN segments
An ACK is sent in each direction along with a FIN to guarantee all data has arrived before the connection is terminated

Connection Termination
Source: Douglas, C (2016) Computer Networks and Internets

Lesson 20 Outline
UDP Transport Service
TCP Transport Service
Flow & Congestion Control
Three-way Handshake
TCP Segment Components

TCP Segment Format
TCP uses a single format for all messages including data messages, ACKs, SYN and FIN
TCP uses the term segment to refer to a message
TCP connection contains two streams of data, one in each direction
Some fields in the segment refer to the data stream traveling in the forward direction
While others refer to data stream traveling in the reverse direction

TCP Segment Format Breakdown
Source: Douglas, C (2016) Computer Networks and Internets

TCP Segment Fields (1/2)
ACKNOWLEDGEMENT NUMBER: sequence number of the data that is expected next
WINDOW: how much additional buffer space is available beyond ACKed data
SEQUENCE NUMBER: sequence number of the first byte of data being carried in the segment
Receiver uses this to compute an acknowledgement number

TCP Segment Fields (2/2)
DESTINATION PORT: identifies which application program on the receiving computer should receive the data
SOURCE PORT: identifies the application program that sent the data
CHECKSUM: checksum that covers the TCP segment header and the data

Practice 20.2
How does TCP deals with flow control?

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 25, 26

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 16: Packet and Circuit Switching

Lesson 16 Learning Outcomes
Distinguish circuit and packet switching
Explain the IEEE 802 model and standard
Describe the concept of unicast, broadcast and multicast
Describe the delivery of broadcast and multicast
Explain the purpose of bit and byte stuffing

Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing

Multi-access Network
Multi-access Network: multiple computers to share a medium in such a way that any computer on the LAN can communicate with any other
In general, LAN technologies provide direct connection among communicating entities
However, professionals say that LANs connect computers with the understanding that a device such as a printer can also connect to a multi-access LAN

LAN Topologies
Each network is classified into a category according to its topology or general shape
Bus Topology
Ring Topology
Mesh Topology
Star Topology

Bus Topology
Bus topology consists of a single cable
Ends of bus network must be terminated to prevent electrical signals from reflecting back
Computers must coordinate to ensure only one computer sends a signal at any time

Source: Douglas, C (2016) Computer Networks and Internets
Advantage Disadvantage
All nodes share access to a common medium Not resilient to failures
Easy to implement
Single cable fault will split the network in two
Adding new node is easy Isolating a fault is difficult

Ring Topology
Ring topology: computers are connected closed loop
In practice, cables can run along hallways or rise vertically from one floor of a building to another
Advantage Disadvantage
Resilience to single failure provided data travel around the ring in either direction Harder to implement
Faults are easier to isolate Use more cable
Adding new node is difficult

Source: Douglas, C (2016) Computer Networks and Internets

Mesh Topology
Mesh topology: provides a direct connection between each pair of computers
Costly to connect n computers:

Advantage Disadvantage
Relatively easy to manage Expensive if there are large number of nodes
Relatively easy fault isolation Adding new nodes is difficult and expensive
Extremely resilient to multiple failures

Source: Douglas, C (2016) Computer Networks and Internets

Star Topology
Star topology: all computers attach to a central point often called a hub
Hub is typically a network device
Source: Douglas, C (2016) Computer Networks and Internets
Advantage Disadvantage
Relatively easy to add new nodes Not particularly resilient to failures especially at hub or central site
Faults are easier to isolate
Easy to manage as network equipment is centralized

Practice 16.1
For each of the following, determine a suitable network topology to use:
Resilient against single point of failure
Easy fault isolation
Cost effective

Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing

Packet and Circuit Switching
Source: https://www.youtube.com/watch?v=gB0DCb84T7c

Circuit Switching
Circuit switching: a communication mechanism that establishes a dedicated path between sender and receiver when needed
Guarantees isolation from paths used by other pairs of senders and receivers
Usually associated with telephone technology as it provides dedicated connection between telephones

Virtual Circuit
Circuit switching networks use electronic devices to establish circuits
Instead of a physical path, multiple circuits are multiplexed over shared media
Resulting to a virtual circuit
Source: Douglas, C (2016) Computer Networks and Internets

Properties of Circuit Switching
Point-to-point communication as circuit is formed between exactly two endpoints
Separate steps for circuit creation, use, and termination, which distinguishes switched vs permanent circuit
Performance equivalent to isolated physical path
Communication between two parties is not affected by others
Provide illusion of an isolated path for each pair of communicating entities

Steps of Circuit Switching
Three-step process analogous to placing a phone call
Establish a circuit between two parties
Two parties communicate using the circuit
Two parties terminate use of the circuit

Packet Switching
Packet switching system uses statistical multiplexing
Multiple sources compete for a shared media

Source: Douglas, C (2016) Computer Networks and Internets

Packets
Packet switching system requires a sender to divide each message into smaller blocks of data
Packets: blocks of data
Size of a packet can vary
Each packet switching technology defines a maximum packet size

Properties of Packet Switching (1/2)
Asynchronous communication: allows sender with one or more recipients and recipient with one or more senders at any time
No set-up required: can deliver a packet to any destination at any time and no need for initialisation before communicating
No need to notify underlying system when communication terminates

Properties of Packet Switching (2/2)
Performance varies due to statistical multiplexing among packets
Multiplexing occurs among packets rather than among bits or bytes

Circuit vs Packet Switching
Packet switching incurs lower cost from sharing
To provide communication among N computers:
Circuit-switched network must have a connection for each computer plus at least N/2 independent paths
Packet switching network must have a connection for each computer but only requires one path that is shared

Packet Switching Technologies
Least expensive networks use technologies that span a short distance like inside a single building
Most expensive span long distances like across several cities

Source: Douglas, C (2016) Computer Networks and Internets

Packet Switching Identification
Each packet sent must contain identification of intended recipient
Senders agree on exact details of how to identify recipient and identification field in a packet
Standard organisations specify these details
IEEE organised Project 802 LAN/MAN Standards Committee to produce standards for networking

Practice 16.2
What are the steps to setup, communicate and terminate circuit switching?
Which property of packet switching allows multiple pairs communication to take place? Explain.

Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing

Standard Organisation Focus
IEEE is mostly focus on the lower two layers of the protocol
Other standard organisations focus on other layers of the stack
Source: Douglas, C (2016) Computer Networks and Internets

IEEE Data Link Specification
IEEE divides Data Link into two conceptual sublayers
Logical Link Control (LLC): specifies addressing and the use of addresses for demultiplexing
Media Access Control (MAC): specifies how multiple computers share underlying medium
Source: Douglas, C (2016) Computer Networks and Internets

IEEE Standards
IEEE assigns multi-part identifier of the form XXX.YYY.ZZZ
XXX: category of standard
YYY: subcategory
ZZZ: reserved for 3rd category
Source: Douglas, C (2016) Computer Networks and Internets

IEEE Working Groups
IEEE created many working groups, each intended to standardise one type of network technology
Working group consists of representatives from industrial and academic communities
IEEE allows working group to be active provided the group makes progress and technology is still deemed important
If a working group decides that a technology is no longer relevant, it can decide to disband

Packet Addressing
IEEE has created a standard known as Media Access Control (MAC) address or Ethernet address, each consisting of 48 bits
Each packet that travels across the shared medium contains MAC address of specific recipient
Source: Douglas, C (2016) Computer Networks and Internets

Demultiplexing Packets
Demultiplexing of packets make use of the MAC address
IEEE allocates a unique address for each piece of interface
Each Network Interface Card (NIC) contains a unique address assigned when the device was manufactured

Assigning MAC Address
MAC Address consist of:
3-byte Organisationally Unique ID (OUI) to identify the equipment vendor
3-byte unique values assigned by vendor that identifies a NIC
Source: Douglas, C (2016) Computer Networks and Internets

Address Type
IEEE addressing supports three types of addresses that correspond to different types of delivery
Source: Douglas, C (2016) Computer Networks and Internets

Unicast, Broadcast & Multicast
IEEE address reserves a bit to distinguish between unicast and multicast but does not provide a way to designate a broadcast address
Broadcast address consists of all 1s
Broadcast is a special form of multicast
Multicast is for specific group of computers
Broadcast includes all computers on the network

Broadcast & Multicast Efficiency
In a typical LAN:
Each computer monitors the shared medium
Extracts a copy of each packet
Examines the address in the packet
Determine whether the packet should be processed or ignored
Broadcast and multicast are useful in LANs as they permit efficient delivery to many computers

Practice 16.3
What are the TWO (2) components in a MAC address?

Lesson 16 Outline
Network Topologies
Circuit and Packet Switching
IEEE Standards
Bit and Byte Stuffing

Framing
Framing is the the structure added to a sequence of bits or bytes that allows sender and receiver to agree on exact format of the message
In a packet-switched network, each frame corresponds to a packet that consists of two conceptual parts:
Header: data like address and information used to process the frame
Payload: data being sent

Frames
A message is opaque: network only examines the frame header
Payload can contain bytes that are only meaningful to the sender and receiver
Some technologies represent frames by sending a short prelude before frame and short postlude after

Source: Douglas, C (2016) Computer Networks and Internets

Frame Example
Assume a packet header is 6 bytes and the payload consists of an arbitrary number of bytes
Can use ASCII characters where
Start Of Header (SOH) character for the beginning
End Of Transmission (EOT) character for the end
Source: Douglas, C (2016) Computer Networks and Internets

SOH and EOT Characters
In the ASCII character set:
SOH hexadecimal value is 201
EOT hexadecimal value is 204
If the payload of a frame includes value 201 or 204, can use byte stuffing to allows transmission of without confusion
SOH and EOT cannot appear in the payload

Bit and Byte Stuffing
Frame delimiters can be used to distinguish between data and control information
Sender changes a data to replace each control byte with a sequence
Receiver replaces sequence with the original
This is known as byte/data/character stuffing
Bit stuffing: a related technique used with systems that transfer a bit stream

Byte Stuffing Example
Byte stuffing reserves a third character to mark occurrences of reserved characters in the data
Sender can replace the special characters with a two-character sequence
An example is using the ESC character:

Source: Douglas, C (2016) Computer Networks and Internets

Byte Stuffing on Receiver
Receiver reverses the mapping
Looks for ESC followed by one of A, B, or C
Replace the 2-character combination with either SOT, EOT or ESC
Once byte stuffing has been performed, SOH EOT and ESC will not appear anywhere in the payload during transmission
Only appear after receiver reverse the mapping

Byte Stuffing Illustration
(a) is the original message
(b) is the message after byte stuffing
Source: Douglas, C (2016) Computer Networks and Internets

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 13

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 15: Interconnection Technologies

Lesson 15 Learning Outcomes
Explain upstream and downstream
Explain narrowband, broadband and DSL technologies
Describe the characteristics of ADSL and local loops
Explain the evolution of access technologies
Describe SONET

Lesson 15 Outline
Broadband and Narrowband
Cable Modem Technologies
Core Technologies

Internet Access Technology
Internet access technology: a data communications system that connects an Internet subscriber to an ISP
Most Internet users follow an asymmetric pattern
Subscriber receives more data than sending
Browser sends a URL that comprises a few bytes
Web server can respond with content that are more than a few bytes

Downstream and Upstream
Downstream: data traveling from an ISP in the Internet to a subscriber
Upstream: data traveling from a subscriber to an ISP
downstream
upstream
Source: Douglas, C (2016) Computer Networks and Internets

Narrowband and Broadband
Technologies are used for Internet access can be divided into two broad categories based on data rate
Narrowband
Broadband
But network bandwidth refers to data rate
Hence, narrowband and broadband are used to reflect industry practice

Narrowband Technologies
Narrowband: technologies that deliver data at up to 128 Kbps
Max data rate for dialup phone lines is 56 Kbps and hence classified as narrowband technology
Source: Douglas, C (2016) Computer Networks and Internets

ISDN
Integrated Services Digital Network (ISDN) offers three separate digital channels:
B, B, and D (D is a control channel supports 16Kbps)
Usually written 2B + D
The 2 B channels (each 64 Kbps) are intended to carry digitised voice, data, or compressed video
B channels can be combined or bonded to produce single channel with data rate of 128 Kbps

Broadband Technologies
Broadband: technologies that offer higher data rates than dialup
Source: Douglas, C (2016) Computer Networks and Internets

Local Loop
Local loop: physical connection between a telephone company Central Office (CO) and a subscriber
Consists of twisted pair and dialup call with 4KHz of bandwidth
Subscriber close to CO may be able to handle frequencies above 1MHz
Source: https://networkencyclopedia.com/local-loop/

Digital Subscriber Line (DSL)
DSL: one of the main technologies used to provide high-speed communication services over local loop
Source: Douglas, C (2016) Computer Networks and Internets

Asymmetric DSL
ADSL most widely deployed variant and most common for residential use
uses FDM to divide bandwidth of the local loop into three regions
one region corresponds to analog phone service, known as Plain Old Telephone Service (POTS)
Source: Douglas, C (2016) Computer Networks and Internets

ADSL Properties
Complex: as no two local loops have identical electrical characteristics
Adaptive: when a pair of ADSL modems are powered on, they probe the line between them to find its characteristics
Use techniques that are optimal

Discrete Multi Tone Modulation
ADSL uses Discrete Multi Tone modulation (DMT)
Combines frequency division multiplexing and inverse multiplexing techniques
FDM in DMT is implemented by dividing the bandwidth into 286 sub-channels
255 sub-channels for downstream
31 sub-channel for upstream
2 upstream are reserved as control channel for control information

Control Channels (1/2)
Each control channel has a separate modem with its own modulated carrier
Carriers are spaced at 4.1325 KHz intervals to keep the signals from interfering with one another
To guarantee that its transmissions do not interfere with analog phone signals
ADSL avoids bandwidth below 26 KHz

Control Channels (2/2)
Two ends assess the signal quality at each frequency
Use the quality to select a modulation scheme
If a frequency has a high signal-to-noise ratio
selects a modulation scheme that encodes many bits per baud
If the quality on a given frequency is low
selects a modulation scheme that encodes fewer bits per baud

ADSL Data Rate
ADSL can achieve:
8.448 Mbps downstream on short local loops
64 Kbps upstream for control channel
576 Kbps upstream for user data
Adaptation property of ADSL does not guarantee a data rate
Only do as well as line conditions allow

ADSL Local Loop Distance
Subscribers far from CO or local loop passes near sources of interference have lower data rates
Subscribers near the CO or local loop does not pass near sources of interference have better data rates
Data rates can vary:
Downstream 32Kbps to 8.448Mbps
Upstream 32Kbps to 640 Kbps

ADSL Splitter
Analog phones operate at frequencies below 4KHz
lifting a receiver can generate noise that interferes with DSL signals
ADSL uses an FDM device known as a splitter
Divides bandwidth by passing low frequencies to one output and high frequencies to another
Usually installed at the location where local loop enters a residence or business
Passive: does not require power

ADSL Installation
Source: Douglas, C (2016) Computer Networks and Internets

DSL-Lite
A variation of ADSL wiring (DSL-lite)
Does not require a splitter to be installed on the incoming line
Subscriber can install DSL by plugging a splitter into a wall jack and plugging a telephone into the splitter

Practice 15.1
How are access technologies classified under broadband and narrowband? Give an example of each.
What is the purpose of control channels? What can be done to prevent their signal from interfering with one another?

Lesson 15 Outline
Broadband and Narrowband
Cable Modem Technologies
Core Technologies

Cable Modem Technologies
Community Antenna TeleVision (CATV): an alternative access technology that uses the wiring already in place for cable television
uses FDM to deliver TV signals over coaxial cable
CATV systems use FDM to deliver many channels
But bandwidth is insufficient to handle FDM scheme that extends a channel to each user
Using a separate channel per subscriber does not scale

Cable Modem Data Rate
In theory, cable system can support:
52 Mbps downstream
512 Kbps upstream.
In practice, the data rate can be much less
Data rate only pertains to communication between the local cable office and the subscriber’s site

Sharing Bandwidth
Bandwidth of cable system is shared with N subscribers
size is controlled by the cable provider
Effective data rate available to each individual subscriber varies over time
if N subscribers share a single frequency
amount of capacity available to an individual subscriber will be 1/N

Cable Modem Installation
Cable modem installation is straightforward
Cable modems attach to the cable wiring directly
FDM hardware in existing cable boxes and cable modems guarantees:
data and entertainment channels will not interfere with one another

Hybrid Fiber Coax (HFC)
HFC provides high-speed data communications
Fiber to connect to the central facilities
Coax to connect to individual subscribers
It is hierarchical
Uses fiber for parts that require highest bandwidth
Uses coax for parts that can tolerate lower data rates

Trunk and Feeder Circuit
Trunk: the high-capacity connections between the cable office and each neighborhood area
can be up to 24km long
Feeder circuit: the connection to an individual subscriber
Feeder circuits are usually less than 1.6km

HFC Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Fiber Access Technologies
A variety of technologies employs optical fiber in a hybrid system
or deploy optical fiber all the way to each subscriber
Source: Douglas, C (2016) Computer Networks and Internets

Fiber to Curb and Building
Fiber To The Curb (FTTC) uses fiber for high capacity trunks
Idea is to run fiber close to subscriber
Use copper for the feeder circuits
Uses two media in each feeder circuit to provide additional service like voice
Fiber To The Building (FTTB) use fiber to allow high upstream data rates for businesses

Fiber to Home and Premises
Fiber To The Home (FTTH) uses fiber to deliver higher downstream for residential subscribers
Emphasis is on many channels of entertainment and video
Fiber To The Premises (FTTP) is a generic term that encompasses both FTTB and FTTH

Head-end & Tail-end Modem
Two types of modem based on location:
Head-end modem: modem used at the CO
Cable Modem Termination System (CMTS): set of head-end modems used by cable providers.
Tail-end modem: modem used at the subscriber
Data Over Cable System Interface Specifications (DOCSIS): specifies format of data that can be sent and messages that are used to request services like pay-per-view)

Wireless Access Technologies
Imagine a farm or remote village:
telephone wiring to such locations exceeds the max distance for technologies like ADSL
Also unlikely to have cable TV
Local loop may not work on all types of lines
Need wireless access technologies
Source: Douglas, C (2016) Computer Networks and Internets

Lesson 15 Outline
Broadband and Narrowband
Cable Modem Technologies
Core Technologies

What is a T1 Line?
Source: https://www.youtube.com/watch?v=o5zSxG-Atsc

Last Mile Problem
Access technologies handle last mile problem
Last mile: the connection to a typical residential subscriber or a small business
Access technology provides sufficient capacity for a residential subscriber or a small business
Small Office Home Office (SOHO)

Core Technologies
Connections among providers and enterprises require substantially more bandwidth
Core: connections at the backbone of Internet
Core technologies: high-speed technologies
Source: Douglas, C (2016) Computer Networks and Internets
Last mile
Core

Point-to-Point Digital Circuit
Point-to-point high-capacity digital circuit can be leased from a telephone company
Can be used to transfer data
Monthly fee depends on data rate of the circuit and the distance spanned
Telephone companies have the authority to install wiring that crosses municipal streets
Between two buildings, across a city, from one city to another

Leasing a Digital Circuit
Subscribers must follow the rules of telephone system and adhere to standards for transmitting digitised voice
Computer industry and the telephone industry developed independently
Need a hardware interface to work between computer and digital circuit

DSU and CSU
Data Service Unit/Channel Service Unit (DSU/CSU): hardware interface for a computer to a digital circuit
Two parts combined into a chassis
CSU handles line termination and diagnostics
DSU handles digital format translation for data
Between format used on circuit and computer

CSU Test Facility
A CSU contains a loopback test facility
To transmit a copy of all data that arrives across the circuit back to sender
Excessive 1s can cause excessive current on the cable due to voltage levels
To prevent problems, can use an encoding that guarantees a balance like differential encoding
Or use a technique known as bit stuffing

Interface Standard
Interface standard used on the computer depends on rate that circuit operates
If < 56 Kbps, the computer can use RS-232 If > 56 Kbps, use RS-449 or V.35 standards
One additional piece of equipment may be used
Network Interface Unit (NIU) or Smartjack

Network Interface Unit
NIU forms a boundary between equipment owned by the telco and equipment provided by subscriber
Telco refers to the boundary as the demarc
Digital circuit needs DSU/CSU at each end
To translate between digital representation used by phone and digital representation used by computer
Digital circuit from a telco follows same transmission standards as digital phone calls

Digital Circuit Standards
In US, standards for digital circuits consist of the letter T followed by a number like T1
Many small businesses use a T1 circuit
But T-standards are not universal
Japan adopted a modified version of the T-series standards
Europe uses the letter E

Digital Circuit Capacity
Source: Douglas, C (2016) Computer Networks and Internets

T-standards Data Rate
Data rates of T standards have been chosen to handle multiple voice calls
Capacity of circuits does not increase linearly with their numbers; T3 is much more than 3x T1
Telcos may lease circuits with lower capacity than those listed in the figure
Known as fractional T1 circuits

STS Standards
Telephone companies use trunk to denote a high-capacity circuit,
Synchronous Transport Signal (STS) standards specify the details of high-speed connections
Data rates for STS-24 and above are > 1 Gbps

List of STS & OC Standards
Source: Douglas, C (2016) Computer Networks and Internets
STS standards: electrical signals used in the digital circuit interface (i.e., over copper)
Optical Carrier (OC) standards: optical signals that propagate across the fiber

Concatenated Circuits
STC and OC allows for an optional suffix of the letter C, which stands for concatenated
denotes a circuit with no inverse multiplexing
OC-3 can consist of 3x OC-1 or single circuit that operates at 155.520 Mbps
Single circuit vs multiple circuits
Generally, single circuit provides more flexibility and eliminates need for inverse multiplexing equipment

Digital Transmission Standards
Two major digital transmission standards:
US, Synchronous Optical NETwork (SONET)
Europe Synchronous Digital Hierarchy (SDH)
SONET specifies details like:
How data is framed
How lower-capacity circuits are multiplexed into a high-capacity circuit
How synchronous clock information is sent along with data

SONET Frame on STS-1 Circuit
Source: Douglas, C (2016) Computer Networks and Internets

Practice 15.2
With cable modem technologies, why is there still a need for core technologies?

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 12

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 3: Internet Networking

Lesson 3 Learning Outcomes
Distinguish between connectionless and connection-oriented communication
Distinguish the role of server and client in a client-server model
Identify the characteristics of a server and client
Understand how multiple clients and servers can be used to work together
Define concurrent servers
Describe the motivation for peer-to-peer interaction on the Internet

Lesson 3 Learning Outcomes
Describe what is the purpose of sockets
Identify the key functions of sockets

Lesson 3 Outline
Communication Paradigms
Client Server Model
Network Programming

Internet Rich Diversity of Services
None of the services is part of underlying communication infrastructure
Internet provides a general-purpose mechanism where:
Individual services are supplied by application programs that run on computers attached to the Internet

Programming on the Internet
Possible to create Internet applications without knowing how networks operate
However, understanding network protocols and technologies allows them to write efficient and reliable code
Enables applications to scale across many sites

Internet Communication Paradigms
Source: Douglas, C (2016) Computer Networks and Internets
Stream Paradigms Message Paradigms
Connection-oriented Connectionless
1-to-1 communication Many-to-many communication
Sequence of individual bytes Sequence of individual messages
Arbitrary length transfer Each message limited to 64 Kbytes
Used by most applications Used for multimedia applications
Built on TCP protocol Built on UDP protocol

Stream Transport
Stream: sequence of bytes flows from one application program to another
Without attaching meaning to the bytes and without inserting boundaries
Two streams between pair of communicating applications, one in each direction
Accepts input from one, delivers to the other
Sending application can choose to generate one byte at a time or can generate blocks of bytes
Network can choose to combine smaller blocks into one large block or divide a large block into smaller blocks

Message Transport
Message: the network accepts and delivers messages
Each message delivered corresponds to message transmitted
Never delivers part of a message, nor does it join multiple messages together
Sender places exactly n bytes in outgoing message
Receiver receives n bytes in the incoming message

Message Transport Types
Unicast
a message can be sent from an application on one computer directly to an application on another, 1-to-1
Multicast
a message can be multicast to some of the computers on a network, 1-to-many
Broadcast
a message can be broadcast to all computers on a given network, 1-to-all

Message Transport Reliability
Message transport is unreliable
Unreliable means no guarantee:
Lost: never delivered
Duplication: more than one copy arrives
Ordered: out-of-order
Programmers must ensure the application operates correctly even if packets are lost or reordered

Message vs Stream
Most applications require delivery guarantees
Use stream services
Programmers tend to use stream service except in special situations
E.g. video, where multicast is needed, the application provides support to handle packet reordering and loss

Connection-oriented Communication
Stream service is connection-oriented:
Applications must request connection be created
Once established, connection allows applications to send data in either direction
Finally, when communication finishes, the applications request the connection be terminated
Source: Douglas, C (2016) Computer Networks and Internets
Algorithm for Stream Paradigm

Practice 3.1
Distinguish between stream and message communication with TWO differences
Describe the THREE steps in a stream paradigm communication algorithm

Lesson 3 Outline
Communication Paradigms
Client Server Model
Network Programming

Client-server Model
16
How can a pair of applications that run on two independent computers coordinate to guarantee that they request a connection at the same time?
The answer lies in a form of interaction known as the client-server model
A server starts first and awaits contact
A client starts second and initiates the connection
Application programs known as clients and servers handle all services in the Internet

Client-Server Differences
Source: Douglas, C (2016) Computer Networks and Internets
Server Application Client Application
Starts first Starts second
Does not need to know which client will contact it Must know which server to contact
Waits passively and arbitrarily long for contact from a client Initiates a contact whenever communication is needed
Communicates with a client by both sending and receiving data Communicates with a sever by sending and receiving data
Stays running after servicing one client and waits for another May terminate after interacting with a server

Characteristics of Client Software
A client software:
Invoked directly by a user, executes only for one session
Runs locally on a user’s personal computer
Actively initiates contact with a server
Can access multiple services as needed, but usually contacts one remote server at a time
Does not require especially powerful computer hardware

Characteristics of Server Software
A server software:
A special-purpose, privileged program
Dedicated to providing one service that can handle multiple remote clients at the same time
Is invoked automatically when a system boots, and continues to execute through many sessions
Runs on a large, powerful computer
Waits passively for contact from arbitrary remote clients
Accepts contact from arbitrary clients, but offers a single service
Requires powerful hardware and a sophisticated OS

Server Programs & Server-class Computers
Server: a program that waits passively for communication
Not to the computer on which it executes
When a computer is running server program(s), it is sometimes called a server
Hardware vendors contribute to the confusion as they classify powerful machines as servers

Server Programs and Server-class Computers
Source: Douglas, C (2016) Computer Networks and Internets

Requests & Response
Once contact is established, two-way communication is possible
From a client to a server
From a server to a client
A client can send a series of requests
The server issues a series of responses
A database client might allow a user to look up more than one item at a time

One Computer, Multiple Servers
A computer that operate multiple servers is useful
Hardware can be shared
Has lower system administration overhead than multiple computer systems
Demand for a server is often sporadic:
Server can remain idle for long periods of time and does not use the CPU while waiting for a request to arrive
If demand is low, consolidating servers can reduce cost without reducing performance

Multiple Clients & Servers
A computer can run:
A single client
A single server
Multiple copies of a client that contact a given server
Multiple clients that each contact a server
Multiple servers, each for a service

Multiple Clients Benefits
A computer with multiple clients is useful as services can be accessed simultaneously
For example, a user can have three (3) windows open simultaneously running three (3) applications:
Email
Chat service
Web browser

Server Identification
Internet protocols divide identification to 2 pieces:
Identifier for the computer on which a server runs
Identifier for a service on the computer
Each computer in the Internet is assigned a unique 32-bit identifier known as an Internet Protocol (IP) address
192.168.1.0
It is an Internet layer address

DNS for Server Identification
A client must specify the server’s IP address
To make server identification easy, each computer is assigned a name and the Domain Name System (DNS)
Thus, a user specifies a name such as www.cisco.com rather than an integer address

Server Demultiplexing
Each service available in the Internet is assigned a unique 16-bit identifier known as port number
Email: 25, HTTP: 80
It is a Transport layer address
When a server begins, it registers with OS by specifying the port number for its service
A client contacts a server to request service
the request contains a port number
A request arrives at a server
software uses port number to determine which application on the server will handle request

Server Identification & Demultiplexing
Source: Douglas, C (2016) Computer Networks and Internets

Server Circular Dependencies
Circular dependencies: a server for one service can act as client for another
Before it can fill in a web page, a web server may need to become a client of a database
A server may also become the client of a security service to verify a client is allowed access to the service
Database
Server
Web Page Server
User
Client of
Client of

Concurrent Servers
Most servers are concurrent
A server uses more than one thread of control
Concurrent execution depends on the OS being used and the code is divided into 2 pieces
Main program (thread)
Handler
Main thread accepts contact from client and creates a thread of control for the client
Each thread of control interacts with a single client and runs the handler code

Running Concurrent Servers
After handling one client the thread terminates
Main thread keeps server alive after creating a thread to handle a request
If N clients are simultaneously using a concurrent server, N+1 threads will be running:
N threads for each client
+1 main thread to wait for additional requests
Main Thread
User
User
Web Page Server
Handler 1
Handler 2

Server Bottlenecks
If a single server provides a given service
the network connection between the server and the Internet can become a bottleneck
User
Internet
Server
User
User
Bottleneck

Peer-to-Peer Interactions
One way to avoid a bottleneck forms the basis of file sharing known as a peer-to-peer (P2P) architecture
Data is distributed equally among a set of N servers
Each request is sent to the appropriate server
Each server only provides 1/N of the data
User
Internet
Server 1 of N
User
1/N traffic
Server 2 of N

Practice 3.2
Describe TWO identification methods that Internet Protocols uses to identify servers
Explain what is server circular dependencies and bottleneck.

Lesson 3 Outline
Communication Paradigms
Client Server Model
Network Programming

What is an API?
Source: https://www.youtube.com/watch?v=s7wmiS2mSXY

Socket API
Application Program Interface (API): interface that an application uses to specify communication
Details of an API depend on the OS
de facto standard for software that communicates over the Internet is the socket API
commonly abbreviated sockets
Socket API is available for many OS
Windows, various UNIX, Linux

Sockets & Descriptors
Originally developed as part of UNIX OS, the socket API is integrated with I/O
When an application creates a socket, OS returns a small integer descriptor that identifies the socket
Application passes the descriptor as an argument
when it calls functions to perform an operation on the socket such as to transfer data across the network or to receive data

Network I/O
In many OS, socket descriptors are integrated with other I/O descriptors

An application can use the read and write operations for socket I/O or I/O to a file

Socket Functions
In socket programming an application must specify many details, such as
address, port number, whether the application will act as a client or as a server
To avoid having a single socket function with many parameters, designers of the socket API chose to define many functions
An application creates a socket, and then invokes functions for details

Pros & Cons of Socket Approach
Advantage of socket approach is that most functions have three or fewer parameters
Disadvantage is that a programmer must remember to call multiple functions when using sockets

Major Functions in Socket API
Source: Douglas, C (2016) Computer Networks and Internets

Socket Calls in a Client and Server
This illustrates the sequence of socket calls made by a typical client and server that use a stream connection
In some applications, send and recv are called in the reverse order
Source: Douglas, C (2016) Computer Networks and Internets

Client Connection Functions
Clients call connect to establish a connection with a specific server
connect (socket, saddress, saddresslen)
Argument socket is the descriptor of a socket to use for the connection

Sockets in Message Paradigm
Socket functions used to send and receive messages are more complicated than stream paradigm because many options are available
A sender can choose whether to store the recipient’s address in the socket and merely send data or to specify the recipient’s address each time a message is transmitted
One function allows a sender to place the address and message in a structure and pass the address of the structure as an argument, and another function allows a sender to pass the address and message as separate arguments

Practice 3.3
What is an Application Program Interface (API)?
Discuss one advantage and one disadvantage of using sockets.

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 3

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 11: Error Handling

Lesson 11 Learning Outcomes
Describe the main sources of transmission errors
Understand the effects of transmission errors on data
Describe two strategies to handle channel errors
Describe block error codes and convolutional error codes

Lesson 11 Learning Outcomes
Describe single parity checking
Calculate Hamming distance between pair of words
Understand the error checks used on the Internet

Lesson 11 Outline
Transmission Errors
Channel Coding

Data Communication Issues
Data communications systems are prone to errors
Some are inherent in the physics of the universe
Some from devices that fail
Some from equipment that does not meet the engineering standards
Small errors that occur during transmission are more difficult to detect than complete failures
Need mechanisms to control and recover

Transmission Errors (1/2)
Interference: electromagnetic radiation emitted from devices like electric motors
Background cosmic radiation
Cause noise that can disturb radio transmissions and signals traveling across wires
Attenuation: signal loss over distance
signals on wired media become weaker over long distances
radio signal becomes weaker with distance

Transmission Errors (2/2)
Distortion: all physical systems distort signals
As pulse travels along fiber, it disperses
Capacitance and inductance: block signals at some frequencies while admitting signals at other frequencies
Placing wire near metal object can change the frequencies that can pass through wire
Metal objects can block some frequencies of radio waves, while passing others

Reducing Errors
Shannon’s Theorem suggests to increase SNR:
Either increase signal or lower noise if possible
Mechanisms like shielded wiring can help lower noise
Physical transmission system is always prone to errors
May not be possible to change the SNR

Error Handling
Noise/interference cannot be eliminated completely
Many errors can be detected
Some can be corrected automatically
But Error detection adds overhead
Error handling is a tradeoff
To decide whether a given error is likely to occur
If so, what is the consequence

Types of Errors
Spike: extremely short duration interference,
often the cause of a single bit error
Longer duration interference or distortion can produce burst errors
Sometimes signal is neither clearly 1 nor clearly 0, but falls in an ambiguous region, which is known as an erasure

Types of Errors Summary
Source: Douglas, C (2016) Computer Networks and Internets

Burst Error
For a burst error:
Burst size/length: number of bits from the start of the corruption to the end of the corruption
Source: Douglas, C (2016) Computer Networks and Internets

Lesson 11 Outline
Transmission Errors
Channel Coding
Parity Checking
Hamming Distance and Hamming Code
Row and Column Parity (RAC)
Internet Checksum

Error Correction History
Source: https://www.youtube.com/watch?v=q-3BctoUpHE

Channel Coding
Number of mathematical techniques have been developed to overcome errors
Known collectively as channel coding
These techniques are
Forward Error Correction (FEC)
Automatic Repeat reQuest (ARQ)

Automatic Repeat Request
ARQ requires cooperation of sender and receiver
Receiver needs to send a short acknowledgement (ACK) message back
If A sends message to B, B sends ACK back to A
Once receives ACK, A knows that the message arrived correctly
If no ACK is received after T time, A assumes the message was lost and retransmits a copy

ARQ Limitations
Useful with detecting errors but not for error correction
For error correction, most use Cyclic Redundancy Check (CRC)
ARQ scheme can be added to guarantee delivery if a transmission error occurs
Receiver discards message if an error occurs
Sender retransmits another copy

Forward Error Correction
FEC adds additional information to data to allow receiver to verify data arrives correctly (detection)
Tries to correct errors if possible
FEC mechanisms allow receivers to determine
Which bits have been changed
How to compute correct values

Visualising FEC
Source: Douglas, C (2016) Computer Networks and Internets

Block Error Codes
Block Error Codes is a type of FEC
Divides data sent into set of blocks
Attach information known as redundancy to each block
Encoding for a given block of bits depends on the bits themselves, not on bits sent earlier
Memoryless: does not carry state information from one block of data to the next

Convolutional Error Codes
Convolutional Error Codes is also a type of FEC
Treats data as a series of bits
Computes a code over a continuous series
Code computed for a set of bits depends on current input and some of the previous bits in the stream
Codes with memory

Single Parity Checking
Single Parity Checking (SPC): a type of block error codes
SPC can define a block to be 8-bit (byte) of data
Sender and receiver must be configured for either even parity or odd parity
On sender, an encoder adds an extra bit, parity bit
Total bits: 9-bit (8-bit data, 1-bit parity)
Receiver uses parity bit to check whether bits in the byte are correct

SPC Example
Source: Douglas, C (2016) Computer Networks and Internets
What is the limitation?

Practice 11.1
Complete the table below
Original Data Even Parity Odd Parity
0011 1100
1111 1100
0101 0101
1100 0011
1010 1100
0011 0111
1111 1111

SPC Limitation
SPC can detect errors but cannot correct errors
Even parity fails if odd number of bits changed
Odd parity fails if even number of bits changed
Consider even parity example:
If one-bit changed value, receiver will declare incoming byte is invalid
If two, four, six, or eight bits changed value, receiver will incorrectly declare valid

Lesson 11 Outline
Transmission Errors
Channel Coding
Parity Checking
Hamming Distance and Hamming Code
Row and Column Parity (RAC)
Internet Checksum

Some Definitions
Datawords: set of all possible messages
Codewords: set of all possible encoded versions
Codebook: subset of the possible combinations that are valid codewords

Hamming Distance
Engineers use a measure known as the Hamming distance
named after a theorist at Bell Laboratories who was a pioneer in the field of information theory and channel coding
Given two strings of n-bit each, Hamming distance is defined as the number of differences
Number of bits to change for one string to be identical to another

Hamming Distance Example
Source: Douglas, C (2016) Computer Networks and Internets

Computing Hamming Distance
Straight forward way is manual observation
Another way is:
Perform exclusive or (XOR) between two strings
Counting number of 1 bits in the answer

Input 1 Input 2 Output
0 0 0
0 1 1
1 0 1
1 1 0

XOR Function
String 1: 1 1 0
String 2: 0 1 1
1 0 1

Total number of 1s = 2
Hence, Hamming distance = 2

Hamming Distance for Codebook
Errors can transform a valid codeword into another valid codeword
To measure such transformations, compute the Hamming distance
between all pairs of codewords in a codebook
Source: Douglas, C (2016) Computer Networks and Internets

Minimum Hamming Distance
Minimum Hamming distance (dmin): lowest Hamming distance among pairs in a codebook
Number of bits that cause a transformation from one valid codeword to another valid codeword
In this example, dmin = 2
Source: Douglas, C (2016) Computer Networks and Internets

Hamming Distance Implication
Implication of dmin = 2:
There is at least one valid codeword that can be transformed into another valid codeword,
If 2-bit errors occur during transmission
A large value of dmin is desirable
if < dmin bits are changed, the code can detect that error(s) occurred 33 Bit Error Detection Maximum number of bit errors that can be detected: e = dmin – 1 Code with higher dmin sends more redundant information 34 Hamming Code Hamming code uses multiple parity bits within a code Each parity bit checks a unique combination of bits Parity bit appears in bit positions that are powers of 2 i.e. bits 1, 2, 4, 8 Remaining bit positions are occupied by bits that are to be checked 35 Hamming Code Parity Checks This table shows the bits check by each parity bits b1: checks all the odd bits b2: checks b2, b3, b6, b7, b10, b11 b4: checks b4, b5, b6, b7, b12 b8: checks b8, b9, b10, b11, b12 Binary code for bit number at top of each column can be obtained by reading all the bits in that column from bottom up 20 = 1 21 + 23 = 10 36 Hamming Example (1/3) Code 0110 1110 in even Hamming code Step 1: Place bits from this byte in first row of a table headed with bit numbers in reverse order, leaving spaces for parity bits Step 2: In b1 row, place all the bits that are checked by b1 Step 3: Calculate value of b1 which give even parity and write in b1 column (use hamming code parity table) Step 4: Repeat Step 2 and 3 for b2 in b2 row, b3 in b4 row and b8 in b8 row 37 Hamming Example (2/3) Step 1: Place bits from this byte in first row of a table headed with bit numbers in reverse order, leaving spaces for parity bits Step 2: In b1 row, place all the bits that are checked by b1 Step 3: Calculate b1 and write in b1 (using Hamming parity table) Step 4: Repeat Step 2 and 3 for b2 in b2 row, b3 in b4 row and b8 in b8 row 38 Hamming Example (3/3) With the parity bits calculated, even Hamming code is as follows 0110 0111 1001 As such, 0110 0111 1001 is transmitted 39 Hamming Example Error (1/2) Original: 0110 0111 1001 Error: 0110 0011 1001 Assuming the error byte is received Step 1: Enter the error byte in the first row Step 2: Check parity calculation for b1, b2, b4 and b8 Step 3: If total parity is odd, underline parity bit and place a 1, which is what is needed to ensure even parity, in the “fix” column and a 0 otherwise 40 Hamming Example Error (2/2) Error occurs in parity bit 1, 2 and 4 but not 8 b7 is the only one check by bit 1,2 and 4 Shows that b7 is the one corrupted Interesting fact is reading the bits in fix column upwards gives the decimal number 7, confirming that error is in b7 i.e. 01112 = 710 Correct the 7th bit to get the original byte 0110 1110 41 Practice 11.2 Code 1101 1010 in even Hamming code 42 Hamming Code Implication Hamming codes is a useful FEC technique But overhead is 50% for 8-bit characters Redundant data transmitted However, there is a potential saving in terms of reduced re-transmissions 43 Lesson 11 Outline Transmission Errors Channel Coding Parity Checking Hamming Distance and Hamming Code Row and Column Parity (RAC) Internet Checksum 44 Row and Column Parity (RAC) Imagine an array of 3-rows and 4-columns Row and Column (RAC) code: parity bit added to each row and column Source: Douglas, C (2016) Computer Networks and Internets 45 RAC Error Detection On the receiver, bits are arranged into an array and parity bits are recalculated If single bit error, two of the calculations will disagree with the parity bits received Source: Douglas, C (2016) Computer Networks and Internets 46 Practice 11.3 Assuming even parity, identify the error bit in the following array. 1 0 1 1 0 1 1 0 1 1 0 1 0 0 1 0 0 0 0 0 1 1 0 0 47 RAC Uses and Limitations Two disagreements correspond to the row and column position of the error Receiver uses calculated parity bits to determine exactly which bit is in error and corrects the bit RAC can only correct single-bit errors In cases where two or three bits are changed RAC encoding will be able to detect an odd number of errors 48 Lesson 11 Outline Transmission Errors Channel Coding Parity Checking Hamming Distance and Hamming Code Row and Column Parity (RAC) Internet Checksum 49 Internet Checksum Internet checksum is a coding scheme on the Internet Code consists of 16-bit 1s complement checksum The Internet checksum does not impose a fixed size on a dataword Allows a message to be of any length Computes a checksum over the entire message 50 Padding the Message Internet checksum treats data in a message as a series of 16-bit integers Source: Douglas, C (2016) Computer Networks and Internets 51 Computing Internet Checksum Sender adds the numeric values of the 16-bit integers, transmits the result Checksum computed in 1s complement arithmetic Uses 16-bit integers instead of 32 or 64-bit To validate the message, a receiver performs the same computation 52 Internet Checksum Example On the receiver: If sum is all 1’s, no error If sum has a 0 somewhere, error has occurred 53 Getting the Checksum (1/2) 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 0 1 1 0 1 0 0 1 0 0 1 1 1 1 0 1 0 1 1 1 1 0 1 0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 1 0 0 0 0 0 Step 1: Perform binary addition Step 2: Bring the leftmost carry to rightmost 1 0 0 0 0 1 1 1 0 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 1 0 0 1 0 0 0 0 1 54 Getting the Checksum (2/2) Getting the Checksum Original 1 0 0 0 0 1 1 1 0 0 1 0 0 0 0 1 Complement 0 1 1 1 1 0 0 0 1 1 0 1 1 1 1 0 Step 3: One complement Checksum: 0111 1000 1101 1110 55 Detecting Errors (1/2) Valid 1st word: 1001 1011 0100 1001 Valid 2nd word: 1110 1011 1101 0111 1st word corrupted 3rd bit from right: 1001 1011 0100 1101 Checksum: 0111 1000 1101 1110 11 11 1 11 11 1 11 1 1 1 0 0 1 1 0 1 1 0 1 0 0 1 1 0 1 1 1 1 0 1 0 1 1 1 1 0 1 0 1 1 1 0 1 1 1 1 0 0 0 1 1 0 1 1 1 1 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 Step 1: Perform binary addition with checksum 56 Detecting Errors (2/2) Step 2: Spot the error bit Error bit 11 11 1 11 11 1 11 1 1 1 0 0 1 1 0 1 1 0 1 0 0 1 1 0 1 1 1 1 0 1 0 1 1 1 1 0 1 0 1 1 1 0 1 1 1 1 0 0 0 1 1 0 1 1 1 1 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 Valid 1st word: 1001 1011 0100 1001 Valid 2nd word: 1110 1011 1101 0111 1st word corrupted 3rd bit from right: 1001 1011 0100 1101 Checksum: 0111 1000 1101 1110 57 Practice 11.4 Calculate the Internet checksum for: 10101100 10010101 00100100 11100100 58 Reading Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 8 59 End of Lesson 60

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 17: Ethernet

Lesson 17 Learning Outcomes
Describe the different channelized, controlled access and random access protocols
Identify the frame format used in Ethernet
Describe the evolution of Ethernet
Describe the hardware used on the Ethernet

Lesson 17 Outline
Multi-access Protocols
Ethernet Standards
Ethernet Wiring

Multi-Access Protocols
Channelized access: like multiplexing
Controlled access: need to grant access
Random access: computer’s own judgement
Source: Douglas, C (2016) Computer Networks and Internets

Channelized Access
Channelization: mapping between a given communication and channel in underlying system
1-to-1 or static channel allocation: works well when communicating entities does not change
Dynamic channel allocation: needed where communicating entities varies
E.g. cellular telephony
Source: Douglas, C (2016) Computer Networks and Internets

Controlled Access
Controlled access is a distributed version of statistical multiplexing
Source: Douglas, C (2016) Computer Networks and Internets

Controlled Access: Polling
Polling: centralised controller that cycles through stations on the network and gives each an opportunity to transmit a packet
Round robin order: transmit on rotation basis
Each station equal opportunity to transmit
Priority order: based on highest priority
Priority stations more opportunity to send
Example: IP phone higher priority than PC

Controlled Access: Reservation
Reservation: employs a two-step process which each round of transmissions is planned in advanced
Step 1: each sender specifies whether they have packets to send and controller broadcast a transmission schedule
Step 2: stations use the schedule to transmit
Variation: an alternate channel to gather reservations for next round and main channel for current round of transmission

Controlled Access: Token Passing
Token passing: a special control message (token) circulate the network continuously
Computers who want to transmit must capture the token
Ring topology: order of circulation is defined, clockwise or anti-clockwise
Bus topology: each station is assigned a position in a logical sequence and the token is passed according to the assigned sequence

Random Access Protocols
Computers attempt to access the shared medium without coordination
Random: access only given when station has packet to send
Prevent all computers from using a medium at the same time
Source: Douglas, C (2016) Computer Networks and Internets

ALOHAnet
ALOHAnet: an early network in Hawaii that pioneered the concept of random access
no longer used, but ideas have been extended
ALOHAnet consist of a powerful transmitter in a central geographic location
Surrounded by a set of stations/computer
Stations can reach the central transmitter but not able to reach all the other stations

ALOHAnet Illustration
ALOHAnet used two carrier frequencies for broadcasting:
Outbound: central transmitter to all stations
Inbound: stations to the central transmitter
Source: Douglas, C (2016) Computer Networks and Internets

ALOHA Protocol
When a station has a packet to send, it transmits the packet on inbound frequency
Central transmitter repeats the transmission on the outbound frequency
To ensure transmission is successful, sender listens to outbound channel
Copy arrives: sender moves to next packet
No copy arrives: sender waits and try again

Transmission Collision
Interference can occur if two stations transmit simultaneously
Collision occurs when two signals interfere and becomes will be garbled
OR two transmitted packets collide
The protocol handles a collision by requiring sender to retransmit each lost packet

Handling Collisions in Ethernet
In 1978, Ethernet was created by Digital Equipment Corporation, Intel, and Xerox
Uses shared wired medium (cable) instead of broadcasting through the atmosphere
Ethernet uses 3 mechanisms to handle collisions:
Carrier sense
Collision detection
Binary exponential backoff

Carrier Sense
Carrier sense: each station monitor cable to detect if another transmission is in progress
Prevents most collision problems and improves network utilisation
Limitation: collision occurs if two stations finds cable idle, both transmit
Some time is required for signal to travel down the cable, a station may not know instantly when another station begins to transmit

Collision Detection
Collision detection: if signal on cable differs from signal that is transmitted, collision has occurred
If collision occurs, sender aborts transmission
Many details complicate Ethernet transmission
Following a transmission, stations must wait for an interpacket gap (9.6s for 10 Mbps Ethernet) to ensure that all stations sense an idle network and have a chance to transmit

Backoff Mechanism
Backoff: after a collision, sender must wait for cable to be idle before transmitting
Specifies max delay d, and a random delay less than d after collision
Randomisation is used to avoid multiple stations transmit simultaneously when cable is idle
If two or more sender choose nearly same amount of delay, there’s risk of second collision
Source: Bing, licensed under CC BY-NC-ND

Binary Exponential Concept
Binary exponential: doubling the range of the random delay
To avoid a sequence of collisions each computer will double range of delay after each collision
Random delay 0 – d, 0 – 2d, 0 – 4d etc.
After few collisions, range becomes large, lowers risks of choosing similar delay values

CSMA/CD
Binary exponential backoff doubles range of the random delay for transmission after each collision
Allows Ethernet to recover quickly after collision
Guarantees reduced contention
Carrier Sense Multi-Access with Collision Detection (CSMA/CD) combines all techniques
Carrier Sense, Collision Detection and Binary Exponential backoff

CSMA/CD Limitations
CSMA/CD does not work as well in wireless LANs (WLAN) as transmitter in WLAN has limited range
A receiver > δ distance away from transmitter will not receive signal and cannot detect carrier

Source: Douglas, C (2016) Computer Networks and Internets

Hidden Station Problem
Hidden station problem: some stations are not visible to others in a WLAN (> δ distance away)
WLAN use a modified access protocol CSMA with Collision Avoidance (CSMA/CA)
CSMA/CA triggers a brief transmission from the intended receiver before transmitting a packet

CSMA/CA
When both sender and receiver transmit a message, all computers within range of either will know a packet transmission is starting

Source: Douglas, C (2016) Computer Networks and Internets

CSMA/CA Steps
Comp3 sends a short message to announce it is ready to transmit a packet to Comp2
Computers in range of Comp3 receive this
Comp2 responds by sending a short message announcing it is ready to receive the packet
Computers in range of Comp2 receive this
Comp1 knows a packet transmission is taking place, even though it cannot receive the signal or sense a carrier

CSMA/CA Collision
Collisions of control messages can occur in CSMA/CA, but can be handled easily
If Comp1 and Comp3 each transmit a packet to Comp2 at the same time, their control messages will collide
When collision occurs, both senders apply random backoff before retransmission
Control messages are smaller than a packet, probability of second collision is low

Automotive Ethernet
Source: https://www.youtube.com/watch?v=BW57JpmZEcc

Practice 17.1
Explain why, for CSMA approaches, slower computers might not get an opportunity to transmit.
What is an alternative multi-access protocol for such cases?

Lesson 17 Outline
Multi-access Protocols
Ethernet Standards
Ethernet Wiring

Ethernet Compatibility
Ethernet versions are backward compatible
New version can automatically adapt to accommodate the older technology
Ethernet are also compatible with newer versions
Ethernet frame format has remained constant since it was created in 1970

Ethernet Frame Format
Frame format: the way a packet is organised
including details like size and meaning of individual fields
Ethernet frame consists of a fixed-length header, a variable-length payload, and a fixed-length CRC

Source: Douglas, C (2016) Computer Networks and Internets

Ethernet Multiplexing
Type field in Ethernet frame allows a computer run multiple protocols operating simultaneously e.g.:
hexadecimal 0800 (IP datagrams) & 0806 (ARP messages)
Receiver will use the type field to determine which software module should process the frame
Source: Douglas, C (2016) Computer Networks and Internets

Ethernet 802.3
IEEE developed standard for Ethernet 802.3 in 1983 which interprets the original type field as a packet length
But also adds 8-byte header that is known as a Logical Link Control / Sub-Network Attachment Point (LLC/SNAP)
Source: Douglas, C (2016) Computer Networks and Internets

Ethernet 802.3 Frame Size
Overall frame size in 802.3 Ethernet remains same at 1514 bytes
Payload is reduced from 1500 to 1492 bytes with 8 bytes SNAP header
To keep the two versions of Ethernet compatible, a convention is used:
If bytes 13-14 of a frame contain a numeric value less than 1500, the field is interpreted as packet length and 802.3 standard applies

Practice 17.2
How does Ethernet achieve multiplexing?
What field does it examine to determine which software program should process the frame?

Lesson 17 Outline
Multi-access Protocols
Ethernet Standards
Ethernet Wiring

Ethernet Wiring
Original Ethernet wiring scheme was informally called thick wire Ethernet or Thicknet (10Base5)
Consisted of heavy coaxial cable
Hardware used with Thicknet is divided into two:
Network Interface Card (NIC) to handle digital aspects of communication
Transceiver to handles carrier detection and conversion between digital and analog data

Thicknet Wiring
A physical cable, Attachment Unit Interface (AUI) is connects a transceiver to a NIC
Transceiver is usually separated from a computer
In an office building, transceivers might attach to an Ethernet in a hallway ceiling
Source: Douglas, C (2016) Computer Networks and Internets

2nd Generation Ethernet Wiring
Thicknet evolves to Thinwire Ethernet or Thinnet (10Base2) that use a thinner coaxial cable
Integrates transceiver directly on NIC and runs a coaxial cable from one computer to another
Source: Douglas, C (2016) Computer Networks and Internets

Thinnet Benefits & Issue
Benefits:
Lower overall cost and ease of installation
No external transceivers were needed
Can be installed in a convenient path
Issue:
Entire network is vulnerable if user unplugged a segment of the network, the entire network would stop working

3rd Generation Ethernet Wiring
Instead of coax, it use a central electronic device (Hub) separate from the computers attached to the network and uses twisted pair
Informally known as twisted pair Ethernet
Hubs are available in a variety of sizes with cost proportional to size
Source: Douglas, C (2016) Computer Networks and Internets

Ethernet Wiring Emulation
A hub emulates a physical cable that makes entire system works like a conventional Ethernet
Uses CSMA/CD
Twisted pair Ethernet retains same frame format as the previous versions
Software on computers do not distinguish between thicknet, thinnet or twisted pair Ethernet
NIC handles details and hides any differences

Ethernet Topology
Hub can be thought of as a bus in a box
Logical and physical topologies:
Logically: twisted pair Ethernet employs a bus topology
Physically: twisted pair Ethernet forms a star-shaped topology

Wiring in Office Buildings
Styles of wiring used for LANs make little difference in a machine room or laboratory
Type of wiring makes a difference in terms of:
type
number of wires needed
distance spanned
cost

Wiring Illustration
Twisted pair Ethernet may require many individual cables to go between offices and a central point, wiring closet
Requires careful labeling of cables

Source: Douglas, C (2016) Computer Networks and Internets

Variants of Ethernet
Higher-speed Ethernet use an electronic device known as a switch
To remain backward compatible:
interfaces automatically sense (autosense) the speed at which a connection can operate; slows down to accommodate older devices
Source: Douglas, C (2016) Computer Networks and Internets

Twisted Pair Connectors
Twisted pair Ethernet uses RJ45 connectors
RJ45 are larger versions of the RJ11 connectors used to connect telephones
Source: Douglas, C (2016) Computer Networks and Internets

Twisted Pair Cables
Straight cable connects a computer and a switch
connects each pin of the RJ45 attached to one end of the cable directly to the corresponding pin on the RJ45 at the other end
Crossed cable connects two switches
connects a pin on one end to a different pin on the other end
To ensure correct connections, Cat 5 or Cat 6 cable are coated with colored plastic

Practice 17.3
Why is the 2nd generation of Ethernet known as Thinnet?

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 14, 15

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 5: Internet Applications

Lesson 5 Learning Outcomes
Understand the purpose of File Transfer Protocols
Describe the steps performed in email
Describe the specifications used in email
Understand the role of ISP, Mail server and Mail Access
Compare and contrast SMTP, POP and IMAP
Describe the commonly used Email Representation standards

Lesson 5 Outline
File Transfer Protocol
Email Protocols
Email Transfer Protocols
Email Access & Representation

Transferring Files over the Internet
File Transfer Protocol (FTP): allows sending a copy of a file from one computer to another
FTP provides a powerful mechanism for the exchange of data
Most FTP is one-to-one

Files on the Internet
File: the fundamental storage abstraction
Holds an arbitrary object like document, spreadsheet, computer program, graphic image, or data
File transfer over the Internet is complicated because computers are heterogeneous
Different file representations
Different type information
Different naming
Different file access mechanisms

Heterogenous Computers
Different OS representation:
Extension for a JPEG image can be or
Each line in text file is terminated by a LINEFEED character or CARRIAGE RETURN and LINEFEED
Separator in files names can be slash (/) or backslash (\)
User accounts which are given the right to access certain files
Account information differs among computers
User X on one is not same as user X on another

FTP Characteristics
Arbitrary file contents: can transfer any type of data
Bidirectional transfer: download or upload
Supports authentication & ownership
Each file have ownership & access restrictions
Ability to browse folders
Textual control messages that are sent as ASCII text
Accommodates heterogeneity: hides the details of individual computer OS
FTP is mostly invisible: invoked automatically by browser when user requests for file download

FTP Communication Paradigm
FTP uses a client/server model
Client establishes a command connection to FTP server and sends requests to which the server responds
Control connection: connection used for commands, requests from clients
When FTP server needs to download or upload a file, it opens a data connection
Data connection: connection used to transfer files
This inverts client-server relationship for data connections!

FTP Communication Sequence
Source: Douglas, C (2016) Computer Networks and Internets
1
2
3
4
5
6
7
8
9

10
11

FTP Authentication
After creating the control connection, client must log into the server
USER command for login name
PASS command for password
Server sends a numeric status response over control connection to indicate if login is successful
Client can only send other commands if login is successful

Anonymous Login
For public files, anonymous clients can access
username “anonymous”
password mostly “guest”

Souce: Bing, licensed under CC BY-SA

FTP Port Number
However, what protocol port number should server specify when connecting to the client?
Default FTP port number is 21
Client allocates a protocol port on local OS and sends port number to the server
Client binds to port to wait for a connection
Clients transmits PORT command over control connection to inform server

FTP Exceptions
Transmission of port number may fail if one of the two endpoints lies behind a Network Address Translation (NAT) device
like wireless router in residence or small office
To support FTP, a NAT device will recognise FTP control connection, inspects the contents and rewrites values in a PORT command

FTP Commands
Four common commands:
open and close: for command connection
ls and cd: to list and change directory
get and put: to open download and upload data
quit: to close FTP

FTP Algorithm
Source: Douglas, C (2016) Computer Networks and Internets

Modern FTP
Modern FTP now comes with a GUI
It now more intuitive rather than by commands
More visually understood by user
Source: Dropbox
Source: Google Drive
Source: Microsoft OneDrive

Practice 5.1
A user uses FTP to transfer files between his laptop and a file server.
What kind of communication model does FTP uses?
What format are control messages sent as in FTP?
What are the TWO commands does the user need to provide to the file server in order authenticate himself?

Trivial File Transfer Protocol (TFTP)
Some devices do not have enough memory and processing capability
Hosts within same LAN has lower probability of errors
TFTP is a light-weight file transfer protocol
Based on message paradigm but each message must be acknowledged before another message can be sent

TFTP Usage and Limitations
TFTP is often used by network managers to download or upload router configurations and software
Unlike FTP, it cannot locate files in directories
File to be transferred are specified in the command line
TFTP has no authentication facility
Network managers need to configure to allow only known IP address

Network File System (NFS)
Sometimes it is easier to access files rather than transfer them
NFS allows physically remote directories to be mounted on local systems
Appears to be local to the users
All standard operations carried out by the OS are supported transparently on remote directories and files

NFS Communication Paradigm
NFS can use message or stream paradigm for its transport service
Since it is simple client/server application, best to use message paradigm
NFS is generally not secure but have some simple authentication facilities

Practice 5.2
What is the rational for using the TFTP over FTP?
Distinguish NFS from FTP by giving an explanation to each.

Lesson 5 Outline
File Transfer Protocol
Email Protocols
Email Transfer Protocols
Email Access & Representation

Electronic Mail
Email is one of the most common Internet application
Email software is divided into:
Email interface application: mechanism to compose/edit outgoing and read/process incoming messages
Mail transfer program: acts as client to send a message to a mail server on the destination computer
mail server accepts incoming messages and deposits each in the corresponding mailbox
Source: Douglas, C (2016) Computer Networks and Internets

Email Algorithm
Source: Douglas, C (2016) Computer Networks and Internets

Email Specifications
Specifications for email can be divided into three broad categories

Protocol Type Description
Transfer Move a copy of an email message from one computer to another
Access Allows user to access their mailbox and to view/send email messages
Representation Specifies the format of an email when stored on disk

Lesson 5 Outline
File Transfer Protocol
Email Protocols
Email Transfer Protocols
Email Access & Representation

Simple Mail Transfer Protocol (SMTP)
SMTP: standard protocol used by mail transfer program uses
Simple text-based protocol
SMTP can send a message to multiple recipients
By allowing clients to list users
Sends a copy of a message to all users on list

SMTP Headers and Body
Messages comprise a set of envelope headers
Headers has MAIL FROM and RCPT TO
Message header are followed by a blank line and the actual text
Message is prefixed with DATA followed by the text (content)
Body is terminated by newline with just a full stop followed by another newline

SMTP Characteristics
Follows a stream paradigm
Uses textual control messages
Only transfers text messages
Allows a sender to specify recipients’ names and check each name
Sends one copy of a given message

Multipurpose Internet Mail Extensions (MIME)
MIME extends the functionality of email to allow the transfer of non-text data in a messages
MIME specifies how a binary file can be encoded into printable characters and decoded by receiver
MIME defines several data types
text/plain, text/html, image/jpeg, application/msword

MIME Encoding
MIME permits a sender/receiver to choose a convenient encoding
Sender includes additional lines in the header to specify encoding used
User can attach multiple attachments, each with their own encoding
Base64 encoding is most popular, but MIME does not restrict encoding to a specific form

MIME Header
MIME adds two lines to an email header
To declare that MIME has been used to
To specify how MIME information is included
For example, the header lines:
MIME-Version: 1.0
Content-Type: Multipart/Mixed;
Boundary=Mime_separator
Mime_separator (e.g. “/”) will appear in the message body before each part

MIME Backward Compatibility
MIME is backward compatible with email that do not understand the MIME standard or encoding
Such systems have no way of extracting non-text attachments
But will treat the body as a single block of text

SMTP Session Example
Source: Douglas, C (2016) Computer Networks and Internets

Practice 5.3
What is the limitation of SMTP that resulted in the protocol MIME?
What happen if a system is using an older version of email that do not support MIME but receives a message with an attachment that uses MIME?

Lesson 5 Outline
File Transfer Protocol
Email Protocols
Email Transfer Protocols
Email Access & Representation

Internet Service Providers (ISP)
Most users leave computer running continuously
But do not know how to manage an email server
ISPs began offering email server services
Provides a mailbox for each user
each ISP provides interface to access mailbox
Email access follows one of two forms:
Special-purpose email interface application
Web browser that accesses an email web page

ISP Email Access
Source: Douglas, C (2016) Computer Networks and Internets

Accessing Email via Mail Application
Mail application can download an entire mailbox to a computer when connected to the Internet
Advantages:
Mailbox can be downloaded prior to disconnected from the Internet (process email on plane)
Once Internet connectivity is back, the application can upload email created and download any new email

Accessing Email via Web Browser
Web browser approach is straightforward:
ISP provides a special web page that displays messages from a user’s mailbox
Advantages:
Ability to read email from any computer
Do not need to run special mail interface application

ISP Mail Access Protocols
Variety of mechanisms available for email access:
Some ISPs provide free email access software to their subscribers
In addition, two standard email access protocols have been created

Mail Access Protocols
Access protocol is distinct from transfer protocol
Access only involves a user interacting with a single mailbox
Transfer protocols allow users to send mail to others
Two common access protocols
each provides its own authentication mechanism that user follows to identify themselves
Acronym Expansion
POP3 Post Office Protocol version 3
IMAP Internet Mail Access Protocol

POP3 vs IMAP
Source: https://www.youtube.com/watch?v=MrZIzZQvZ0c

Access Protocols Characteristics
Provide access to a user mailbox
Permit preview of headers, download, delete, or send messages
Client runs on user personal computer
Server runs on a computer that stores user mailbox

Email Preview
View a list of messages without downloading the message contents is useful
Good when link between two parties is slow
Example: user browsing on a cell phone may look at headers and delete spam without waiting to download the message contents

POP3 Characteristics
Downloads email onto the device, delete message on mail server
Has 3 basic phases
Authentication: authenticate user with username and password
Transaction: download messages from server
Update: delete messages (if required) after it is downloaded to the client

IMAP Characteristic
IMAP allows preview of mail
Useful when bandwidth are expensive and limited
Reduce wastage for spam or unwanted messages
Keeps original message on mail server
Saves a copy on local computer

Mail Representation Standards
Other than MIME, another standard is RFC (Request For Comments) 2822 Mail Message Format
From IETF standards document RFC 2822
Message is represented as a text file & consists:
header section, blank line and a body
Header lines each have the form:
Keyword: information
Set of keywords is defined to include From:, To:, Subject:, Cc:

Fixed Representation Problem
Traditional application protocols employs a fixed representation
Main problem is the difficulty involved when making changes
Example: SMTP restrict message content to text, major change was needed to add MIME extensions

Extensible Representation
Alternative to fixed representation is the extensible system that allows sender to specify format of data
One widely accepted extensible representation is the Extensible Markup Language (XML)
XML resembles HTML in the sense that both languages embed tags into a text document

Tags in XML
Tags in XML are not specified a priori and do not correspond to formatting commands
XML describes the structure of data and provides names for each field

XML Tags Characteristics
Tags in XML are well-balanced
Each occurrence of a tag must be followed by an occurrence of
XML does not assign any meaning to tags
Tag names can be created as needed
Tag names can be selected to make data easy to parse or access

XML Example
Suppose two companies agree to exchange corporate telephone directories
Can define an XML format that has data items
Employee’s name, phone number, office
Can further divide a name into last & first name
Source: Douglas, C (2016) Computer Networks and Internets

Practice 5.4
Distinguish between POP3 and IMAP email protocols based on the following characteristics:
Location of the original message
Bandwidth consumption

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 4

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 18: Wireless Networking

Lesson 18 Learning Outcomes
Describe the different PAN technologies and standards
Explain the wireless LAN architecture
Explain how contention is resolved in a wireless LAN
Describe wireless WAN technologies
Explain the concept of cell clusters
Understand the different generations of cellular technologies
Describe the role of GPS and VSAT satellites

Lesson 18 Outline
PAN Technologies
LAN & MAN Technologies
WAN Technologies

Wireless Networks Types
Government regulations make specific ranges of the electromagnetic spectrum available for communication
License is required to operate transmission equipment in some parts of the spectrum
Some parts of the spectrum are unlicensed
Source: Douglas, C (2016) Computer Networks and Internets

Personal Area Networks (PANs)
PAN provides communication over a short distance intended for devices owned and operated by a single user
Example: wireless headset and cell phone
Source: Douglas, C (2016) Computer Networks and Internets

Bluetooth Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Ultra Wideband (UWB)
UWB consumes low power to reach same distance
Idea behind is spreading data across many frequencies
Uses wide spectrum of frequencies
Short distance: 2m-10m
Signal permeates obstacles such as walls
Data rate of 110 at 10 meters, and up to 500 Mbps at 2 meters

Zigbee
Zigbee: arose from desire to standardise wireless remote-control technology especially for industrial equipment
Only send short command, high data rates are not required
Wireless standard for remote control, not data
Frequency bands, 868 MHz 915 MHz, 2.4 GHz
Data rate of 20-250 Kbps, depending on frequency band
Low power consumption

Zigbee Applications
Source: Douglas, C (2016) Computer Networks and Internets

InfraRED
InfraRED: often used in remote controls
Range of 1 to several meters
Directional transmission with a cone covering 30o
Data rates 2.4Kbps (control) to 16Mbps (data)
Generally low power consumption
Signal may reflect from surfaces but cannot penetrate solid objects

RFID
Radio Frequency Identification (RFID)
A small tag contains identification information that a receiver can “pull” from the tag
Passive tags: draw power from the signal sent by the reader
Active tags: contain a battery which may last up to 10 years
Can use frequencies < 100MHz to 868-954 MHz Used for inventory control, sensors, passports, and other applications 11 RFID Applications Source: Douglas, C (2016) Computer Networks and Internets RFID tag 12 ISM Wireless Bands ISM Wireless is a region of electromagnetic spectrum reserved for use by Industrial, Scientific, and Medical purposes Not licensed to specific carriers and are used for LAN/PAN Source: Douglas, C (2016) Computer Networks and Internets 13 Practice 18.1 Suggest appropriate PAN technology for the following criteria: Multiple devices, line of sight not required, public use Smart home purposes, line of sight not required Industrial, scientific and medical purposes 14 Lesson 18 Outline PAN Technologies LAN & MAN Technologies Wi-Fi Technology WiMax Technology WAN Technologies 15 Wireless LAN Technologies IEEE provides most wireless LAN standards (IEEE 802.11) Group of vendors who build wireless equipment formed Wi-Fi Alliance to test and certify equipment meant for 802.11 standards Thus, most consumers associate wireless LANs with Wi-Fi Source: Douglas, C (2016) Computer Networks and Internets 16 Spread Spectrum Spread spectrum: multiple frequencies to send data sender spreads data across multiple frequencies receiver combines information from multiple frequencies to reproduce original Can be used to achieve one of the following: Increase overall performance Make transmission more immune to noise 17 Spread Spectrum Techniques When a wireless technology is defined, designers choose appropriate multiplexing technique Source: Douglas, C (2016) Computer Networks and Internets 18 Wireless LAN (WLAN) Architecture Building blocks of WLAN: Access Points (AP): also called base stations Interconnection mechanism: like switch or router used to connect APs Set of wireless hosts/nodes Two possible WLAN configurations: Ad hoc: wireless hosts communicate amongst themselves without a base station Infrastructure based: wireless host only communicates with AP that relays all packets 19 WLAN Illustration Source: Douglas, C (2016) Computer Networks and Internets 20 Overlaps & Dead Zone Dead zone: physical location with no wireless connectivity Overlap: wireless host can reach multiple APs To handle this, host can associate with an AP Source: Douglas, C (2016) Computer Networks and Internets 21 AP Coordination APs communicated amongst themselves to ensure smooth handoff AP also allows measuring signal strength and move a host to AP that have stronger signal Some vendors offers lower cost, less complex APs that do not coordinate Argued that signal strength does not provide a valid measure of mobility Mobile computer can handle changing from one AP to another 22 Handling Contention in WLAN Wi-Fi employs collision avoidance methods. CSMA/CA triggers a brief transmission from the intended receiver before transmitting a packet Source: Douglas, C (2016) Computer Networks and Internets 23 Lesson 18 Outline PAN Technologies LAN & MAN Technologies Wi-Fi Technology WiMax Technology WAN Technologies 24 WiMax WiMax: World-wide Interoperability for Microwave Access WiMAX Forum promote use of the technology Can be used as an Internet access technology Two main versions of WiMAX, Fixed WiMAX: does not provide for handoff among access points Mobile WiMAX: technology offers handoff among APs 25 WiMax Deployment Can be used as backhaul connection between ISP facility and remote locations like cell towers Use frequencies with clear Line-Of-Sight (LOS) Internet access can use frequencies with Non-Line-Of-Sight (NLOS) deployment Source: Douglas, C (2016) Computer Networks and Internets 26 WiMax Key Features Uses licensed spectrum (i.e., offered by carriers) Each cell can cover a radius of 3 to 10 Km Uses scalable orthogonal FDM Guarantees quality of services (for voice or video) Can transport 70 Mbps in each direction at short distances Provides 10 Mbps over a long distance (10 Km) 27 Lesson 18 Outline PAN Technologies LAN & MAN Technologies WAN Technologies Cellular Technologies Satellite Technologies 28 How Do Satellites Stay in Orbit Source: https://www.youtube.com/watch?v=IC1JQu9xGHQ 29 Cellular Communication Systems Cellular communication system: provides voice and data services and Internet Connectivity Each cell contains a tower and group of cells are connected to a Mobile Switching Center (MSC) MSC tracks mobile users and manages handoff as the user passes from one cell to another Source: Douglas, C (2016) Computer Networks and Internets 30 Handoff Approaches Handoff can occur: Same MSC if moving within same group of cells Another MSC if moving to another group of cells Perfect coverage occurs if each cell is a hexagon But in practice, coverage is imperfect 31 Cell Tower Signals Most cell towers use omnidirectional antennas that transmit in a circular pattern Cells overlap and gaps exist with no coverage Theoretical In practice Source: Douglas, C (2016) Computer Networks and Internets 32 Cell Density Variability of cell density is possible Rural areas: each cell is large, a single tower is adequate for a large area Urban areas: various size cells with smaller cells covering metropolitan areas Designers break a region into many cells to handle more calls 33 Cell Clusters Interference can be minimized if adjacent cells do not use the same frequency Cellular planners employ a cluster approach in which a small pattern of cells is replicated Source: Douglas, C (2016) Computer Networks and Internets 34 Cell Clusters Approach For a cell assigned to a unique frequency, the repeated pattern will not be assigned to any adjacent cells Source: Douglas, C (2016) Computer Networks and Internets 35 Cellular Technologies Four generations: 1G, 2G, 3G, and 4G with intermediate versions like 2.5G and 3.5G 1G (1970s-1980s): analog signals to carry voice 2G & 2.5G (1990s): digital signals to carry voice with limited data 3G & 3.5G (2000s): addition of higher-speed data 4G (2008): focuses on real-time multimedia 36 Cellular Standards European: TDMA technology called Global System for Mobile Communications (GSM) Motorola: TDMA technology called iDEN Japan: TDMA technology called PDC Source: Douglas, C (2016) Computer Networks and Internets 37 Cellular Services General Packet Radio Service (GPRS) & Wireless Application Service (WAP): Internet access Enhanced Data rate for GSM Evolution (EDGE) & Enhanced GPRS (EGPRS): higher rates EDGE Evolution: even higher rates Short Message Service (SMS): textual messaging Multimedia Messaging service (MMS): multimedia messaging 38 Practice 18.2 Explain the THREE (3) building blocks of WLAN. What are the TWO (2) possible WLAN configurations? 39 Lesson 18 Outline PAN Technologies LAN & MAN Technologies WAN Technologies Cellular Technologies Satellite Technologies 40 Parabolic Antenna Key to satellite communication is a parabolic antenna (dish) Aiming dish at satellite and placing detector at focus point guarantees strong signal Incoming energy is reflected from dish towards receiver Source: Douglas, C (2016) Computer Networks and Internets 41 VSAT Satellite Technology Very Small Aperture Terminal (VSAT) satellites use three frequency ranges that differs in Strength of the signal delivered Sensitivity to atmospheric conditions Satellite footprint Source: Douglas, C (2016) Computer Networks and Internets 42 GPS Satellites Global Positioning System (GPS) satellite provide location-based services Obtaining position is straightforward: receiver can determine location on earth by calculating distance to the satellites Source: Douglas, C (2016) Computer Networks and Internets 43 GPS Process Source: Douglas, C (2016) Computer Networks and Internets 44 Reading Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 16 45 End of Lesson 46

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 14: Multiplexing

Lesson 14 Learning Outcomes
Explain the concept of multiplexing
Understand the concept of multiplexing and demultiplexing
Describe the concepts behind FDM, TDM and WDM
Describe the variations of FDM and TDM
Describe the framing used in telephone system version of TDM

Lesson 14 Learning Outcomes
Describe inverse multiplexing and code division multiplexing

Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing

Multiplexing
Multiplexing: combination of information streams from multiple sources for transmission over a shared medium
Multiplexor: mechanism that implements multiplexing
Demultiplexing: separation of a combination back into separate information streams
Demultiplexor: mechanism that implements demultiplexing

Multiplexing Illustration (1/2)
Each sender communicates with a receiver
All pairs share a transmission medium
Multiplexor combines information from senders
Demultiplexor separate information for receivers
Source: Douglas, C (2016) Computer Networks and Internets

Multiplexing Illustration (2/2)
Main purpose is sharing the medium
Source: Douglas, C (2016) Computer Networks and Internets

Multiplexing Example
Source: Douglas, C (2016) Computer Networks and Internets

Types of Multiplexing
Frequency Division Multiplexing (FDM)
Wavelength Division Multiplexing (WDM)
Time Division Multiplexing (TDM)
Code Division Multiplexing (CDM)
TDM and FDM are widely used
WDM is a form of FDM used for optical fiber
CDM is a mathematical approach used in cell phone mechanisms

Multiple Access and Multiplexing
Source: https://www.youtube.com/watch?v=oYRMYSIVj1o

Frequency Division Multiplexing (FDM)
Radio stations/TV can transmit electromagnetic signals simultaneously
With little interference by using separate channel (carrier frequency)
Possible to send simultaneously multiple carrier waves over a single copper wire
Demultiplexer applies filters that each extract a small range of frequencies near one of carrier frequencies

FDM Illustration
Filters used in FDM only examine frequencies
FDM mechanism will separate the frequency from others without modifying the signal
Source: Douglas, C (2016) Computer Networks and Internets

FDM Advantages & Limitations
Advantages: simultaneous use of a transmission medium by multiple pairs of entities
FDM provides each pair with a private transmission path like separate physical transmission medium
Limitations: If the frequencies of two channels are too close, interference can occur
Demultiplexing hardware that receives combined signal must divide signal into separate carriers

FDM Guard Band
Source: Douglas, C (2016) Computer Networks and Internets
Designers choose a set of carrier frequencies with a gap between them known as a guard band

FDM Independent Channels
Source: Douglas, C (2016) Computer Networks and Internets

FDM Characteristics (1/2)
Long-lived: the idea of dividing the electromagnetic spectrum into channels came from early experiments in radio
Widely used: used in broadcast radio and television, cable television, and some cellular telephone
Versatile: it filters on ranges of frequency without examining other aspects of signals

FDM Characteristics (2/2)
Analog: multiplexing and demultiplexing hardware accepts and delivers analog signals
Even if a carrier has been modulated to contain digital information, FDM hardware treats the carrier as an analog wave
But makes FDM susceptible to noise and distortion

Using Range of Frequencies
Most FDM systems assign each sender and receiver a range of frequencies
Can choose how frequencies can be used
Two primary ways:
Increase the data rate
Increase immunity to interference

Increasing Overall Data Rate
To increase the overall data rate:
Sender divides the frequency range of the channel into K carriers
Sends 1/K of the data over each carrier

Subchannel and Subdivision
Sender can perform FDM within allocated channel
subchannel allocation refers to subdivision
To increase immunity to interference
Use a technique known as spread spectrum

Godmother of Spread Spectrum: Hedy Lamarr

Source: Douglas, C (2016) Computer Networks and Internets

Multiplexing with FDM
Basic idea:
divide the range of the channel into K carriers
transmit data spread, using unique patterns, over multiple channels
allow receiver to use a copy compose data that arrives using same spread pattern used by sender
Works well in cases where noise is likely to interfere with some frequencies at a given time

Hierarchical FDM
If a set of incoming signals all use frequency 0-4 KHz
map first first onto the range 0-4 KHz
map second onto the range 4-8 KHz
map third onto the range 8-12 KHz
Hierarchy in FDM multiplexors is that each maps its inputs to a larger continuous band of frequencies

Hierarchical FDM in Telephone
Source: Douglas, C (2016) Computer Networks and Internets

Practice 14.1
Frequency Division Multiplexing (FDM) allocates different frequencies to each pair of communication.
What is the advantage and limitation of such approach?
How can the limitation of FDM be overcome?

Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing

Wavelength Division Multiplexing (WDM)
WDM: application of FDM to optical fiber
Dense WDM (DWDM): many wavelengths of light employed
Inputs and outputs of multiplexing are wavelengths of light denoted by the Greek letter λ, and informally called colors
Prisms are used in optical multiplexing and demultiplexing

WDM Idea
When white light passes through a prism colors of the spectrum are spread out
WDM demultiplexor uses a prism to separate the wavelengths.
If several colored light beams are each directed into a prism at the correct angle, the prism will combine the beams to form a beam of white light
WDM multiplexor accepts beams of light of various wavelengths to combine them into a beam

WDM Illustration
Source: Douglas, C (2016) Computer Networks and Internets
WDM Multiplexor
WDM Demultiplexor

Each color can be used as a channel

Practice 14.2
What is the purpose of a prism in WDM for:
A multiplexor
A demultiplexor

Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing

Time Division Multiplexing (TDM)
TDM: assigns time slots to each channel repeatedly
Sent in a round-robin fashion
TDM Multiplexing: transmitting an item from one source first before transmitting an item from another source
Source: Douglas, C (2016) Computer Networks and Internets

Synchronous TDM
Synchronous TDM: when TDM is applied to synchronous networks, no gap occurs between items
Source: Douglas, C (2016) Computer Networks and Internets

Synchronous TDM in Telephone
Telephone systems use synchronous TDM to multiplex digital streams from multiple phone calls
Demultiplexer is synchronized with multiplexer
A synchronous TDM sends one slot after another without any indication of the output to which a given slot occurs
Slight difference in the clocks used to time bits can cause a demultiplexer to misinterpret the bit stream

Synchronous TDM Framing
To prevent misinterpretation, phone system includes an extra framing channel as input
Instead of a complete slot, framing inserts a single bit in the stream on each round
Demultiplexor extracts data from the framing channel and checks for alternating 0 and 1 bits
If an error causes a demultiplexor to lose a bit, likely that framing check will detect the error and allow the transmission to be restarted

Framing Illustration

Source: Douglas, C (2016) Computer Networks and Internets

Hierarchical TDM
Each successive stage of a TDM hierarchy uses N times the bit rate
In FDM, each successive stage uses N times the frequencies
Additional framing bits are added to the data
The bit rate of each successive layer of hierarchy is slightly greater than the aggregate voice traffic

Hierarchical TDM Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Synchronous TDM Unfilled Slots
Synchronous TDM works well if each source produces data at a uniform
Many sources generate data in bursts, with idle time between bursts
In real life, a slot cannot be empty as the underlying system must continue to transmit data
the slot is assigned a value (such as zero) with an extra bit set to indicate that the value is invalid

Unfilled Slots Illustration
Sources on the left produce data items at random
Synchronous multiplexor will leave a slot unfilled if the corresponding source has not produced an item by the time, the slot must be sent
Source: Douglas, C (2016) Computer Networks and Internets

Statistical TDM
One technique to increase utilisation of shared medium is statistical TDM or statistical multiplexing
Instead of leaving a slot unfilled, skip any source that does not have data ready
By eliminating unused slots, statistical TDM takes less time to send the same amount of data

Statistical TDM Illustration
Use only 8 slots instead of 12

Source: Douglas, C (2016) Computer Networks and Internets
Synchronous TDM
Statistical TDM

Statistical TDM Overheads
Statistical multiplexing incurs extra overhead
Synchronous TDM system: every Nth slot corresponds to which receiver
Statistical TDM: any slot can correspond to any receiver
Hence, each slot must contain the identification of the receiver to which the data is being sent
Overhead: extra information sent with data

Inverse Multiplexing
On the Internet, a connection between two points can consists of multiple transmission media
but no single medium has a bit rate that is sufficient as service providers need higher bit rates than what is available
Multiplexing can be used in reverse
Spread a high-speed digital input over multiple lower-speed circuits for transmission and combine the results at the receiving end

Inverse Multiplexing Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Inverse Multiplexor Design
Inverse multiplexor cannot be constructed by connecting the pieces of a conventional multiplexor backward
Hardware must be designed so that sender and receiver agree on how data arriving from the input will be distributed over the lower-speed connections
To ensure ordered delivery, it must be designed to handle cases like different latency on different lower-speed connections

Practice 14.3
How does synchronous TDM handle cases where a sender has nothing to send?
What technique can be used with TDM to better utilise the shared medium?
On the Internet, explain why is multiplexing not such a suitable approach for multiple pairs of communication? Explain an alternative that can be used.

Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing

Code Division Multiplexing (CDM)
CDM is used in parts of the cellular telephone system and for some satellite communication
Version of CDM used in cell phones is known as Code Division Multi-Access (CDMA)
CDM does not rely on physical properties
such as frequency or time

CDM Idea
CDM relies on values from orthogonal vector spaces can be combined and separated without interference
Each sender is assigned a unique binary code Ci, known as a chip sequence
Chip sequences are selected to be orthogonal vectors
the dot product of any two chip sequences is zero

CDM Steps
Each sender’s value to transmit is Vi
Each ith senders transmit (Ci) x (Vi)
Each sender transmit at the same time and the values are added together
To extract value Vi, receiver multiplies the sum by Ci

CDM Example (1/5)
Consider an example
Use a chip sequence that is only 2-bit long and data values that are 4-bit long
think of the chip sequence as a vector
Source: Douglas, C (2016) Computer Networks and Internets

CDM Example (2/5)
The first step consists of converting binary values into vectors that use -1 to represent 0

Source: Douglas, C (2016) Computer Networks and Internets

CDM Example (3/5)
Resulting values is a sequence of signal strengths to be transmitted at the same time
Resulting signal is the sum of two signals

Source: Douglas, C (2016) Computer Networks and Internets

Normal arithmetic addition

CDM Example (4/5)
A receiver treats sequence as a vector
Computes dot product of vector and chip sequence
Treats result as sequence, converts result to binary by interpreting positive values as binary 1 and negative values as 0
Thus, receiver 1 computes:
Source: Douglas, C (2016) Computer Networks and Internets
C1

Received data
(1(0) + (-1)(-2))
(1(2) + (-1)(0))

CDM Example (5/5)
Interpreting the result as a sequence produces:
(2 -2 2 -2)
which becomes the binary value: (1 0 1 0)

1010 is the correct value of V1
Receiver 2 will extract V2 from the same transmission using C2

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 11

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 13: Modulation

Lesson 13 Learning Outcomes
Explain carriers, frequency and propagation
Explain the three different types of modulation
Explain the shift keying
Understand what modulation and demodulation means
Describe the different types of modems used for modulation and demodulation

Lesson 13 Outline
Analog Modulation
Digital Modulation
Modulation and Demodulation Hardware

Carriers
Long-distance communication use an oscillating electromagnetic wave called carrier
Makes small changes to carrier that represent information being sent

Frequency and Propagation
Frequency of electromagnetic energy determines how the energy propagates
One motivation for the use of carriers arises from the desire to select a frequency that will propagate well
Independent of the rate that data is being sent

Analog Modulation
Modulation: changes made to a carrier according to the information being sent
Takes two inputs
Carrier
Information signal
Sender must change one of the fundamental characteristics of the wave

Analog Modulation Schemes
Three primary techniques that modulate an electromagnetic carrier according to a signal:
Amplitude modulation
Frequency modulation
Phase shift modulation
First two methods of modulation are most familiar and used extensively

What is Amplitude Modulation
Source: https://www.youtube.com/watch?v=fGf_ng7qljI

Modulation Concept
Diagram show the concept of modulation with two inputs
Source: Douglas, C (2016) Computer Networks and Internets

Amplitude Modulation (AM)
AM: varies amplitude of a carrier in proportion to the information being sent (according to a signal)
Carrier continues oscillating at fixed frequency, but amplitude of the wave varies
Only amplitude of the sine wave is modified

AM Illustration
(a) unmodulated carrier wave
(b) analog information signal
(c) resulting AM carrier
Source: Douglas, C (2016) Computer Networks and Internets

AM and Shannon’s Theorem
Modulation only changes amplitude of a carrier slightly, depending on a constant, modulation index
Practical systems do not allow for a modulated signal to approach zero
In Shannon’s Theorem, signal-to-noise ratio will approach zero as the signal approaches zero
Keeping carrier wave near maximum ensures signal-to-noise ratio remains as large
Permits the transfer of more bits per second

Frequency Modulation (FM)
FM: amplitude of the carrier remains fixed but frequency changes according to signal:
Signal stronger, carrier frequency increase slightly
Signal weaker, carrier frequency decrease slightly
FM is more difficult to visualize as slight changes in frequency are not as clearly visible
But can notice that modulated wave has higher frequencies when signal used for modulation is stronger

FM Illustration

Analog Information Signal
Resulting FM Carrier
Source: Douglas, C (2016) Computer Networks and Internets

Sine Wave Phase Properties
One of the properties of sine wave is its phase, the offset from reference time which sine wave begins
Possible to use changes in phase to represent signals
The term phase shift characterise such changes

Phase Modulation (PM)
If phase changes after cycle k, next sine wave will start slightly later than the time at which cycle k completes
Slight delay resembles a change in frequency
PM: a special form of frequency modulation
Phase shifts are important when a digital signal is used to modulate a carrier

Practice 13.1
Define modulation and describe THREE (3) different ways a signal can be transformed by a modulator into a resulting carrier that is transmitted over a medium.

Lesson 13 Outline
Analog Modulation
Digital Modulation
Modulation and Demodulation Hardware

Digital Modulation
Modifications to the modulation schemes are needed for digital modulation
Instead of modulation that is proportional to a continuous signal, digital schemes use discrete values
Digital modulation is referred to as shift keying

Shift Keying
Shift keying operates like analog modulation
But instead of continuous values, digital shift keying has a fixed set
For example, AM allows amplitude of carrier to vary by small amounts in response to change in signal
Amplitude Shift Keying (ASK): uses a fixed set of possible amplitudes
Frequency Shift Keying (FSK): uses a fixed set of possible frequencies

Shift Keying Illustration
Carrier wave
Digital input signal
Amplitude Shift Keying
Frequency Shift Keying
Source: Douglas, C (2016) Computer Networks and Internets

Phase Shift Keying (PSK)
AM and FM require at least one cycle of a carrier wave to send a single bit
unless a special encoding scheme is used
Number of bits sent per unit time can be increased
if encoding scheme permits multiple bits to be encoded in a single cycle of the carrier
PSK: changes phase of the carrier wave abruptly
Each such change is called a phase shift

PSK Illustration
There are three abrupt phase changes
A phase shift is measured by angle of the change
Left most portion of sine wave changes its phase by π/2 radians or 180o
Second phase change corresponds to 180 shift
Abrupt changes
Source: Douglas, C (2016) Computer Networks and Internets

Encoding Data Using Phase Shifts
A sender and receiver can agree on the number of bits per second
No phase shift denote logical 0
Presence of a phase shift to denote a logical 1
A system can use a 180o phase shift to represent presence of a phase shift

Constellation Diagram
Constellation diagram: to express exact assignment of data bits to specific phase changes
A possible phase shift mechanism can permit sender to transfer one bit at a time
Called Binary Phase Shift Keying (BPSK) or 2-PSK
2-PSK is also used to denote the two possible values

Constellation Diagram Illustration
A constellation diagram showing logical 0 as 0o phase shift and logical 1 as a 180o phase shift
Source: Douglas, C (2016) Computer Networks and Internets

Detecting Specific Phase Shift
Receiver can measure amount a carrier shifted during a phase change
Can devise system that recognizes a set of phase shifts each to represent specific values of data
Systems are designed to use power of 2 possible shifts
Sender can use bits of data to select among shifts

Specific Phase Shift Illustration
Diagram shows a system that uses 4-PSK
At each stage of transmission, sender uses 2-bits of data to select among 4-possible shift values
Source: Douglas, C (2016) Computer Networks and Internets

Data Rate in Phase Shifts
Possible to increase the data rate by increasing the range of phase shifts
A 16-PSK mechanism can send twice as many bits per second as a 4-PSK mechanism
But in practice, noise and distortion limit the ability of hardware
to distinguish among minor differences in phase shifts

Increasing Data Rate Further
Data rate can be increased further by using combination of modulation techniques
Change two characteristics of carrier at one time
Most sophisticated technology combines ASK and PSK
Known as Quadrature Amplitude Modulation (QAM) or Quadrature Amplitude Shift Keying (QASK)

Quadrature Amplitude Modulation (QAM)
QAM: uses both change in phase and amplitude
To represent QAM on a constellation diagram
Use distance from the origin as a measure of amplitude

QAM Illustration
Diagram shows a variant known as 16QAM with dark gray areas indicating the amplitudes
Source: Douglas, C (2016) Computer Networks and Internets

Practice 13.2
What is digital modulation known as? Distinguish between analog and digital modulation.

Lesson 13 Outline
Analog Modulation
Digital Modulation
Modulation and Demodulation Hardware

What is a Modem
Source: https://www.youtube.com/watch?v=T4oeyn9Pxsc

Modulator and Demodulator
Modulator: accepts a sequence of data bits, applies modulation to carrier wave according to the bits
Demodulator: accepts a modulated carrier wave and recreates sequence of data bits
Most communication systems are full-duplex
Each location needs a modulator to send and a demodulator to receive data

Modem
To keep cost low and make the pair of devices easy to install and operate
Manufacturers combine modulation and demodulation into a device: modem (modulator and demodulator)
Modems are also designed to provide communication over long distances

Modem Illustration
Diagram shows how a pair of modems use a 4-wire connection to communicate
Source: Douglas, C (2016) Computer Networks and Internets

Modem Variations
Modems can be used with other media like Radio Frequency (RF) transmission and optical fibers
RF modems and optical modems
Modems can use entirely different media, but the principle remains the same:
at sender, a modem modulates a carrier
at receiver, extract data from modulated carrier

Dialup Modems
Dialup modem uses an audio tone
Like conventional modems, carrier is modulated at sender and demodulated at receiver
Uses data to modulate an audible carrier
which is transmitted to the phone system
Difference with conventional modems is the lower bandwidth of audible dialup modems

Modern Telephone System
Modern telephone system used today is digital
Phone system digitises incoming audio
Transports a digital form internally
Converts the digitised version back to analog audio for delivery
Receiver demodulates analog carrier
Extracts original digital data

Internal & External Modems
Dialup modem can also be embedded in a computer
Internal modem: denote an embedded device
External modem: denote separate physical device

Source: Douglas, C (2016) Computer Networks and Internets

QAM on Dialup Modem
QAM is used with dialup modems to maximise data rate
Most telephone connections transfer frequencies 300-3000Hz but a given connection may not handle the extremes well
To guarantee better reproduction and lower noise, dialup modems use 600-3000Hz
Available bandwidth = 3000-600= 2400 Hz

QAM on Dialup Modem Illustration
Source: Douglas, C (2016) Computer Networks and Internets

V.32 and V.32bis Modem
V.32 modem that uses 32 combinations of ASK and PSK to achieve a data rate of 9600 bps in each direction
V.32bis modem uses 128 combinations of ASK and PSK to achieve 14400 bps in each direction
Sophisticated signal analysis is needed to detect the minor change that occurs
from a point in the constellation to a neighboring point

V.32 and V.32bis Constellations

Source: Douglas, C (2016) Computer Networks and Internets
V.32 Dialup Modem
V.32Bis Dialup Modem

Practice 13.3
How many modems are required in a full-duplex communication system?
What are optical modems and RF modem? Describe one similarity between them.

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 10

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 4: Internet Protocols

Lesson 4 Learning Outcomes
Identify the purpose of the Application-Layer Protocols
Distinguish the two main types of Application-layer protocols
Distinguish between the three key standards of web protocols
Identify the components in a URL
Describe the four main HTTP request type functions

Lesson 4 Learning Outcomes
Describe the components in HTTP requests
Describe the architecture of a Web browser

Lesson 4 Outline
Application-layer Protocol
Web Protocols
Caching in Browser

Application-layer Programming
When programmer creates network applications, the programmer specifies some details:
Syntax and semantics of messages to be exchanged
Client or server initiates interaction
Error Control
When to terminate communication

Application-layer Protocol
Two broad types of application-layer protocols that depend on intended use:
Private communication
Standardised service
Size of a protocol specification depends on the complexity of the service

Private Communication
Private communication: communicate over the Internet with intention for private use
Straightforward interaction: code can be written without formal protocol specification
Source: https://www.publicdomainpictures.net/en/view-image.php?image=23003

Standardised Service
Standardised service: expectation that many programmers will create server software to offer the service or client software to access the service
Must be documented independent of implementation
Specification must be precise and unambiguous

Representation and Transfer
Application-layer protocols specify two aspects of interaction:
Representation
Transfer
Aspect Description
Data Representation Syntax of data items that are exchanged, specific form used in transfer, translation of integers, characters and files between computers
Data Transfer Interaction between client and server, message syntax and semantics, valid and invalid exchange error handling, termination of interaction

Source: Douglas, C (2016) Computer Networks and Internets

Web Protocols
World Wide Web (WWW) is the most widely used services on the Internet but the web is complex
Many protocol standards have been devised to specify various aspects and details
Source: Douglas, C (2016) Computer Networks and Internets
Standard Description
HyperText Markup Language (HTML) Representation standard to specify the contents and layout of a web page
Uniform Resource Locator (URL) Representation standard to specify the format and meaning of web page identifiers
HyperText Transfer Protocol (HTTP) Transfer protocol to specify how a browser interacts with a web server to transfer data

Practice 4.1
For each of the following scenario, suggest the appropriate web protocols to use.
To specify how a web page look like
To specify the address of the web page
To specify how data are transferred to a web server

Lesson 4 Outline
Application-layer Protocol
Web Protocols
HyperText Markup Langauge (HTML)
Uniform Resource Locator (URL)
HyperText Transfer Protocol (HTTP)
Caching in Browser

HTML Representation
HTML specifies the syntax of a web page
HTML allows complex web page that contains graphics, audio, video text
Hypermedia rather of hypertext

HTML Characteristics
Uses textual representation
Describes pages that contain multimedia
Follows a declarative rather than procedural paradigm
Provides markup specifications instead of formatting
Permits embedded hyperlink in an arbitrary object
Allows a document to include metadata

Declarative Markup Language
Declarative: allows one to specify what is to be done, not how to do it
Markup language: gives general guidelines for display and does not include detailed formatting instructions
Allows a page to specify level of importance of a heading
Does not require author to specify the exact font, typeface, point size, or spacing for the heading

HTML Extension
HTML Extensions: created to allow the specification of an exact font, typeface, point size, and formatting.
A browser chooses all display details

Importance of Markup Language
Two advantages:
Allows a browser to adapt the page to the underlying display hardware
Web page can be formatted for a high-resolution or low-resolution display
Large screen or small like smartphone

HTML Tags
To specify markup, HTML uses tags embedded in the document
Tags provide structure as well as formatting
Example: IMG tag to encode a reference to an external image
Additional parameters can be added to specify the alignment of the figure with surrounding text

HTML Tags Properties
Tags control all display
White space (extra lines or blank characters) can be inserted at any point in the HTML document
Without effect on the formatted version that a browser displays
Case insensitive: does not distinguish between uppercase and lowercase letters

HTML Example

Code Output

<br /> text that forms the document title<br />

body of the document appears here

Here is an icon of a house.

Here is an icon of a house
Source: Douglas, C (2016) Computer Networks and Internets

Practice 4.2
Why is HTML consider as a declarative language?
2. Why is HTML consider as a Markup language?

Lesson 4 Outline
Application-layer Protocol
Web Protocols
HyperText Markup Langauge (HTML)
Uniform Resource Locator (URL)
HyperText Transfer Protocol (HTTP)
Caching in Browser

URL Components
URL: syntactic form used by the Web to specify a web page
protocol://host_name:port/document?parameters
protocol: protocol used to access the document (HTTP,FTP)
host_name: domain name of the computer on which the document resides
port: port number that server is listening (default 80 for HTTP)
document: name of the document
%parameters: options for the page
Example: https://www.google.com/search?q=hello
Optional
Optional

Reason for URL
URL contains information that browsers need to retrieve a page
Browser uses the separator characters like “:”, “/”, and “%” to identify the components of a URL
Host name and protocol port are used to form a connection to the server on which the page resides
Browser uses the document and parameters to request a page

URL Hyperlinks
User can omit many of the parts of URL
protocol (http is assumed)
port (80 is assumed)
document name (index.html is assumed)
parameters (none are assumed)

Lesson 4 Outline
Application-layer Protocol
Web Protocols
HyperText Markup Langauge (HTML)
Uniform Resource Locator (URL)
HyperText Transfer Protocol (HTTP)
Caching in Browser

HTTP Transfer Protocol
HTTP: primary transfer protocol that a browser uses to interact with a web server
A browser is a client that extracts a server name from a URL and contacts the server
Most URLs contain an explicit reference of “http://” or omit the protocol (HTTP is assumed)

HTTP Characteristic
Uses textual control messages
Transfers binary data files
Can download or upload data
Incorporates caching

HTTP Methods
Source: https://www.youtube.com/watch?v=guYMSP7JVTA

HTTP Request Type
Once connection is established, client sends an HTTP request to server
Request Description
GET Requests a document; server responds by sending status information followed by a copy of the document
HEAD Request status information; server responds by sending status information but does not send a copy of the document
POST Sends data to a server; the server appends the data to a specified item i.e. a message appended to a list
PUT Sends data to a server; the server uses the data to completely replace the specified item i.e. overwrites the previous data

Source: Douglas, C (2016) Computer Networks and Internets

HTTP GET Request
Most common form of interaction begins with client requesting a page from the server
Client sends a GET request over
Server responds by sending a header, a blank line, and the requested document
Source: Bing, licensed under CC BY-SA

HTTP GET Request Form
GET /item version CRLF
Item: the URL for the item being requested,
Version: version of the protocol (HTTP/1.0 or HTTP/1.1)
CRLF: two ASCII characters, carriage return and linefeed
to signify the end of a line of text
Version information is important in HTTP
allows protocol to change but remains backward compatible
client sends version information that allows server to choose highest version that both understand

HTTP Response Header
HTTP/1.0 status_code status_string CRLF
Server: server_identification CRLF
Last-Modified: date_document_was_changed CRLF
Content-Length: datasize CRLF
Content-Type: document_type CRLF
CRLF
Source: Bing, licensed under CC BY-SA
Source: Douglas, C (2016) Computer Networks and Internets

HTTP Status Code
First line of a response header contains a status code that tells the browser whether the server handled the request
If the request is not available, status code pinpoints the problem

Status Code Message Description
200 Ok The request is OK
400 Bad Request Server did not understand the request
404 Not Found Server cannot find the requested page
403 Forbidden Access is forbidden to the requested page

HTTP GET Response Example
Sample output from an Apache web server
Item requested is a text file containing 16 characters
“This is a test.” plus a NEWLINE character
GET request specifies HTTP version 1.0 but the server runs version 1.1
Server returns 9 lines of header, a blank line, and the contents of the file

HTTP POST Request
Another common HTTP request is POST
Mainly used to send data to server
POST: Parameters is sent in body of the HTTP request
POST /test/demo.php HTTP/1.1
Host: host_name
name1=value1&name2=value2
GET: Parameters is piggybacked on the URL
/test/demo.php?name1=value1&name2=value2

HTTP POST vs GET
HTTP POST HTTP GET
Never cached Can be cached
Do not remain in browser history Remain in the browser history
Cannot be bookmarked Can be bookmarked
No restrictions on data length Have length restrictions
Should not be used for sensitive data
Should only be used to request data not modify

HTTP PUT Request
Similar to POST request
But PUT is meant to replace data in server
PUT: Parameters is sent in body of the HTTP request

HTTP HEAD Request
Similar to GET request
For the same GET request, if HEAD is used, the headers are returned without the body
Useful to check what a GET will return before making the request
i.e. before downloading a large file

Practice 4.3
Distinguish between HTTP GET and HTTP POST functions. Detailing at least TWO differences.

Lesson 4 Outline
Application-layer Protocol
Web Protocols
Caching in Browser

Browser Structure
Browser structure is complex
A browser must understand HTTP and:
Contain client code for each protocols used
Must know how to interact with a server
How to interpret responses from server
How to access FTP service
Ability to cache frequently visited web pages

Browser Architecture
Source: Douglas, C (2016) Computer Networks and Internets

Caching in Browser
Caching is an important optimization for web access
users tend to visit the same web sites repeatedly
Much of the content on web consists of large images
Graphics Image Format (GIF)
Joint Picture Encoding Group (JPEG)
Such images often contain backgrounds or banners that do not change frequently

Cached Copy
Browser can reduce download times significantly
by saving a copy of image in a cache on user’s disk and using cached copy
What happens if the document on server is modified after browser cache a copy?
How can a browser tell whether its cached copy is stale?

Updating Cache
When browser contacts server, response header shows the time document was modified
Browser saves Last-Modified date along with the cached copy
Browser makes HEAD request to the server
Server’s Last-Modified vs Cache’s Last-Modified
If cached version is stale, the browser downloads the new version

Updating Cache Algorithm
Source: Douglas, C (2016) Computer Networks and Internets

Cache Header
HTTP allows web site to include No-cache header that specifies a given item should not be cached
Browsers do not cache small items
Time to download the item with a GET request is approximately same as time to make a HEAD request
Keeping many small items in cache increases cache lookup times

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 4

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 10: Wireless Media

Lesson 10 Learning Outcomes
Understand the motivation for infrared communication
Explain infrared communication
Explain point-to-point laser communication
Explain radio communication
Describe the different types of satellites
Distinguish the different types of satellite orbit

Lesson 10 Learning Outcomes
Compare and contrast wireless media with wired media
Use appropriate measures to measure performance of media

Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance

Energy Types
Source: Douglas, C (2016) Computer Networks and Internets

Infrared Data Association (IrDA)
Infrared Data Association (IrDA): founded in 1993
To develop and promote a standard for infra-red data communications
Earliest members were HP, IBM and Sharp
Designed for short-range digital communications between PC and devices
Keyboard, digital cameras, smartphones

InfraRed(IR) Communication
InfraRed (IR): uses same type of energy as a TV remote control
Behaves like visible light but falls outside the range that is visible to a human eye
Like visible light, infrared disperses quickly
Infrared signals can reflect from a smooth, hard surface

IR Limitations
Opaque object can block the signal
Thin sheet of paper or moisture in atmosphere
Up to 1 meter in normal light conditions within a 30o cone from the transmitter
Speed ranging from 9.6 kbit/s to 4 Mbit/s
IR commonly used to connect to a nearby peripheral
Attractive for laptop users as can move around
Now mostly uses Bluetooth technology

IR Technologies
Source: Douglas, C (2016) Computer Networks and Internets

Point-to-Point Communication
Pair of devices with beam that follows line-of-sight
IR is a type of point-to-point communication
Other point-to-point communication technologies also exist
One form of point-to-point communication uses a beam of coherent light produced by a laser

Laser Communication
Laser communication requires unobstructed path between the communicating sites
Laser beam does not cover broad area and is only a few centimeters wide
Sender and receiver must be aligned precisely to ensure that sender beam hits sensor in the receiver
Suitable for outdoors and span great distances
Useful in cities to transmit from building to building

Bluetooth
Low-cost, low power short-range radio developed by Bluetooth Special Interest Group made up initially of Ericsson, Intel, Nokia and Toshiba
Now include Microsoft and many others
Designed to carry voice and data between devices within noisy radio environment
Smartphones, hands-free devices, laptops
In unlicensed 2.4 GHz frequency band

Bluetooth Piconet
Piconet: network of 8 active Bluetooth devices
One act as master
Communication between master and slave
2 slaves cannot communicate directly
Piconet supports up to 255 non-active devices

Bluetooth Scatternet
Scatternet: network of ten piconets
Slave of a piconet becomes a master of another
Each bridging traffic between two piconets
Severe interference is likely to reduce data rate
but will not stop Bluetooth from working together

Service Discovery
Bluetooth includes service discovery protocols
Allows applications to discover what functions are supported by Bluetooth devices
Creates a database of trusted devices
following authentication
Pair up devices by entering PIN numbers

Bluetooth and WiFi
Bluetooth uses same frequency range as WiFi
Likely to cause interference when used near WiFi hotspots
IEEE has set up a task group to make recommendations that will help these two coexist

Practice 10.1
For each of the following, say whether an obstructed direct path will prevent communication entirely and give an example.
InfraRed (IR) communication
Laser communication
Bluetooth communication

Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance

Radio Communication
Most common form of unguided communication consists of wireless networking technologies
Electromagnetic energy in the Radio Frequency (RF) range
RF transmission has distinct advantage over light
Can traverse long distances and penetrate objects like walls of a building

Radio Frequencies
Properties of electromagnetic energy depends on frequency
Spectrum: range of possible frequencies
Organizations allocate frequencies for specific purposes
Federal Communications Commission (FCC) sets rules for how frequencies are allocated
Limits on amount of power that communication equipment can emit at each frequency

Electromagnetic Spectrum
One part of the spectrum corresponds to IR
Spectrum used for RF communications spans frequencies from approximately 3KHz-300GHz
Includes frequencies allocated to radio and television broadcast, satellite and microwave communications

Electromagnetic Frequencies
Source: Douglas, C (2016) Computer Networks and Internets

Signal Propagation
Amount of information an electromagnetic wave can represent depends on the frequency
Frequency of electromagnetic wave determines how wave propagates
Source: https://www.earthmagazine.org/article/ionospheric-charge-could-forewarn-earthquakes

Radio Waves Types
Surface waves follows surface of earth
Space (or tropospheric) waves follows line of sight paths but can also be reflected off the ground and other large objects.
Sky (or Ionospheric) waves are reflected off the ionosphere and can be carried over long distance
Scattered waves: broadcast and scattered it in all directions
some are scattered back to earth and picked up, often used in off-shore oil platforms

Signal Frequency Types
Lowest frequencies: electromagnetic radiation follow the earth’s surface
If terrain is relatively flat, possible to place receiver beyond horizon of transmitter
Medium frequencies: signal can bounce off the ionosphere to travel
transmitter and receiver can be farther apart
Highest frequencies: signal propagates in a straight line
Path between transmitter and receiver must be free from obstructions

Signal Frequencies Summary
Source: Douglas, C (2016) Computer Networks and Internets

Wireless Propagation Types
Terrestrial: Communication uses equipment like radio or microwave transmitters that is close to earth surface
Typical locations include tops of hills, man-made towers and tall buildings
Non-terrestrial: some of the communication is outside the earth’s atmosphere
satellite in orbit around earth

Frequency & Power
Frequency and amount of power used can affect following:
Speed at which data can be sent
Maximum distance for communication to occur
Characteristics like if signal can penetrate solid objects

Practice 10.2
There are two kinds of wireless propagation types. For each of the following, say what kind of propagation type does it use.
Satellite dishes on top of buildings
Satellite in space
Antennas on top of a hill

Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance

How Satellite Internet Works
Source: https://www.youtube.com/watch?v=QpO0FwN9Png

Laws of Physics
Kepler’s Law govern the motion of an object that orbits the earth
Satellite
Period: time required for a complete orbit
Depends on the distance from the earth
Communication satellites are classified according to their distance from the earth

Satellite Networks
Satellites have been used for communications since early days of space travel in 1960s
Provide high bandwidth between distant points on earth
Transmitter and receivers can be large or small dishes or even portable satellite telephones
Transponder: signal relay equipment on satellite (transmitter/responder)

Satellite Footprint
Each satellite can transmit and receive signals from an area of earth surface known as satellite footprint
Size of footprint depend on height of orbit and to what degree the satellite focuses the beam of its signal
Source: Douglas, C (2016) Computer Networks and Internets

GEO Satellites
Geostationary Earth Orbit (GEO): orbital period is the same as the rate that earth rotates
If positioned above equator, GEO satellite always remains in the same location over earth surface
Stationary satellite position means that once a ground station has been aligned with the satellite
Equipment never needs to move

GEO Communication
Source: Douglas, C (2016) Computer Networks and Internets
GEO satellite and ground stations are permanently aligned

GEO Distance from Earth
Distance required for GEO is 35,785 km
about 1/10 the distance to the moon
Implication:
Consider radio wave traveling to GEO and back
At speed of light, 3×108 m/s , trip takes:
Source: Douglas, C (2016) Computer Networks and Internets
Distance:
Speed:

Delays from GEO
Delay of about 0.2s can be significant for some applications
For electronic transactions like stock exchange offering a limited set of bonds
Delaying an offer by 0.2s may mean difference between successful and unsuccessful offer

Constraints in GEO
Limited space available in GEO above equator
Satellites using same frequency must be separated to avoid interference
Separation distance depends on power of transmitters
As technology evolves, possible allocate more satellites on orbit

Coverage of the Earth
Three satellites are needed to cover the earth
Positioned around the equator (120o)
Source: Douglas, C (2016) Computer Networks and Internets

Medium Earth Orbit (MEO)
Medium Earth Orbit (MEO): between 8,000-20,000Km above earth, orbital period of 2-12hours
Global Positioning System (GPS): uses 24 satellites in six MEOs
Triangulation: receivers measures delays from 4 GPS satellites in MEO to calculate their position on earth

Low Earth Orbit (LEO)
Low Earth Orbit (LEO): altitudes up to 2000Km
Satellite must be placed above fringe of the atmosphere to avoid drag produced by encountering gases
LEO satellites are typically placed at altitudes of 500-600Km or higher
LEO offers short delays, typically 1 to 4 ms

Problems with LEO
Orbit does not match the rotation of the earth
LEO satellite appears to move across the sky
Ground station must have an antenna that can rotate to track the satellite
Tracking is difficult because satellites move rapidly
Lowest altitude LEO satellites orbit the earth in approximately 90 minutes
Higher LEO satellites require several hours

Satellite Cluster
LEO satellites in clustering or array deployment
Group of LEO satellites designed to work together
Satellite in groups can communicate with one another
Members of the group stay in communication and agree to forward messages as needed

Satellite Cluster Example
Suppose a user in Europe sends a message to a user in USA
A ground station in Europe transmits message to satellite currently overhead (above it)
A cluster of satellites communicate to forward message to satellite in cluster that is currently over a ground station in USA
Finally, satellite currently over USA transmits the message to a ground station

Practice 10.3
What is a satellite footprint?
For Global Positioning System (GPS), how many satellites are needed and what kind of orbits are these satellites on?
Explain the idea that GPS uses to calculate the position on earth.

Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance

Media Choice Factors (1/2)
Cost: materials, installation, operation, and maintenance
Data rate: number of bits per second that can be sent
Delay: time required for signal propagation or processing

Media Choice Factors (2/2)
Affect on signal: attenuation and distortion
Environment: susceptibility to interference and electrical noise
Security: susceptibility to eavesdropping

Quantitative Characteristics (1/2)
Propagation delay: time required for a signal to traverse the medium
Queuing delay: time required for a signal to wait for its turn to be transmitted
Channel capacity: maximum data rate that the medium can support
Shannon’s Law, Nyquist Theorem

Quantitative Characteristics (2/2)
Attenuation: loss of signal power over distance
Utilisation: proportion of time that channel is fully occupied
Throughput: number of bits carried by the channel per second as experienced by end user

Practice 10.4
Describe each of the following:
Attenuation
Utilisation
Throughput
Security
Channel capacity

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 7

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 1: Overview of Communication and Networking

Lesson 1 Learning Outcomes
Understand the evolution of computer networking
Identify the five key aspects of networking
Define the term data communications
Define interoperability and communication protocol
Describe how packet switching and the Internet revolutionise data communication and computer networking
Compare and contrast the OSI and TCP/IP model

Lesson 1 Outline
Evolution of Computer Networking
Key Aspects of Networking
Interoperability and Standards
OSI and TCP/IP Model

Network & Internet
Network: system for connecting computer using a single transmission technology
Internet: set of networks connected by routers that are configured to pass traffic among any computers attached to networks in the set
Source: Bing images licensed under CC BY-NC-ND

Evolution of Internet (1/2)
Roots in military network called ARPANET
Fundamental changes from centralized to distributed computing
Incorporated reliability and robustness
Multiple links & distributed routing

Source: Bing images licensed under CC BY-SA

Evolution of Internet (2/2)
Ethernet standard made local networking feasible
TCP/IP protocol made internetworking possible
Developed after Arpanet
Switchover occurred in 1983
Exponential growth
doubling every 18 months

History of Internet
Source: https://
www.youtube.com
/
watch?v
=h8K49dD52WA

Growth of Computer Networks
Computer networks (NW) growing explosively
used in advertising, production, shipping, planning, billing, and accounting

Continued growth of the global Internet is one of the most exciting phenomena in NW
Source: Bing images licensed under CC BY-SA

Economic Impact
Data NW made telecommuting available to individuals and have changed business communication
Popularity & importance of NW has produced a demand in all industries for people with more NW expertise
Companies need workers to plan, acquire, install, operate, and manage the hardware (HW) and software (SW) systems that comprise computer NW and Internets

Why Network is Complex
Diverse technologies, each has features that distinguish it from the others
Multiple NW standards, some are incompatible
Various commercial NW products and services that use the technologies in unconventional ways
Multiple technologies required to interconnect two or more NW
Many combinations of NW are possible!

Challenges for Beginners
No simple and uniform terminology for NW concepts
Multiple terms exist for a given concept
Large set of terms & acronyms that contains many synonyms
NW jargon contains terms that are often abbreviated, misused, or associated with products
Source: Bing images licensed under CC BY-SA

Network Conceptual Models
Various conceptual models to be used to explain the differences and similarities among NW HW and SW systems
Simplistic models: do not distinguish among details
Complex models: do not help simplify the subject

Simplistic Models

Complex Models

Practice 1.1
Define network and Internet. How are networks and the Internet related?
Describe 4 reasons why network is complex.

B
13

Lesson 1 Outline
Evolution of Computer Networking
Key Aspects of Networking
Interoperability and Standards
OSI and TCP/IP Model

Five Key Aspects of Networks

1. Network Applications

2. Data Communications

3. Packet Switching

4. Internetworking with TCP/IP

5. Public and Private Networks

1. NW Applications & Programming
Network applications: program that communicates across a network
Email, file transfer, web browsing
Network programming: write codes for network applications to communicate across a network
Similar to conventional programming

2. Data Communication
Data communication: study of low-level technologies used to send information across a medium like wire and radio wave
Provides foundation of concepts on which networking is built upon

3. Packet Switching
Packet switching: a way to allow multiple senders to transmit data over a shared network
Divides data into small blocks call packets
Each packets has an identification for the intended recipient
Source: Bing images licensed under CC BY-SA

4. Internetworking with TCP/IP
Transmission Control Protocol (TCP)
Transport layer protocol that receives data and divide them into smaller packets
Internet Protocol (IP)
Network layer protocol that deals with the routing of the packets through the Internet

5. Public & Private Networks
Internet Service Providers (ISP): are vendors who offer Internet access for a fee
Public network: a network owned by a ISP and offers service to any individual or organisation that pays the subscription fee
Private network: a network that is restricted to one group
Can be leased from a provider

Private Network Types
Consumer: smallest and least expensive network
Small Office/Home office (SOHO): slightly larger than consumer network
Small-to-medium Business (SMB): larger than SOHO, usually multiple offices in a building
Large Enterprise: large network with multiple buildings

Practice 1.2
What is data communication?
Describe how does packet switching works

B
22

Lesson 1 Outline
Evolution of Computer Networking
Key Aspects of Networking
Interoperability and Standards
OSI and TCP/IP Model

History of Communication
Source: https://
www.youtube.com
/
watch?v
=0ay2Qy3wBe8

Communications
Communication must involved at least two entities
One to sends the information
One to receive it
Can also contain intermediate entities
Devices to forward packets

Interoperability
Misunderstanding can occur in communications
Real-world: different language, different context etc.
Network: disruptions, corrupted data, congestion
Interoperability: ability of two entities to communicate without misunderstanding
Source: Bing images licensed under CC BY-SA-NC

Interoperability for Communication
Communication between entities must agree on the details
Electrical voltage used to the format and meaning of messages
To ensure entities can interoperate, rules for all aspects of communications are written down

Communication Protocol
Protocols are specification or set of rules
Communication protocol: specifies details for one aspect of computer communication
Actions taken when errors arises
Voltage and signals to be used
Format of messages that applications exchange

Networking Standards
Networking standards ensure interoperability between product vendors
de jure: if they have been produced or accepted by a recognized standards body
de facto: if they have not been accepted by a recognized body but have gained widespread use by market forces

de jure Standards
Hardware and communications standards are often de jure
Network routers
Software standards are often de facto
Microsoft Windows
Competitive markets will arise after de jure standards have been agreed and stabilized
Achieved by having different vendors work together in standard bodies to agree standards for new technologies

de facto Standards
Can be divided into two kinds
Proprietary standards which are produced, owned and controlled by a commercial organization
Microsoft Windows
Open standards may originally produced by a commercial organization but since transferred to the public domain
Unix

Practice 1.3
What is interoperability?
Distinguish between the two main types of standards.

B
32

Lesson 1 Outline
Evolution of Computer Networking
Key Aspects of Networking
Interoperability and Standards
OSI and TCP/IP Model

Network Architecture
Engineers like to define an architecture for complex system
Easier to design, build, test and maintain
Network architecture
Structured framework within which networks can be analysed, designed and implemented, incorporating a defined set of layers and protocols

Cooperating Protocols
Protocol suite: set of protocols that work together to fulfil a communication
Each protocol fulfil one aspect
Abstract version of protocol suite is a layering model

Multi-layered Architecture
Set of layers each serving different function
Can be implemented separately
Lower layers provides a service to the layer directly above it by means of an interface
Imagine layer as software implementations that perform specific functions
Imagine interface as a set of procedure calls with parameters

Layered Architecture Benefits
Divide complex operations into manageable groups
Can change one layer without affecting others
Can mix different technologies and suppliers for different layers

Layered Architecture Model
Layered architecture advantages is clear but:
How many layers is needed?
What function should be at each layer?
International Organization for Standardization (ISO)
Seven layer model: Open Systems Interconnection (OSI)
Developed in the 1980s
To free users from proprietary standards which were being used by vendors to lock customers into their network architectures

TCP/IP vs OSI Model

Source: Douglas, C., 2016. Computer Networks and Internets
Internet TCP/IP Model
Open System Interconnection Networking Model
TCP/IP Layer 5 maps to Layer 5-7 in OSI
Course is focused on TCP/IP model

Layer 1: Physical Layer Protocols
Concern with transmission medium and the associated hardware
Specification about:
Electrical properties
Radio frequencies
Signals

Layer 2: Network Interface
Concern with communication between higher layers of protocols, which are usually software
And the underlying network, which is implemented in hardware
Specifications about:
Network addresses
Maximum packet size that a network can support
Protocols used to access the underlying medium
Hardware addressing

Layer 3: Internet
Concern with communication between two computers across the Internet
Across multiple interconnected networks
Also known as network layer
Specifications about:
Internet addressing structure
Format of Internet packets
Methods for dividing packets into smaller packets for transmission
Mechanisms for reporting errors

Layer 4: Transport
Concern with communication between two application programs
Specifications about:
Maximum receiver rate
Congestion control
Delivery of data
Ordered, no duplication, no error

Layer 5: Application
Concern with how a pair of applications interact when they communicate
Specifications about:
Email exchange
File Transfer
Web Browsing
Telephone services
Video conferencing

Communicating Between Layers
Source: Douglas, C., 2016. Computer Networks and Internets

Layered Communication
Application
Transport
Internet
Network Interface
Application
Transport
Internet
Network Interface
Communication Flow
Host 1
Host 2
Interfaces
Layers
Peers
Physical
Physical

Headers and Layers
Each layer will add their own information
Header: additional information added/removed by the corresponding layer
Not necessary of the same size for each header
Example: Port number (transport), IP address (Internet)
Source: Douglas, C., 2016. Computer Networks and Internets

Practice 1.4
Briefly describe the functions of the five layers in the Internet model.

B
48

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 1

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 19: Internet Addressing

Lesson 19 Learning Outcomes
Describe the addressing scheme used on the Internet
Explain the IP address hierarchy
Explain the motivation behind CIDR
Distinguish between IPv4 and IPv6 addressing

Lesson 19 Outline
IPv4 Addressing
Classless Addressing
Berkeley Address
IPv6 Addressing

Seamless Communication
To achieve seamless communication system
Protocols must hide the details of physical networks and offer the illusion of a single, large network
The Internet is created entirely by protocol software
Addresses, packet formats, and delivery techniques independent of underlying hardware

Internet Addressing
Addressing is a critical component of the Internet
All host must use uniform addressing scheme
MAC addresses does not suffice as each network technology on the Internet defines its own
Internet Protocol (IP) address: supplied by protocols
Application programs can communicate without knowing the underlying hardware
Many protocols use IP addresses

IP Addressing Scheme
IP address (or Internet address) is a unique 32-bit number
Sender’s protocol must specify source IP address (itself) and destination IP address (receiver)
Network number (prefix) assignments must be coordinated globally
Suffixes are assigned locally without global coordination

Prefix and Suffix
Prefix: physical network of the host
Each network is assigned a unique prefix
Suffix: specific host/node on the network
Each host/node is assigned a unique suffix
Source: Douglas, C (2016) Computer Networks and Internets

Classful IP Addressing
Difficult choice to allocate bits for prefix and suffix
Large prefix: many networks, limited size each
Large suffix: many computers, limited networks
Original classful IP addressing divided IP address space into three primary classes
Each class has a different size prefix and suffix
Leading bits are used to identify class

IP Address Authority
ICANN handle address assignment and disputes but authorise registrars to assign prefixes
Registrars make blocks of addresses available to ISPs
ISPs provide addresses to subscribers
To obtain a prefix, a corporation usually contacts an ISP

Dotted Decimal Notation
Dotted Decimal Notation express each 8-bit section of a 32-bit number as a decimal value
Use periods to separate the sections
Treats each byte as an unsigned binary integer
0.0.0.0 to 255.255.255.255
Source: Douglas, C (2016) Computer Networks and Internets

Classful Addressing Limitations
Classful address are divided into unequal sizes to accommodate variety of scenarios
Class A is limited to 128 networks meant for ISP
Class C allows few hosts on a LAN
Source: Douglas, C (2016) Computer Networks and Internets

Meeting Growing Needs
Many demanded class A or class B address for future growth but many addresses were unused
Many class C addresses remained, but few wanted to use them
Can use variable prefix and suffix length:
Subnet addressing
Classless addressing

Subnet and Classless Addressing
Subnet addressing was originally used within large organizations
Classless addressing extended subnet addressing to all Internet
If a customer requests a prefix for a network that contains 55 hosts, a class C will be wasteful
Classless addressing allows the ISP to assign:
Prefix that is 26 bits long
Suffix that is 6 bits long

Classless Addressing
With classless addressing the prefix can be divided into several prefixes and assign each to a subscriber
Instead of wasting addresses, ISP can assign each of the four classless prefixes to a subscriber

Classful Class C
Classless
Source: Douglas, C (2016) Computer Networks and Internets

Address Masks
Classless and subnet require hosts and routers to store additional piece of information:
Boundary of prefix and suffix
To mark the boundary, IP uses a 32-bit value known as address mask or subnet mask
Hosts and routers compare prefix portion of the address to a value in their forwarding tables
bit-mask makes comparison efficient

Address Masks Comparison
Suppose a destination address (D), prefix (N) and address mask (M)
To test whether destination lies on specified network, router tests condition: N == (D & M)
Router uses mask with a “logical and (&)” operation to set the host bits of address D to zero (0)
Compares the result with the network prefix N

Address Masks Example
Consider the 32-bit prefix:
10000000 00001010 00000000 00000000= 128.10.0.0
32-bit mask:
11111111 11111111 00000000 00000000= 255.255.0.0
32-bit destination address:
10000000 00001010 00000010 00000011= 128.10.2.3
”AND” operation for destination address and address mask to extracts high-order 16-bits
10000000 00001010 00000000 00000000= 128.10.0.0

Practice 19.1
Why is there a need for classless addressing?

Lesson 19 Outline
IPv4 Addressing
Classless Addressing
Berkeley Address
IPv6 Addressing

Classless Inter-domain Routing
Source: https://www.youtube.com/watch?v=rJXvFWY4Ak0

Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR) specifies addressing and forwarding
General form of CIDR: ddd.ddd.ddd.ddd/m
ddd: decimal value for an octet of the address
m: number of one bits in the mask
192.5.48.69/26 specifies a mask of 26 bits

CIDR Example
Assume an ISP has 128.211.0.0/16
customer1 CIDR: 128.211.0.16/28
customer2 CIDR: 128.211.0.32/28
The binary value customer1 is:
10000000 11010011 00000000 0001 0000
The binary value assigned to customer2 is:
10000000 11010011 00000000 0010 0000
No ambiguity as each customer has unique prefix
Retains most of original address to allocate remaining to other customers

CIDR Host Address
Assume organization is assigned 128.211.0.16/28
Source: Douglas, C (2016) Computer Networks and Internets

Special IP Address
IP defines a set of address that are reserved; never assigned to hosts
Network Address
Directed Broadcast Address
Limited Broadcast Address
This Computer Address
Loopback Address
Source: Douglas, C (2016) Computer Networks and Internets

1. Network Address
IP reserves host address zero (all 0s in the suffix) to denote a network
Address 128.211.0.16/28 denotes a network as bits beyond 28 are 0s
A network address should never appear as the destination address in a packet

2. Directed Broadcast Address
When packet sent to directed broadcast address (DBA)
A copy of the packet travels across Internet until it reaches the specified network
Delivered to all hosts on the specific network
Directed broadcast address for a network is formed by adding a all 1s suffix to the prefix
Prefix: 10000000 11010011 00000000 0000000
Result: 10000000 11010011 11111111 11111111

DBA Requirements
If network hardware supports broadcast
directed broadcast will be delivered using the hardware broadcast capability
If a network does not have hardware support for broadcast
software must send a separate copy of the packet to each host on the network

3. Limited Broadcast Address
Limited broadcast: broadcast on directly-connected network
Normally used during system startup by computer that does not know network number
Limited broadcast address are 32-bits of 1s
IP will broadcast any packet sent to the all-1s address across the local network

4. This Computer Address
A computer needs to know its IP address before it can send or receive Internet packets
The startup protocols use an IP to communicate
To obtain IP address automatically when the computer boots
IP reserves the address that consists of all 0s to mean this computer

5. Loopback Address (1/2)
Loopback address used to test network applications
Used for preliminary debugging
Instead of executing each program on a separate computer
Runs both programs on a computer and use a loopback address when communicating
Programmers often use host number 1
127.0.0.1 is most popular loopback address

5. Loopback Address (2/2)
Allows testing the program logic quickly
IP reserves the network prefix 127/8 for use with loopback
During loopback testing no packet leaves computer
IP software forwards packets from one application to another
Never appears in a packet traveling across a network

Practice 19.2
For CIDR, how can an address be split into multiple subnetworks?

Lesson 19 Outline
IPv4 Addressing
Classless Addressing
Berkeley Address
IPv6 Addressing

Berkeley Broadcast Address
The University of California at Berkeley developed an early implementation of TCP/IP protocols
Berkeley Software Distribution (BSD)
BSD contained a nonstandard feature
Uses a host suffix of all 0s identical to the network address
this address form is known as Berkeley broadcast

Berkeley Contribution
Initially many computer manufacturers derived their early TCP/IP software from Berkeley implementation
Few still uses Berkeley broadcast today
TCP/IP implementations often include a configuration parameter
To select between TCP/IP standard and Berkeley form

Routers IP Address
Router has connections to multiple physical networks
Each IP address for a physical network
Each router is assigned two or more IP addresses
One address for each network attached
IP address does not identify a device but identifies a connection between a device and a network

Router IP Address Illustration
A device with multiple network like a router must be assigned one IP address for each connection

Source: Douglas, C (2016) Computer Networks and Internets

Multi-homed Hosts
A host computer with multiple network connections is said to be multi-homed
Multiple addresses, one for each network connection
Reliability: can reach the Internet through either connections
Performance: can send traffic directly and avoid routers that are sometimes congested

Lesson 19 Outline
IPv4 Addressing
Classless Addressing
Berkeley Address
IPv6 Addressing

IPv6 Addressing
IPv6 includes addresses with a multi-level hierarchy
highest level corresponds to an ISP
next level corresponds to an organization
next to a site
IPv6 defines a set of special addresses
does not include a special address for broadcasting on a given remote network

IPv6 Address Types
Packets will be only delivered to just one of a group of addresses
Nearest one, measured by routing protocol, will be chosen
Source: Douglas, C (2016) Computer Networks and Internets

Colon Hexadecimal Notation
IPv6 address occupies 128 bits represented in Colon hexadecimal notation (colon hex)
Reduce number of characters used to write addresses
Each group of 16 bits is written in hex with a colon separating groups
69DC : 8864 : FFFF : FFFF : 0 : 1280 : 8C0A : FFFF

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 21, 24

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 7: Analogue and Digital Signals

Lesson 7 Learning Outcomes
Distinguish between analogue and digital signal
Distinguish between periodic and aperiodic signals
Define sine waves
Identify the four important characteristics of signals
Define composite signals

Lesson 7 Learning Outcomes
Discuss the time and frequency domain representation of signals
Use Shannon law and Nyquist theorem to calculate the channel theoretical capacity

Lesson 7 Outline
Data Communications
Communication Model
Physics of Transmission
Types of Signal
Representing Signals

Information Source & Destination
Communication system accepts input from one or more sources and delivers to a destination
On the Internet, source and destination of information are pair of application programs
Source: generate data
Destination: consume data
Source: Bing, licensed under CC BY-SA

Data Communications
Data communications theory concentrates on low-level communications systems
Sources: microphones, sensors, measuring devices like thermometers and scales and computer peripherals like keyboards, mice
Destinations: audio output devices like earphones and loud-speakers, LEDs that emit light

Data vs Computer Communications
In most cases, the terms are interchangeable
To be specific:
Data communication: for lower layer aspects like signaling, device interfaces, hardware related issues
Computer communication: for higher layer aspects such as network protocols, applications, software related issues

Communication Model
Provides an understanding of how information is transferred
Shows how information is sent and received
Shows the parties and factors involved in a communication

Claude Shannon & Warren Weaver
In 1948, Shannon and Weaver came up with an idea
Successful communication requires:
Information source: generates data
Messages: data transmitted
Transmitter: converts information to signals
Channel: medium in which signal are transmitted
Receiver: translate signal back to original message
Destination: consume data

Shannon’s Communications Model
Information
Source
Transmitter
Channel
Receiver
Destination
Noise Source
Noise
Message
Signal
Received Signal
Message

Complicating Factor
Transmission of signal can be disrupted by noise
Noise could be caused by the medium/channel used
Noise source: generates noise
Increase the signal, increase the noise
If signal is amplified, noise is amplified too

Amplifying Signal
Source: Douglas, C (2016) Computer Networks and Internets
Signal to noise ratio will remain constant
P1 : N1 = P2 : N2

Noise level N1
Noise level N2

Communication Model
Source: https://
www.youtube.com
/
watch?v
=OY1JsGFZprc

Practice 7.1
Describe the SIX components in the Shannon’s communication model.

Lesson 7 Outline
Data Communications
Communication Model
Physics of Transmission
Types of Signal
Representing Signals

Physics of Transmission
Most functions on physical layer depends on characteristics of physical medium
Each physical medium requires own physical layer
Before looking into the functions, we need to consider some basic physics
like electromagnetism including electric conduction and electromagnetic radiation

Transmitting Information
Data communications deals with two types of signals to transmit information:
analog
digital
Analog signal is characterized by continuous signal levels
When input changes from one value to next, it does so by moving through all possible intermediate values

Analog vs Digital Signal
A digital signal has a fixed set of valid levels
Each change consists of an instantaneous move from one valid level to another
Source: Douglas, C (2016) Computer Networks and Internets
Analog
Digital

Signal Classification
Signals are broadly classified as
periodic – repeated
aperiodic – sometimes called nonperiodic
Classification depends on whether they repeat
Left: aperiodic as the signal does not repeat
Right: periodic as the signal repeats

Source: Douglas, C (2016) Computer Networks and Internets

Sine Waves
Much analysis in data communications involves the use of sinusoidal trigonometric functions
Especially sine, abbreviated sin
Sine wave: periodic wave that oscillates regularly and smoothly between negative and positive value

Source: Bing, licensed under CC BY-SA

Sine Waves: Superimposed
Two sine waves are superimposed exactly on each other
If they are in the same phase
Two sine waves cannot superimpose on each other
If one of the waves of the same frequency has crest slightly later than the other
They are said to be out of phase

Sine Waves Importance
Sine waves are especially important in information sources
Natural phenomena produce sine waves
A microphone picks up an audible tone, the output is a sine
Electromagnetic radiation can be represented as sine wave
We are interested in sine waves that correspond to a signal that oscillates in time

Sine Waves Shape
Importance of sine waves is not just that signals often carried by them
Like in fibre optics, wireless transmission and communications using modems
The shapes are also important
Square shaped digital signal can be represented by a series of sine waves of different frequencies
Help engineers to analyse and design transmission systems

Signal Characteristics
Frequency: the number of oscillations per unit time (usually seconds)
Amplitude: the difference between the maximum and minimum signal heights
Phase: how far the start of the sine wave is shifted from a reference time
Wavelength: length of a cycle as a signal propagates across a medium
Determined by speed which signal propagates

Speed of Waves

Electromagnetic Spectrum
Source: Douglas, C (2016) Computer Networks and Internets

Visualising Signal Characteristics
Signal characteristics can be expressed mathematically
Source: Douglas, C (2016) Computer Networks and Internets

Signal Observations (1/3)
Frequency can be calculated as inverse of the time required for one cycle, which is known as period
The example (a) has:
Period T = 1 seconds
Frequency of 1 / T or 1 Hertz
The example (b) has
Period T = 0.5 seconds
Frequency of 2 Hertz

Source: Douglas, C (2016) Computer Networks and Internets

Signal Observation (2/3)
Both (a) and (b) are extremely low frequencies
Typical communication systems use high frequencies measured in millions of cycles per second

Source: Douglas, C (2016) Computer Networks and Internets

Signal Observation (3/3)
To clarify high frequencies, engineers express time in fractions of a second
OR express frequency in units like megahertz (MHz)
There are other orders of time to express frequency

Time & Frequency Units
Source: Douglas, C (2016) Computer Networks and Internets

Practice 7.2
Describe the FOUR signal characteristics.

Lesson 7 Outline
Data Communications
Types of Signal
Representing Signals

Simple Signals
Signals like the ones illustrated below are classified as simple
They consist of a sine wave that cannot be decomposed further
Source: Douglas, C (2016) Computer Networks and Internets

Composite Signals
However, most signals are classified as composite
signal can be decomposed into a set of simple sine waves
Source: Douglas, C (2016) Computer Networks and Internets

Importance of Composite Signal
Data communications concepts mostly relates to sine functions and composite signals
In modulation and demodulation, one of the primary reasons:
Signals that result from modulation are usually composite signals

Fourier Analysis (1/2)
A mathematician named Fourier discovered that
Possible to decompose a composite signal into its constituent parts
Each part is a sine function with frequency amplitude and phase

Fourier Analysis (2/2)
Analysis by Fourier shows that if composite signal is periodic, the constituent parts will also be periodic
Most systems use composite signals to carry information
Composite signal is created at the sending end
Receiver decomposes the signal into simple components

Fourier Analysis Result
Harmonics: frequencies from multiples of a base frequency
Mathematics behind this is known as Fourier Analysis
Signal of any shape can be represented by series of sine waves of different harmonic frequencies

Lesson 7 Outline
Data Communications
Types of Signal
Representing Signals

Representing Composite Signals
Several methods have been invented to represent composite signals
Time domain: graph of signal as function of time
Frequency domain: graph of frequency vs amplitude

Frequency Domain
Frequency domain graph shows a set of simple sine waves that constitute a composite function
y-axis gives the amplitude
x-axis gives the frequency

Source: Douglas, C (2016) Computer Networks and Internets
Frequency domain of

Frequency Domain Benefits
Frequency domain representation can also be used with nonperiodic signals
Compactness: frequency domain graph is small and easy to read as each sine wave occupies a single point along the x-axis
Good when a composite signal contains many simple signals

Bandwidth
Range of frequencies that can be effectively carried by the channel
Measures the difference between the upper and lower level frequency of transmission
In other words, it measures the number of times a signal oscillates per second
Unit of measurement: Hertz (Hz)

Maximum Bit Rate Achievable
Dependent on ability of receiver to discern level of signals received without errors
Depends on the effects of transmission impairments, particularly noise
There are two useful theorem:
Nyquist’s theorem
Shannon’s law

Nyquist Theorem
Nyquist Theorem: considers a noiseless channel when calculating maximum theoretical capacity of a channel
Nyquist Theorem: C = 2B log2V
V is the number of signaling levels used to carry the signals
C is the maximum theoretical capacity in bit/s
B is the bandwidth in Hertz (Hz)

Nyquist Theorem Example
Suppose a 3.1 kHz telephone circuit in which signal is carried using 32 signaling levels
Using Nyquist Theorem:
C = 2B log2V
= 2 x 3100 x log232
= 6200 x 5
= 31,000 bit/s
= 31 kbit/s

Practice 7.3
A signal making use of 64 separate signaling states is transmitted in a channel between the frequencies of 2kHz and 2.4 kHz.
What is the maximum achievable data rate in this channel?

Shannon’s Law
Shannon’s Law: considers a noisy channel by using signal to noise ratio when calculating maximum theoretical capacity of a channel
Shannon’s Law: C = B log2 (1 + S/N)
C is the maximum theoretical capacity in bit/s
B is the bandwidth in Hertz (Hz)
S is the signal power in Watts (W)
N is the noise power in Watts (W)

Signal to Noise Ratio
Signal to Noise Ratio (SNR) is calculated using
SNR = 10 log10 (S/N)
S is the power of signal
N is the power of noise
Unit of measurement: deciBel (dB)

Shannon’s Law Example (1/2)
Suppose a 3.1 kHz telephone circuit with a signal to noise ratio of 40dB
Converting 40dB to real ratio:
40 = 10 log10 S/N
4 = log10 S/N
104 = S/N
S/N = 10,000

Shannon’s Law Example (2/2)
Using Shannon’s law to calculate the theoretical maximum capacity
C = B log2(1 + S/N)
= 3,100 log2 (1 + 10000)
= 3,100 log2 (10,001)
= 3,100 x 13.2879
= 41,192 bit/s
= 41.2 kbit/s

Practice 7.4
A microwave radio signal is broadcast between the frequencies of 1 GHz and 1.1 GHz between two radio towers where it is affected by noise of power 2mW.
If the signal must carry 1 Gbit/s of data, calculate minimum level of power that will be required in the transmitter.

Extreme Cases to Consider
If signal do not change at all over time, frequency range of the component sine waves is 0
Requires no bandwidth
If the signal change abruptly from one level to another, like a square wave, frequency range of the component sine waves is infinite
Requires infinite bandwidth

Infinite Range of Frequencies
No medium can support infinite range of frequencies
All media must remove higher frequency components from the signal
No medium can perfectly carry a square shaped wave
Shape will be distorted when higher frequency components are removed
Shape of the signal received will not have perfectly straight edges

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 6

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 2: History of the Internet

Lesson 2 Learning Outcomes
Describe the motivation for resource sharing
Describe how the Internet has evolved to the present state
Describe the evolution in audio that has occurred in the Internet
Describe the impact of the Internet on cable television industry
Describe the significance of wireless Internet access

Lesson 2 Outline
Brief History
Changes on the Internet
Impact on Cable TV Industry

Early Computer Networks
Early computers networks when computers were large and very expensive, and the main motivation was resource sharing
To connect multiple users
Each with a screen & keyboard (terminals) to a large centralized computer
Allows sharing of peripheral devices
Permitted sharing of expensive, centralized resources

Birth of the Internet
In 1960s, Department of Defense for Advanced Research Projects Agency (DARPA or ARPA) wanted to find ways to share resources
Researchers needed powerful computers, but were very expensive
ARPA budget was insufficient
ARPA planned to interconnect all computers with a network!

ARPANET
ARPA devise a software that would allow a researcher to use whichever computer was best suited to perform a given task
ARPA did the following to achieve the visionary work:
gathered some of the best minds
focused them on computer network research
hired contractors to turn the designs into a working system: ARPANET
Research turned out to be revolutionary

Growth of the Internet
In less than 30 years…
the Internet has grown from ARPANET
Connecting a handful of sites to a global communication system
The rate of growth has been phenomenal!

Exponential Growth
Source: Douglas, C (2016) Computer Networks and Internets
x-axis: the year
y-axis: number of computers attached to the Internet, in millions
Exponential growth between 1980s-2000s

Early Internet Timeline
Source: Douglas, C (2016) Computer Networks and Internets

Lesson 2 Outline
Brief History
Changes on the Internet
Overview
Technological Changes
Application Changes
Impact on Cable TV Industry

Changes in the Internet
The Internet changed in two ways:
Communication speeds increased dramatically
New applications arose that appealed to many parts of the society
No longer dominated by scientists and engineers, scientific applications, or access to computational resources

Technological Changes
Internet was born out of resource sharing
More for communication now
Two technological changes that shifted the Internet
Higher communication speeds
Enable large data to be transferred quickly
Affordable personal computers
Powerful computational power and graphical display eliminates needs for resource sharing

Text to Multimedia
Data being sent across the Internet has changed
Multimedia: combination of text, audio, graphics and video
Originally only text but change to images, videos and audio
Source: Douglas, C (2016) Computer Networks and Internets
Email, files
1970s
Mixed Media
1980s
Multimedia
1990s
On-demand
Real-time
2000s

Audio Content on the Internet
Much of the content available is now multimedia
Quality of media also improved due to higher bandwidth
High resolution video and high-fidelity audio
Source: Douglas, C (2016) Computer Networks and Internets

Static to Dynamic
The Internet has transitioned from transfer of static textual documents
Now the Internet can transfer dynamic high-quality multimedia
Offline vs real-time requirements

Changes in Technologies
New technologies continue to emerge
Most significant transitions are the traditional communication systems
Voice and television moved from analog to digital
Support for mobile communication
Although the Internet applications changes, the underlying technologies remain the same

Analogue vs Digital
Analogue: analogue signals
Sounds waves, which vary continuously over time are analogue data
Digital: digital signals (bits)
Each bit can be represented by one of a number of discrete energy values
Superior performance
Computers produce digital data that is in binary form, that is, it is represented as a series of 1s and 0s

Practice 2.1
Describe the two motivations of resource sharing
What are the two ways in which the Internet has changed since 1960s?

B
18

Digital and Analogue Signals
Source: https://www.youtube.com/watch?v=RQc_frX50BA

Lesson 2 Outline
Brief History
Changes on the Internet
Overview
Technological Changes
Application Changes
Impact on Cable TV Industry

Technology Changes
The Internet has enabled some technology changes
Learning points
Traditional way of doing certain task
Modern way of doing certain task

Telephone System
Before: analog transmission of audio
Use the analog telephone circuits to communicate
Known as the Public Switched Telephone Network (PSTN)
After: digital transmission of audio
Uses the Internet infrastructure to communicate
Voice over Internet Protocol (VoIP)

How Telephone Works
Source: https://www.youtube.com/watch?v=hDCSMN-h8RU

Television
Before: wired analog channel
Can also make use of satellite dishes to receive TV broadcasts
After: wired/wireless digital
Can be received wireless
Digital transmission
Uses Internet Protocol (IP) for transmission

Cellular
Before: analog wireless cellular services
Satellite dishes on Earth
No connection to the Internet
After: digital cellular services
3G onwards
High speed mobile data

Internet Access
Before: wired, dial-up modem
Make use of telephone circuits
Cannot use telephone while connected
After: wireless, broadband, fibre optic
Wi-Fi
Separate/shared channel from telephone

Significance of Wi-Fi
Before: mobile devices has to have mobile data connectivity
Additional hardware needed for Internet Access
Incurs mobile data charges
After: any device near a Wi-Fi broadcast can access the Internet
Allows for less powerful electronic devices to connect to the Internet
Save on mobile data charges

Data Access
Before: centralised
Single server, multiple users
Bottleneck performance
After: distributed, peer-to-peer (P2P)
User can act as servers (Torrent)
Can be used as backups

Content Delivery
Before: requires download to view
Sometimes slow download
Impact work efficiency
After: streaming
Can view content on browser
Video streaming

Applications
Before: standalone, download and install
Updates requires additional download
Manual updates
After: cloud, auto-updates
Low cost computers, good productivity
No need to install applications, save space
Auto updates

Technology on the Internet
Area Before After Technology
Telephone System Analog voice Voice over IP VoIP
Television Analog delivery Digital delivery Internet Protocol (IP)
Cellular Analog cellular services Digital cellular services 3G
Internet Access Wired, Dial-up Wireless Wi-Fi
Data Access Centralised Distributed P2P
Content Delivery Download Streaming Video streaming
Applications Standalone Cloud Cloud services

Practice 2.2
How has the Internet changed the telephone system?
What are the 2 benefits of wireless Internet access?

B
32

Lesson 2 Outline
Brief History
Changes on the Internet
Overview
Technological Changes
Application Changes
Impact on Cable TV Industry

Application Trends
Technological advances has lead to many new Internet applications
Learning points
Definition
Benefits
Examples

Teleconferencing Systems
Teleconferencing system: combination of software and hardware to allow virtual meetings
Made possible by technological changes
Benefits
Reduces travel expenses/costs
Communicate anywhere

Navigation System
Navigation system: combination of software and hardware that provides users with map functions
Enabled by cloud technologies
Benefits
Can navigate to new locations
No need for standalone GPS system

Social Media
Social Media: community or platform that facilitates sharing and creation of multimedia
Enabled by availability and accessibility of the Internet
Benefits
Creates and maintains social connections
Meet new people
Entertainment purposes

Forums
Forums: platform that engage participants in some form of discussions
Contributes to community development
Benefits
Get help easily
Mostly free to use

Wikis
Wikis: platforms facilitates and maintain knowledge creation and sharing
Enabled by growing capacity and reduction in price of storage devices available on the Internet
Benefits
Public access knowledgebase
Contains plenty of content

Blogs
Blogs: customisable website that allows individual to share and create contents
Can be used to review products or act as a personal diary
Benefits
Can be designed to suit individual needs
Often free to create
Easily accessible to most people

Email
Email: electronic mail system that facilitates sending of multimedia messages
Important form of communication and can be used in courts as evidence
Benefits
Supported on most devices
Almost instant delivery

E-commerce
E-commerce: online retail platform that facilities sales transactions
Enabled by online payment services
Benefits
Shop anywhere
Shop anytime
Cost effective transactions

Remote Working
Remote Working: ability to work and remain productive anywhere
Enabled by remote access services
Benefits
Can access to complex software running on powerful computers
Allows mobile devices to be productive

Applications on the Internet
Area Uses Example
Teleconferencing Business-to-business communication Skype, Zoom
Navigation Systems Military, shipping industry, consumers Google Map
Social Media Consumers, volunteer organisations, businesses Instagram, Facebook
Forums Discussions Reddit, Stack Overflow
Wikis Knowledge sharing Wikipedia
Blogs Branding, diary Wordpress
Email Textual communication Gmail, Hotmail
E-commerce Consumers, businesses Amazon, Lazada, Shopee
Remote Working Remote access TeamViewer, Google Docs

Practice 2.3
Define E-commerce and how has the Internet help to enable E-commerce?
What are the two benefits of remote working?

Lesson 2 Outline
Brief History
Changes on the Internet
Impact on Cable TV Industry

Cable TV Companies
Often laid networks across metropolitan areas
Primarily to carry signals from cable TV head-end to individual homes
Most have been designed as cascaded star networks with head-end as central hub
Can usually support about 400,000 subscribers from a Regional Cable Head (RCH)
Cable TV Company
RCH

RCH

Cable TV Network
Original cable TV networks used copper coaxial cables
From RCH via a series of splitters and amplifiers to homes
Modern networks have been re-engineered
To use optical fibre cables
But cost of running it to each home is prohibitive
As such, optical network is converted back to copper at Fibre Nodes in street cabinets

Cable TV Network Architecture
RCH feeds a number of Distribution Hubs
Can support about 40,000 subscribers via switches and Fibre Nodes
Fibre Node will split TV signals and transmit over coaxial cable
Can provide service to about 1,000 homes using a tree topology

Hybrid Network
Cable TV networks are known as Hybrid Fibre-Coaxial networks
Can be adapted to provide a backwards channel to support interactive services
Most cable TV companies have adapted their networks to support broadband services using cable modems
Coaxial cables carrying TV signals also carry downstream and upstream data channel

Cable TV as Service Providers
Cable TV companies also supports voice services
But usually via a separate cable
Carried over separate channels to RCH where these voice channels are connected to the telephone network circuits
Cable TV companies offer similar products to enterprises as network operators
Like private circuits, LAN extension and IP VPNs
Access charges for use of the cable TV network are subsumed within prices charged for the WAN services

Practice 2.4
What type of cables did cable TV provider used to provide services to individual homes?
Why is cable TV network sometimes known as hybrid networks?

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 2

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 8: Signal Encoding

Lesson 8 Learning Outcomes
Define analogue bandwidth
Describe how information can be represented as digital signal
Define baud and bit rate
Describe how signals are synchronised
Define line coding
Describe Manchester encoding
Define lossy and lossless data compression

Lesson 8 Outline
Analog Signal Bandwidth
Digital Signal Bandwidth
Converting Analog to Digital Signal
Data Compression

The Electromagnetic Spectrum
Source: https://www.youtube.com/watch?v=cfXzwh3KadE

Analog Bandwidth
In networking and communication, the definition of bandwidth varies
Bandwidth: difference between highest and lowest frequencies of the constituent parts
Obtained by Fourier analysis
Known as analog bandwidth
OR bandwidth of an analog signal

Analog Bandwidth Example
Consider the frequency domain plot below
audible frequencies to humans
Bandwidth = 5 KHz – 1 KHz = 4 KHz
Source: Douglas, C (2016) Computer Networks and Internets

Digital Signals
Some systems use voltage to represent digital values
positive voltage correspond to logical one
zero voltage correspond to logical zero
Example
+5 volts can be used for a logical one
0 volts for a logical zero

Two Signal Levels
If only two levels of voltage are used
each level corresponds to one data bit (0 or 1)
Some physical transmission mechanisms can support more than two signal levels
When multiple digital levels are available each level can represent multiple bits

Four Signal Levels
Consider a system that uses four levels of voltage:
-5 volts, -2 volts, +2 volts, +5 volts
Each level can correspond to two bits of data
Source: Douglas, C (2016) Computer Networks and Internets
Digital Signal, Two levels
Digital Signal, Four levels

Number of Levels Required
Relationship between number of levels required and the number of bits to be sent is straightforward
There must be a signal level for each possible combination of bits
There are 2n combinations possible with n bits
Communication system must use 2n levels to represent n bits

Determining Number of Levels
Numbers of levels can be determined by dividing voltage into small increments
Millions of levels between 0 and 1 volts
0.0000001 volts for one level, 0.0000002 for the next level etc.
However, in real world, systems cannot distinguish between signals that differ by small amounts
They are restricted to a few signal levels

Practice 8.1
Suppose a system sent the following voltage:
+5, -1, +1, -5, +1, -1, +5, +2, -2, +3, -3
How many bits are sent by this system?
2. Suppose a system need to represent 4 bits, how many number of levels should it have?

Bits Per Second
How much data can be sent in each time depends on two aspects
Number of signal levels
Amount of time the system remains at a given level before moving to the next
2)
1)
Source: Douglas, C (2016) Computer Networks and Internets

Increasing Bits Per Second
If the communication system is modified to use half as much time for a given bit
Twice as many bits will be sent in the same amount of time
Source: Douglas, C (2016) Computer Networks and Internets
1
1
1
1

Limitation on Hardware
As with signal levels, hardware in real-world system places limits on how short the time can be
If signal does not remain at a given level long enough, the receiving hardware will fail to detect it

Baud Rate
Baud: how many times the signal can change per second
If a system requires signal to remain at a given level for .001 seconds, it operates at 1000 baud
Both baud and number of signal levels affects bit rate

Baud and Bits Per Second
System with two signal levels operates at 1000 baud
can transfer 1000 bits per second
System with four signal levels operates at 1000 baud
can transfer 2000 bits per second (as four signal levels can represent two bits)
Source: Douglas, C (2016) Computer Networks and Internets

Practice 8.2
Suppose a system with eight levels with 2000 baud, how many bits per second can the system transfer?
Suppose a system with four levels with 500 baud, how many bits per second can the system transfer?
Suppose a system with two levels with 1000 baud, how many bits per second can the system transfer?

Converting Digital to Analog
According to Fourier, any curve can be represented as a composite of sine waves
where each sine wave has specific amplitude, frequency, and phase
Fourier’s theorem also applies to a digital signal
However, accurate representation of a digital signal requires infinite set of sine waves

Engineer Approach
Engineers adopt a compromise:
conversion of a signal from digital to analog is approximate
generate analog waves that closely approximate the digital signal
approximation involves building a composite signal from only a few sine waves

Approximating Digital Signal
Source: Douglas, C (2016) Computer Networks and Internets

Lesson 8 Outline
Analog Signal Bandwidth
Digital Signal Bandwidth
Converting Analog to Digital Signal
Data Compression

Bandwidth of Digital Signal
Suppose Fourier analysis is applied to a square wave
such as the digital signal illustrated before
A digital signal has infinite bandwidth
as Fourier analysis of a digital signal produces an infinite set of sine waves with frequencies that grow to infinity

Synchronised Communication
Electronics at both ends of a physical medium must have circuitry to measure time precisely
if one transmits a signal with 10 elements per second,
Receiver must expect 10 elements per second
At slow speeds, making both ends agree is easy

Synchronisation Difficulty
Building electronic systems that synchronise at high speeds is extremely difficult
Problem is how data is represented which will affect synchronisation of sender/receiver
Suppose receiver misses first bit that arrives and starts interpreting data starting at the second bit
Mismatch in interpretation can produce errors

Synchronisation Errors
Source: Douglas, C (2016) Computer Networks and Internets
Synchronisation error where receiver allows slightly less time per bit than sender

Avoiding Synchronisation Errors
Several techniques have been invented to avoid synchronization errors
Signal agreement: sender transmits a known pattern of bits for receiver to synchronize
typically a set of alternating 0s and 1s,
Avoid ambiguity: Data is represented in a way that there can be no confusion about the meaning

Line Coding
Line coding: the way data is encoded in a signal
Eliminates ambiguity
Consider a transmission mechanism that supports three signal levels
Source: Douglas, C (2016) Computer Networks and Internets
three signal levels
two bits

Line Coding Efficiency Issues
Using multiple signal elements to represent a single bit means fewer bits can be transmitted per unit time
Designers prefer schemes that transmit multiple bits per signal element
Source: Douglas, C (2016) Computer Networks and Internets

Line Coding Variety
Line coding choice depends on the specific needs of a given system
A variety of line coding techniques are available that differ in
How they handle synchronization
Other properties like the bandwidth used

Line Coding Types
Source: Douglas, C (2016) Computer Networks and Internets

Detecting Signal Transition
One important line coding:
The Manchester Encoding used in Ethernet
Detecting a transition in signal level is easier than measuring the signal level

Manchester Encoding
Manchester Encoding uses transitions rather than levels to define bits
1 corresponds to a transition from negative voltage level to positive voltage level
0 corresponds to a transition from a positive voltage level to a negative level
Transitions occur in the “middle” of the time slot of a bit

Visualising Manchester Encoding
Differential Manchester Encoding
Manchester Encoding
Source: Douglas, C (2016) Computer Networks and Internets

Practice 8.3
State the sequence of bits for the following Manchester Encoding.

Differential Manchester in 2mins
Source: https://www.youtube.com/watch?v=du_boiwX1yU

Differential Manchester Encoding
Differential Manchester Encoding (or Conditional DePhase Encoding) uses relative transitions rather than absolute
representation of a bit depends on previous bit
Each bit time slot contains one or two transitions
transition always occurs in middle of the bit time

Differential Manchester Process
Bit is represented by presence or absence of a transition at the beginning of a bit time:
logical 0 is represented by a transition
logical 1 is represented by no transition
Important property from a practical consideration:
encoding works correctly even if the two wires carrying the signal are accidentally reversed

Lesson 8 Outline
Analog Signal Bandwidth
Digital Signal Bandwidth
Converting Analog to Digital Signal
Data Compression

Converting Analog to Digital
Many sources of information are analog which means they must be converted to digital form
further processing such as before they can be encrypted
There are two basic approaches:
Pulse Code Modulation (PCM)
Delta Modulation (DM)

Pulse Code Modulation
In PCM, level of an analog signal is measured repeatedly at fixed time intervals (sampling) and converted to digital form
Source: Douglas, C (2016) Computer Networks and Internets

PCM Steps
Sampling: each measurement is known as a sample
A sample is quantised by converting it into a small integer value
quantized value is not a measure of voltage or any other property of the signal
range of the signal from minimum to maximum levels is divided into a set of slots (in power of 2)
Encoded into a specific format

Visualising Sample Quantisation
Source: Douglas, C (2016) Computer Networks and Internets

PCM Samples & Quantisation
Six samples are represented by vertical gray lines
each sample is quantized by choosing the closest quantum interval
Example, the third sample, taken near the peak of the curve, is assigned a quantized value of 6

Avoiding Inaccuracy
In practice, slight variations in sampling have been invented
To avoid inaccuracy caused by a brief spike or dip in the signal, averaging can be used
instead of relying on single measurement for each sample
Averaging: mean of three measurements taken close together is computed

Delta Modulation
Delta modulation (DM) takes samples like PCM
But does not do quantization for each sample
DM sends one quantization value followed by a string of values that give difference between the previous value and current value

DM vs PCM
Transmitting differences requires fewer bits than sending full values
Especially when signal does not vary rapidly
Main issue with DM is the effect of an error
if any item in sequence is lost or damaged, subsequent values will be misinterpreted
Communication systems that expect lost or changes during transmission usually use PCM

Sampling Rate
Undersampling: taking too few samples
Gives crude approximation of the original signal
Oversampling: taking too many samples
more digital data will be generated which uses extra bandwidth

Nyquist Theorem
Mathematician named Nyquist discovered the answer to how much sampling is required
Nyquist Theorem provides a practical solution to the problem
sample a signal at least twice as fast as the highest frequency that must be preserved
fmax is the highest frequency in the composite signal

Source: Douglas, C (2016) Computer Networks and Internets

Lesson 8 Outline
Analog Signal Bandwidth
Digital Signal Bandwidth
Converting Analog to Digital Signal
Data Compression

Data Compression
Data compression: technique that reduces the number of bits required to represent data
Data compression is useful for communication system:
Reduce number of bits used to represent data reduces the time required for transmission
Communication system can be optimised by compressing data

Lossy Data Compression
Lossy: some information is lost during compression
generally for data that human consumes, like image and video/audio
Only need to preserve details to level of human perception
change is acceptable if humans cannot detect the change
JPEG (JPG) for images or MPEG-3 (MP3) for audio recordings employ lossy compression

Lossless Data Compression
Lossless: all information is retained in the compressed version
Can be used for documents or in any situation where data must be preserved exactly
For communication, sender compresses data before transmission and receiver decompresses the result
arbitrary data can be compressed by a sender and decompressed by a receiver to recover an exact copy of the original
GIF is lossless

Practice 8.4
Differentiate between lossy and lossless data compression. Give an example for each.

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 6

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 9: Wired Media

Lesson 9 Learning Outcomes
Distinguish between guided and unguided transmission media
Identify the different classification of transmission media based on forms of energy
Understand the motivation for using different forms of wired media
Distinguish the three different types of twisted pair copper wiring
Explain the motivation for shielding copper wiring

Lesson 9 Learning Outcomes
Identify the categories of twisted pair cables
Compare and contrast optical fibre with copper wiring

Lesson 9 Outline
Wired Transmission Media
Electrical Energy Transmission
Light Energy Transmission

Transmission Media Classes
There are two broad approaches:
Type of path: communication can follow an exact path such as a wire, or can have no specific path, such as a radio transmission
Form of energy: electrical energy is used on wires, radio transmission is used for wireless, and light is used for optical fiber

Guided Media
Guided: constrain the propagation of signals into a solid cable
Copper wiring or optical fibers provide a specific path
Signals do not easily stray from the cable
Energy requirements are relatively low
Relatively secured from eavesdropping
Good error performance

Unguided Media
Unguided: little to no constraint to the propagation of signals
At higher frequencies, signals tends to follow line of sight but still strays
Require more energy
Vulnerable to eavesdropping
Subject to interference and errors
Often fail in certain atmospheric conditions

Taxonomy by Forms of Energy
Taxonomy: the practice and science of classification
classification are not perfect and exceptions exist
Example:
Space station in orbit around the earth might employ non-terrestrial communication that does not involve a satellite

Energy Types
Source: Douglas, C (2016) Computer Networks and Internets

Practice 9.1
What are the TWO (2) ways to classify transmission media?

Lesson 9 Outline
Wired Transmission Media
Electrical Energy Transmission
Light Energy Transmission

Electrical Energy Transmission
Electrical current flows along a complete circuit
All transmissions of electrical energy need two wires to form a circuit
One wire to receiver
One wire back to the sender
Simplest form of wiring consists of a cable that contains two copper wires

Insulating Wires
Each wire used for electrical energy transmission is wrapped in plastic coating
To insulates the wires electrically
The outer coating on the cable holds related wires together to make it easier for humans who connect equipment
Prevent electrical shock

Interferences and Noise (1/2)
Random electromagnetic radiation, called noise, permeates the environment
Most communication systems generate minor amounts of noise as side-effect under normal operation
When noise hits metal, electromagnetic radiation induces a small signal
interfere with signals used for communication

Interferences and Noise (2/2)
Metal absorbs radiation, acting as shield
Placing enough metal between source of noise and communication medium can prevent noise from interfering
Source: Bing, licensed under CC BY-SA

Parallel Wiring
If two wires are parallel, high probability that one is closer to radiation than other
One wire act as shield, absorbs radiation
Second wire receives less radiation
Total of 32 units of radiation strikes
Top wire absorbs 20 units,
Bottom wire absorbs 12, producing difference of 8

Source: Douglas, C (2016) Computer Networks and Internets

Twisted Pair Wiring
Twisting two wires makes them less susceptible to electrical noise than leaving them parallel
Total of 32 units of radiation strikes
Each is on top half the time
Each absorbs same amount of radiation
Source: Douglas, C (2016) Computer Networks and Internets

Why Wires are Twisted
Source: https://www.youtube.com/watch?v=P7WfY9P2uNY

Practice 9.2
What is the rational to twisting wires?

Unshielded Twisted Pair (UTP)
Otherwise known as telephone cables
Pairs of metal cables twisted around each other in a regular fashion
Each metal conductor is surrounded by a plastic sheath to insulate it from other conductors
Several (usually 6) twisted pairs are then surrounded by an outer plastic sheath for protection

Source: Douglas, C (2016) Computer Networks and Internets

Twisted Pair Problems
Although immune to most background radiation, twisted pair wiring does not solve all problems
Twisted pair tends to have problems with:
Strong electrical noise when close physical proximity to the source of noise
High frequency communication

Electrical Noise Source
Electrical noise source:
factory that uses electric arc welding equipment
Cable runs above the ceiling in an office building on top of a florescent light fixture
Difficult to build equipment that can distinguish between valid signals and noise
even small amount of noise can cause interference when high frequencies are used

Cable Television Wiring
One familiar form of wiring that have extra metal shielding is the wiring for cable television
known as coaxial cable (coax)
It has a thick metal shield formed from braided wires that surround a center (inner) wire that carries the signal
Provides barrier to electromagnetic radiation from any direction

Coaxial Cable (Coax)
Early 10Mbit/s Ethernets and long-distance telecommunication circuits used Coax
Provide excellent performance
Outer plastic sheath for protection
But bulky, heavy and expensive
Difficult to install

Source: Douglas, C (2016) Computer Networks and Internets

Coax Benefits
Metallic shield prevents signals on inner wire from radiating electromagnetic energy
that could affect other wires
Coax can be:
Placed adjacent to sources of electrical noise and other cables
Can be used for high frequencies

Shielding of Coax
Source: Douglas, C (2016) Computer Networks and Internets

Shielding Wires
Using braided wire instead of solid metal shield keeps coaxial cable flexible but making it heavy
less flexible than twisted pair wiring
Variations of shielding have been invented that provide a compromise:
cable is more flexible, but has slightly less immunity to electrical noise

Making Shielded Cable Flexible
One popular shielded variation is known as Shielded Twisted Pair (STP)
STP has a thinner, more flexible metal shield surrounding one or more twisted pairs of wires
In most STP cable, the shield consists of metal foil like aluminum foil used in a kitchen

Shielded Twisted Pair (STP)
Like UTP except outer sheath contains a metal mesh connected to the earth
Compare to UTP:
Pairs inside are protected from electromagnetic interference
Less subject to impulse noise
Higher speed or longer distance
More expensive
Heavier and harder to install
Better performance
Source: Douglas, C (2016) Computer Networks and Internets

Standards for Twisted Pair
Standards organizations worked together to create standards for twisted pair cables used in computer networks
Mostly American orgnisations:
American National Standards Institute (ANSI)
Telecommunications Industry Association (TIA)
Electronic Industries Alliance (EIA)

Categories of Twisted Pair
Source: Douglas, C (2016) Computer Networks and Internets

Practice 9.3
Group the following type of media based on firstly, COST and secondly, immunity NOISE, in ascending order.
Coaxial cable (Coax)
Unshielded Twisted Pair (UTP)
Shielded coaxial Cable (STP)

Lesson 9 Outline
Wired Transmission Media
Electrical Energy Transmission
Light Energy Transmission

How Fiber Optics is Made
Source: https://www.youtube.com/watch?v=6CqT4DuAVxs

Using Light Energy Transmission
Three media use light energy to carry information:
Optical fibers
InfraRed transmission
Point-to-point lasers
Most important and widely used type is optical fiber

Optical Fiber
Each fiber consists of a thin strand of glass or transparent plastic encased in a plastic cover
One end connects to a laser or Light Emitting Diode (LED) used to transmit light
Other end connects to a photosensitive device used to detect incoming light
Two fibers are needed for two-way communication
One to carry information in each direction
Source: Douglas, C (2016) Computer Networks and Internets

Traveling Round a Bend in Fiber
When light encounters boundary between two substances, its behavior depends on:
density of the two substances
angle at which the light strikes the boundary
Given a pair of substances, there exists a critical angle, theta θ
measured with respect to a line that is perpendicular to boundary

Angle of Incidence
Angle of incidence: angle between the transmitted light and perpendicular line to the boundary
angle of incidence < θ, light crosses the boundary and is refracted angle of incidence = θ, light travels along the boundary angle of incidence > θ, light is reflected as if boundary was a mirror

Angle of Incidence Illustration
Angle of incidence < θ Angle of incidence = θ Angle of incidence > θ
Source: Douglas, C (2016) Computer Networks and Internets

Reflection in Fiber
Reflection in optical fiber is not perfect
It absorbs a small amount of energy
If a photon takes a zig-zag path that reflects from the walls of the fiber many times
This photon will travel slightly longer distance than another that takes straight path
Is dispersed (stretched) over time
Serious problem for long optical fibers

Dispersion Illustration
Source: Douglas, C (2016) Computer Networks and Internets
Light pulse sent and received over an optical fiber

Types of Fiber
Multimode, Step Index: least expensive and used when performance is unimportant
Reflect frequently and high dispersion
Multimode, Graded Index: slightly more expensive
Reduced reflection and lowered dispersion
Single Mode: most expensive and least dispersion
used for long distances and higher bit rates
has smaller diameter and other properties that help reduce reflection

Types of Fiber Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Transmitting Light
Single mode fiber and the equipment used at each end are designed to focus light
Pulse of light can travel long distances without becoming dispersed
Minimal dispersion helps increase the rate at which bits can be sent
A pulse corresponding to one bit does not disperse into the pulse that corresponds to a successive bit

Receiving Light
Devices used for transmission must match the fiber
Transmission: LED or Injection Laser Diode (ILD)
Reception: photo-sensitive cell or photodiode
Multimode fiber generally use LEDs and photo-sensitive cells
Single mode fiber generally use ILDs and photodiodes

Fiber Optic Transmission

Source: Douglas, C (2016) Computer Networks and Internets

Optical Fiber Properties
Optical fiber:
immune to electrical noise
higher bandwidth
Ends of an optical fiber must be polished before they can be used
Requires special equipment and expertise for installation
Can easily break if accidentally pulled or bent

Optical Fiber vs Copper Wiring
Source: Douglas, C (2016) Computer Networks and Internets

The Internet’s Undersea World
Source: https://visual.ly/community/Infographics/technology/internets-undersea-world

Practice 9.4
For each of the following requirements, each suggest ONE type of media that can be used.
Electrical energy transmission, lowest cost
Light energy transmission, high bandwidth
Electrical energy transmission, flexible cabling
Radio transmission, non-terrestrial propagation

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 7

End of Lesson

52
Google

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 12: Transmission Modes

Lesson 12 Learning Outcomes
Distinguish between parallel and serial transmission
Distinguish between asynchronous and synchronous transmission
Define bits, bytes, blocks and frames
Distinguish between simplex, half-duplex and full-duplex transmission
Explain what is meant for an equipment to be DCE and DTE

Lesson 12 Outline
Transmission Modes
Serial Transmission Types
Channel Types

Serial vs Parallel Transmission
Source: https://www.youtube.com/watch?v=myU2x27FIIc

Transmission Modes
Transmission mode: manner in which data is sent over the underlying medium
Two fundamental categories:
Serial: one bit is sent at a time
Further categorised according to timing of transmissions
Parallel: multiple bits are sent at the same time

Transmission Modes Types
Source: Douglas, C (2016) Computer Networks and Internets

Parallel Transmission
Allows multiple data bits sent at the same time over separate media
Mostly used with a wired medium that uses multiple, independent wires
Signals on all wires are synchronised
a bit travels across each of the wires at precisely the same time

Parallel Transmission Illustration
Source: Douglas, C (2016) Computer Networks and Internets
Each wire carries one bit

Parallel Transmission Notes
In addition to parallel wires that each carry data, a parallel interface usually contains other wires
To allow sender and receiver to coordinate
To make installation and troubleshooting easy, wires for parallel transmission system are placed in a single physical cable

Parallel Transmission Benefits
High speed: can send N bits at the same time
parallel interface can operate N times faster than an equivalent serial interface
Match to underlying hardware: computer and communication hardware uses parallel circuitry
parallel interface matches internal hardware well

Serial Transmission
Parallel is superior but most communication systems use serial mode
Serial can be extended over long distances at much less cost
Using only one wire means there will never be a timing problem caused by one being slightly longer than another
However, devices must contain a hardware to converts data from parallel circuitry to serial form for the wire

Serial Transmission Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Parallel & Serial Interface
Source: Douglas, C (2016) Computer Networks and Internets
MSB: Most Significant Bit
LSB: Least Significant Bit
Diagram shows the conversion

Serial Transmission Hardware
Hardware needed to convert data between parallel and serial can be straightforward or complex
Depend on type of serial communication
Universal Asynchronous Receiver and Transmitter (UART): chip that performs the conversion
Universal Synchronous-Asynchronous Receiver and Transmitter (USART): chip that handles conversion for synchronous networks

Serial Transmission Order
What should sender transmit first:
Most Significant Bit (MSB): big-endian to describe a system that sends the MSB first
Least Significant Bit (LSB): little-endian to describe a system that sends the LSB first
Either form can be used, but sender and receiver must agree

Ethernet Transmission Order
Order in which bits are transmitted does not settle the question of transmission order
Data in a computer is divided into bytes, and each byte is further divided into 8bits
Possible to choose a byte order and a bit order independently
Ethernet technology specifies data is sent byte big-endian and bit little-endian

Byte Big-endian Bit Little-endian
Source: Douglas, C (2016) Computer Networks and Internets
What happens if bit big-endian and byte little-endian

Serial Transmission Timing
Asynchronous: transmission can occur at any time
arbitrary delay between transmission of two data items
Synchronous: transmission occurs continuously
no gap between transmission of two data items
Isochronous transmission occurs at regular intervals
fixed gap between transmission of two data items

Practice 12.1
Suppose each character is represented by 8-bit, suggest the possible transmission modes for the following and explain why.
Achieve high speed
Achieve long distance at low cost

Lesson 12 Outline
Transmission Modes
Serial Transmission Types
Channel Types

Asynchronous Transmission
System is asynchronous if it allows physical media to be idle for some time between two transmissions
Asynchronous style of communication is well-suited to applications that generate data at random
User typing on a keyboard or clicks on a link

Asynchronous Transmission Issues
Issues arises from the lack of coordination between sender and receiver
Receiver don’t know how long medium will remain idle before more data arrives
Asynchronous usually arrange for sender to transmit a few extra bits before each data item
Preamble or Start bits: extra bits to inform data transfer is starting and allow receiver to synchronise with incoming signal

RS-232 Character Transmission
Consider transfer of characters across copper wires between a computer and a device keyboard
Each data item represents one character
Standardized by Electronic Industries Alliance (EIA)
Most widely used for character communication
Known as RS-232-C, and commonly abbreviated RS-232

EIA Specification for RS-232
EIA standard specifies the details like:
physical connection size: max cable length 50 feet long
electrical details: between -15v +15v
the line coding being used
Can be configured to control exact number of bits per second
Can be configured to send

RS-232 Illustration
Voltage varies at different stages
When a start bit, eight bits of a character, and a stop bit are sent
Diagram shows voltage for 8-bits character
Source: Douglas, C (2016) Computer Networks and Internets

Synchronous Transmission
Synchronous mechanism transmits bits continually
No idle time between bits
after transmitting final bit of one data byte, sender transmits bit of the next data byte
Sender and receiver constantly remain synchronized
Less synchronization overhead

Synchronous vs Asynchronous
Asynchronous RS-232 requires an extra start bit and stop bit
8-bit character requires minimum of 10-bit time, even if no idle time
On a synchronous system
each character is sent without start or stop bits

Synchronous Transmission Illustration

Async:
Sync:
Source: Douglas, C (2016) Computer Networks and Internets

Framing
Sometimes sender does not have data ready to send
Can make use of the idea of framing
Framing: interface added to a synchronous mechanism that accepts and delivers a block of bytes
Frame: blocks of bytes in framing

Framing Synchronisation
To ensure sender and receiver stay synchronized, a frame starts with a special sequence of bits
Idle sequence or idle byte: transmitted when sender has no data to send
Source: Douglas, C (2016) Computer Networks and Internets

Isochronous Transmission
Isochronous transmission: delivering data at a steady rate
provide steady bit flow for multimedia applications
Important to reduce jitter that disrupt reception
cause pops or clicks in audio/make video freeze for a short time
Jitter: variability of delay

Isochronous Network
Isochronous network is designed to accept and send data at a fixed rate, R
Data must be handed to the network for transmission at exactly R bits per second
Suppose an isochronous mechanism designed to transfer voice operates at a rate 64000bits/second
Sender generate digitized audio continuously
Receiver must be able to accept and play the stream

Practice 12.2
Propose and explain for each of the following type of scenarios, what will be a suitable serial transmission to use.
Minimum idle time in the physical medium
User typing on a keyboard
Real-time streaming video

Lesson 12 Outline
Transmission Modes
Serial Transmission Types
Channel Types

Channels Types
Depending on direction of transfer:
Simplex: transfer data in a single direction
analogous to broadcast radio or television
Full-Duplex: allows transmission in two directions simultaneously
analogous to a voice telephone conversation
participant can speak even if they are able to hear background music at the other end
Half-Duplex: transmission in two direction but one at a time

Channel Types Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Half-duplex Transmission
Uses shared transmission medium
Shared medium can be used for communication in each direction
Analogous to using walkie-talkies where only one side can transmit at a time
Additional mechanism is needed at each end to coordinate transmission to ensure only one side transmits at a given time

DTE and DCE
Source: https://www.youtube.com/watch?v=62tQoiqt2cc

DTE and DCE Terms
Data Communications Equipment (DCE) and Data Terminal Equipment (DTE) were originally created by AT&T to distinguish between
DCE are communications equipment owned by phone company
DTE are terminal equipment owned by a subscriber

DTE and DCE Example
If a business leases a data circuit from a phone company
Phone company installs DCE equipment at business
Business purchases DTE equipment that attaches to the phone company’s equipment

Ownership Concept
Concept of DCE-DTE distinction is not ownership of the equipment
It lies in the ability to define an arbitrary interface for a user
If underlying network uses synchronous transmission
DCE equipment can provide either synchronous or isochronous interface to user equipment

DCE and DTE Illustration
Source: Douglas, C (2016) Computer Networks and Internets

Practice 12.3
Suppose a user sign up with an Internet Service Provider (ISP) for a fiber plan. The ISP will have to setup a modem in his premise. On this modem, the user plugs in his own network device onto the modem and connects his end-user devices to the network device.
What are some examples of end-user devices?
What kind of network device can the user attach to the modem. How can his end-user devices connect to it?
Which device(s) is a DCE and which is a DTE?

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 9

End of Lesson

Communications and Networks

version 1.0

Diploma in Information Technology

Lesson 6: Domain Name System

Lesson 6 Learning Outcomes
Explain what is DNS
Describe the purpose of DNS
Explain how DNS works
Describe how DNS cache
Describe the different types of DNS entries
Identify how abbreviations is handled in DNS

Lesson 6 Outline
DNS Basics
Implementing DNS Servers
DNS Mapping Process

Human-Readable Names
Domain Name System (DNS): provides a service that maps human-readable names to computer addresses
Browsers, mail software, most other Internet applications use the DNS
DNS is an example of client-server interaction!

Mapping of Domain Names
Traditionally, mapping of names was done via centrally managed text file
helloworld.com => 123.151.121.131
Inefficient: Text file is distributed to all hosts
DNS allows applications to use domain names
Domain names is then translated into network layer address (IP address)
Hierarchical and fully distributed

Resolving Hostname
Mapping is not performed by a single server
Fully distributed: Naming information is distributed to multiple servers on the Internet
Whenever an application needs to translate a name, it becomes a client of the naming system
client sends request message to name server
server finds corresponding address and sends a reply message

Temporary Client
Mapping may not be always available
If a server cannot answer a request,
Server temporarily becomes the client of another name server
Until a server is found that can answer the request

DNS Syntax
Each name consists of a series of alpha-numeric segments separated by periods
Domain names are hierarchical, most significant part of the name is on the right
Top layer of the hierarchy is the last field
.com, .org, .edu, .net

Domain Name Hierarchy Example
Example: mordred.cs.purdue.edu, anakin.cisco.com
Left-most segment “mordred” and “anakin” is the name of individual computer
Other segments identifies the group that owns it
“purdue” gives the name of a university
“cisco” gives the name of a company

Top-level Domain
DNS specifies values for the most significant segment called a top-level domain (TLD)
Controlled by Internet Corporation for Assigned Names and Numbers (ICANN)
ICANN designates one or more domain registrars to administer a given top-level domain and approve specific names
But ICANN is responsible for managing 13 top DNS root server

TLDs Servers
For each top-level domains are at least 2 DNS servers
Below this level are other delegated authorities to managed them
Some TLDs are generic: they are generally available
Other TLDs are restricted to specific groups or government agencies

Domain Registration
An organisation applies for a name under one of the existing top-level domains
most US corporations choose to register under the “.com” domain
DNS allows organisations to use a geographic registration
Corporation For National Research Initiatives
cnri.reston.va.us

List of TLDs
Source: Douglas, C (2016) Computer Networks and Internets

Mnemonic Names
Domain names are assigned to reflect the service provided
FTP server: ftp.foobar.com
Web server: www.foobar.com
Such names are mnemonic, but not compulsory

Using WWW
Use of www for a web server is merely a convention
Any computer can run a web server, even if it does not contain www
A computer with domain name beginning with www is not required to run a web server

DNS Example
doc.gold.ac.uk
.uk managed by UK registration authority (Nominet)
.ac.uk and gov.uk managed by UK Education and Research Networking Assocation (UKERNA)
.gold.ac.uk managed by Goldsmiths (a school in a university)
doc.gold.ac.uk managed by Department of Computing in Goldsmiths

DNS Explained
Source: https://www.youtube.com/watch?v=72snZctFFtA

DNS Process Summary
If local server can resolve name, returns the address
If unable, make request to one of the root servers
If root server can resolve, returns the address
Otherwise, return address of another server that can help
Repeats until name is resolved OR firmly established that name cannot be resolved
Recursive or iterative

Practice 6.1
What happens to a request when a name server receives it but does not have an answer to it?
In the URL, “cnri.reston.va.us”, which segment is the TLD? What does the TLD represent?
What does it mean when domain names are mnemonic?

Lesson 6 Outline
DNS Basics
Implementing DNS Servers
DNS Mapping Process

Implementing the DNS
Each organization is free to choose the details of its servers
A small organization that only has a few computers can contract with an ISP to run a DNS server
Computer
Computer
Computer
Hub
DNS Computer
Computer
Computer
Computer
Hub
ISP DNS Computer
ISP Hub

DNS Choices
An organization that runs its own DNS
Can choose to place all names in a single physical server
Can choose to divide its names among multiple servers
candy.foobar.com
soap.foobar.com
foobar.com
candy
soap

DNS Server Hierarchy Example
A hypothetical Foobar Corporation could choose to structure servers if the corporation had a candy division and a soap division
Source: Douglas, C (2016) Computer Networks and Internets

DNS Autonomy
DNS is designed to allow each organisation to assign or modify domain names without informing a central authority
Autonomy: each organization is permitted to operate DNS servers for its part of the hierarchy
Purdue University operates a server for names ending with purdue.edu
IBM Corporation operates a server for names ending with ibm.com

Replicating DNS Servers
Each DNS server can link to other domain name servers up and down the hierarchy
a server can be replicated; multiple physical copies of the server exist
Replication is useful for heavily used servers (like root servers) that provide information about TLDs
Administrators must guarantee all copies are coordinated to provide identical information

Name Resolution
Translation of a domain name into an address is called name resolution
The name is said to be resolved to an address
Software that perform this is known as a name resolver or resolver
In socket API, the resolver is invoked by calling function gethostbyname

DNS Request and Reply
Resolver becomes a client by contacting a DNS server
DNS server returns an answer to the caller
Each resolver is configured with the address of one or more local domain name servers
Resolver forms a DNS request message
Sends the message to the local server
Waits for server to send DNS reply message

Name Resolution Paradigm
Resolve can choose to use either stream or message paradigm when communicating with a DNS server
Most resolvers use message paradigm as it imposes less overhead for a small request
Example: chocolate.candy.foobar.com
Resolver will send request to local DNS server for foobar.com
Even if it cannot answer the request, the server knows to contact the server for candy.foobar.com, which can generate an answer

Locality of Reference
Locality of reference principle that forms the basis for caching applies to the Domain Name System in two ways
Spatial: A user tends to look up the names of local computers more often than the names of remote computers
Temporal: A user tends to look up the same set of domain names repeatedly

Exploiting Locality
DNS exploits spatial locality
Resolver contacts a local server first
To exploit temporal locality
a DNS server caches all lookups

DNS Lookup Algorithm
Source: Douglas, C (2016) Computer Networks and Internets

Caching in DNS Server
When a request arrives for a name outside the set for which the server is an authority
Server temporarily becomes a client of another name server
When the other server returns an answer, original server caches the answer
Sends a copy of the answer back to the resolver from which the request arrived

Server 1
(authority)
Resolver
i) Help resolve name
ii) Can’t resolve name
Server 2
iii) Ask others
iv) Can resolve name
v) Returns answer
vi) Cache answer
vii) Returns answer

Caching Consideration
In addition to knowing the address of all servers down the hierarchy
Each DNS server must know address of a root server
How long items should be cached?
If cached too long, the item will become stale (may become outdated)
DNS will specify a cache timeout for each item

Practice 6.2
How does DNS allow autonomy?
Why is there a need to replicate a DNS server?
What kind of communication paradigm does name resolution uses? Which is more commonly used and why?

Lesson 6 Outline
DNS Basics
Implementing DNS Servers
DNS Mapping Process

Entry Fields
Each entry in a DNS database consists of three fields:
Domain name: human readable names
Record type: specifies how the value is to be interpreted
Value: such as IP address
A query specifies both a domain name and a type
Server only returns a binding that matches the type of the query

Binding Classifications
Principal type maps a domain name to an IP address
Known as type A bindings
used by applications such as FTP, or a browser
ftp.web.com -> 192.168.1.0
Another is type MX that specifies a Mail eXchanger
SMTP uses type MX to look up the domain name in an email address
hello@myemail.com

Entry Type
Each entry in a DNS server has a type
Resolver must specify the type desired when looking up a name
DNS server returns only entries that match the specified type

Mapping Entry Type Efficiently
DNS type system can produce unexpected results
Address returned depends on the type
the name corporation.com can be used for both web and email services
Possible to divide the workload between separate computers
Mapping of type A lookups to one computer and type MX lookups to another

Alias and CNAME
The DNS offers a CNAME
Analogous to shortcut in file systems
This provides an alias for another DNS entry
aliases can be useful, suppose Foobar Corporation has two computers, named as:
hobbes.foobar.com and calvin.foobar.com

Alias and CNAME Example
Suppose that foobar runs a web server on computer hobbes, and wants to follow the convention of using the name www
EITHER: rename computer hobbes
OR: create a CNAME entry for www.foobar.com that points to hobbes
When resolver sends a request for www.foobar.com, server returns address of computer hobbes

Alias Benefits (1/2)
Alias permits an organization to change the computer used for a service without changing the names or addresses:
Foobar Corporation can move its web service from hobbes  calvin
changing the CNAME record in DNS server allows two computers retain their original names and IP addresses

Alias Benefits (2/2)
Aliases allows an organization to associate multiple aliases with a single computer
Can run an FTP server and a web server on the same computer
Create CNAME records for:
www.foobar.com
ftp.foobar.com

Abbreviations
DNS does not incorporate abbreviations
Server only responds to a full name
Most resolvers can be configured with a set of suffixes that allow a user to abbreviate names
Each resolver at Foobar Coporation might be programmed to look up a name twice:
once with no change
once with the suffix foobar.com appended

Abbreviations and DNS
If user enters full domain name
Local server will return address and proceed
If user enters an abbreviated name
Will first try to resolve the name
Receive error because no such name exists
Then it will try appending suffix and looking up the resulting name

DNS Character Set Limitations
DNS uses the ASCII character set
Can represent mostly English characters
Languages like Russian, Greek, Chinese, and Japanese each contain special characters
No ASCII representation exists
Many European languages use diacritical (accent) marks that cannot be represented in ASCII

IETF Approach to DNS Characters
IETF debated modifications and extensions of the DNS to accommodate international domain names
Chose an approach known as Internationalizing Domain Names in Applications (IDNA)
IDNA uses ASCII to store all names
If a domain name contains a non-ASCII character, IDNA translates the name into a sequence of ASCII characters
Stores the result in the DNS

IDNA Translation
IDNA relies on applications to translate between international characters and internal ASCII form
Rules for translating international domain names are complex and uses Unicode
Latest versions of Firefox and Internet Explorer can accept and display non-ASCII domain names as they each implement IDNA

Practice 6.3
Describe each of the three fields in a DNS entry.
Describe the type of domain name binding that can be used for a mail exchanger. Give an application example that uses such binding.

Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 4

End of Lesson

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