MI0026

MI 0026

Computer Networks (MBA IS)

Contents

Unit 1

Introduction to Computer Networks 1

Unit 2

Reference Models 19

Unit 3

Data Communications 40

Unit 4

Physical Medium 61

Unit 5

Communication Satellites 79

Unit 6

Network topologies and networking devices 88

Unit 7

Data Link Layer & MAC Sub-Layer 107

Unit 8

Network and Transport Layer 121

Revised Edition: Fall 2009

BKID B1041 27th

June 2009

Unit 9

Network Layer in Internet 134

Unit 10

Internet Applications and Network Security 149

Bibliography 167

Prof. S. Kannan Director & Dean (In-charge) Directorate of Distance Education Sikkim Manipal University of Health, Medical & Technological Sciences (SMU-DDE) Board of Studies

Mr. Shanath Kumar (Chairman) Mr. Shankar Jagannathan Head- Management & Commerce Consultant (ex-Treasurer)-WIPRO SMU DDE, Bangalore 560 008

Mr. K. Ashok Kumar Mr. Pankaj Khanna Additional Registrar Director HR SMU DDE, Manipal 576 104 Fidelity Mutual Fund

Mr. M.K.N. Prasad Mr. Abraham Mathews Controller of Examinations CFO Infosys BPO SMU DDE, Manipal 576 104

Dr. T.V. Narasimha Rao Ms. Sadhana Rao Adjunct Faculty & Advisor Senior Manager HR SMU DDE, Bangalore 560 008 Microsoft India Corporation (Pvt) Ltd.

Prof. K. V. M. Varambally Special Invitee Director Prof. Ramu Iyer Manipal Institute of Management Ex-Professor Manipal 576 104 IIM Calcutta

Prof. Sunderrajan IIM Bangalore

Content preparation Team Content Writing and Compilation

Mr. Ramachandra Ms. Ramya S. Gowda Assistant Professor Lecturer, Dept. of Management & Nagarjuna College of Engineering Commerce, SMU DDE Bangalore Bangalore

Edition: Fall 2007 Revised Edition: Fall 2009

This book is a distance education module comprising a collection of learning materials for our students. All rights reserved. No part of this work may be reproduced in any form by any means without permission in writing from Sikkim Manipal University of Health, Medical and Technological Sciences, Gangtok, Sikkim.

Printed and Published on behalf of Sikkim Manipal University of Health, Medical and Technological Sciences, Gangtok, Sikkim by Mr. Rajkumar Mascreen, GM, Manipal Universal Learning Pvt. Ltd., Manipal 576 104.

Printed at Manipal Press Limited, Manipal.

SUBJECT INTRODUCTION

Computer Networks has become vital part of technology in the present

world. A computer network is a group of interconnected computers.

Networks may be classified according to a wide variety of characteristics.

This text also provides a general overview of some types and categories

and also presents the basic components of a network.

This text comprise of ten units as mentioned below:

Unit 1: The introduction to computer networks, use of computer networks to

people, different classification of networks and different design issues for the

layers.

Unit 2: Difference Reference models, the comparison of the different

models and different types of networks, the different standardization of the

networking.

Unit 3: Theoretical basis for communication, concepts of signals and

Fourier analysis, different types of transmission.

Unit 4: Physical medium explains the medium of data transfer and different

types of mediums.

Unit 5: the concepts of communication satellites, wireless communication.

Unit 6: this unit explains the different network topologies and the

comparison between the topologies and the different networking devices.

Unit 7: Explains the Data link layer and MAC sub layer

Unit 8: Design issues in Network layer and transport layer is explained in

this unit.

Unit 9: The role of network layer in internet is explained here.

Unit 10: The different applications of the internet and security issues are

explained in this unit.

Computer Networks Unit 1

Sikkim Manipal University Page No. 1

Unit 1 Introduction to Computer Networks

Structure:

1.1 Introduction

Objectives

1.2 Uses of Computer Networks

Networks for companies

Networks for peoples

1.3 Networks Hardware

Classification of networks

LAN

MAN

WAN

Wireless

Home Networks

1.4 Network Software

Protocol hierarchy

Design issues for the layers

Connection Oriented and Connectionless Services

Service Primitives

1.5 Summary

1.6 Terminal Questions

1.7 Answers to SAQs and TQs

1.1 Introduction

In this unit you will study about the basic concepts of computer networks.

Computer Network consists of a set of devices which are connected via

communication media link. The devices can be Computers, Printers,

Laptops, or any other communication devices capable of sending and / or

receiving data generated by other device on the network.

The term computer network to mean a collection of autonomous

computers interconnected by a single technology, in which each of them can

exchange information. The communication media can be wired or wireless.

Wired media are copper wire, co-axial cable, optical fiber and wireless

media can be microwaves, infrared links and communication links for

satellites. Network can be of different size, shape, form, and structure.



Objectives:

After studying this unit, you will be able to:

(a) Define and explain the computer network.

(b) Explain Different uses of computer network.

(c) Point out Classification of networks.

(d) Discuss the various types of networks hardware and software.

1.2 Use of Computer Networks

Before discussing the technology of the computer network, Let us see why

the people are interested in computer networks and why they can be used

for. Broadly the uses of computer networks can be classified into categories

namely, companies, people, mobile users and home networking.

1.2.1 Networks for companies

Industries have enough computers for performing or managing inventory

and payroll for their employees. If their computers are isolated from others, it

will be difficult to extract and correlate information about the entire company.

The uses of this are:

Resource sharing: All the programs, devices and data will be made

available to everyone without regard to the physical location of the

resources and users. Example: Sharing physical resources such as printers,

scanners and CD burners.

Client-Server model: Some companies will have offices and plants located

in other countries. If these companies are networked, any employee of a

country can access the data of another employee in other country.

In this model, the data are stored on powerful computers called servers.

The employees have simpler machines, called clients, on their desks, with

which they access remote data.



Figure 1.1: A network with two clients and one server

1.2.2 Networks for peoples

Another goal of setting up a computer has to do with people rather than

information or computers. The uses of these are:

Communication medium: The network provides a powerful communication

medium among people. For example, e-mail (electronic mail) can be used

as daily communication application.

Advantages

1. It is easy for two or more people who work far apart to write a report

together.

2. Changes to an online document, the others can see immediately.

Videoconferencing: The people at distant locations can hold a meeting,

conference, and writing on a shared virtual black board. This application

saves lot of cost and time which is devoted to travel.

Electronic business: The business can be done electronically with the help

of network. Many companies is doing business electronically with other

companies. The manufacturers can place order electronically as needed by

the company.

E-commerce: The electronic commerce is doing business with consumers

over the Internet. Many companies provide catalogs of their goods and

services on-line and take orders on-line. Examples: Airlines, bookstores,

etc.



Self Assessment Questions: I

1. The data are stored on powerful computer called ______________.

2. The full form of e-mail is _________________.

3. The term __________________ to mean a collection of autonomous

computers interconnected by a single technology.

1.3 Networks Hardware

The computer network has two criteria for classifying networks:

1. Transmission Technology.

2. Scale.

There are two types of transmission technology that are widely used:

1. Broadcast links.

2. Point-to-point links.

Broadcast networks have a single communication channel that is shared

by all the computers on the network. Packets (short message) sent by a

single computer will be received by all the others on the network.

The addressing system in packet determines the machine to receive a

packet, but other machines just ignore. Broadcast system allows addressing

a packet to all destinations by using a special code in the address field.

When a packet with this special code is transmitted to all destinations, it is

received and processed by every machine on the network. This mode of

operation is known as broadcasting.

If the packets are sent to only a subset or group of machines on the

network, then it is called as multicasting.

Point-to-point networks provide communication link from one source to one

destination. Here packets have to visit one or more intermediate machines

to reach the destination. Large networks are usually point to point.

Transmission of packets with one sender and one receiver is also called as

uncasing.

1.3.1 Classification of networks

Another way of classifying networks is based on scale. If the interprocessor

distance is 1m on the same board, then it is called Personal Area Network.

These are meant for one person. For example, a wireless network



connecting computer with its phone, keyboard and printer can be

considered as PAN.

Next, if the computers are networked with 100 m to 1 kilometer of space,

then it will be Local Area Network (LAN).

The network spans throughout the city of distance 10km can be called as

Metropolitan Area Network (MAN).

If the network covers the distance of 100 km to 1000 km then it can be

considered as Wide Area Network (WAN).

Finally, the connection of two or more networks is called as internetwork or

internet.

1.3.2 LAN

Local Area Network is privately owned networks which spans over a size of

up to a few kilometers in a building or a campus. They are widely used to

connect personal computers (PCs) in companies and academic

laboratories. It is widely used to share resources (Printers) and exchange

information within a group of computers connected.

LANs can be classified with other type of networks by three important

characteristics:

1. Size

2. Transmission Technology

3. Topology

LANs size can be up to few kilometers covering buildings and campus.

Here, in worst-case, transmission time required is known in advance. If the

size is restricted, then it is very easy to manage network.

LANs uses transmission technologies in which cable are used to connect all

the machines. The type of transmission media used will have impact on

speed of the network. Traditional LANs operate at speeds from 10 Mbps to

100 Mbps. Latest LANs operate at very high speeds up to 10 Gbps.

LANs can have different topologies like bus, ring, star, and complete so on.

In case of broadcast LANs of bus topology, at any instant only one machine

is allowed to transmit and only one will be a master (controller). All other

machines must stop transmission when transmission line is busy. There are



some mechanisms for resolving conflict when two or more machines are

required to transmit simultaneously

Figure 1.2: Two broadcast networks. (1) Bus, (2) Ring

1.3.3 MAN Metropolitan Area Networks are bigger than LANs, which span over the city.

MANs provide higher data rate and uses co-oxial cable or optical fiber as

transmission media.

The well known example is cable TV network available in many cites. The

cable TV network operators thought of utilizing un-used parts of the

spectrum for two way internet service. In the following figure we can see

both television signals and Internet being fed into the centralized head end

for subsequent distribution to peoples homes.

Cable (2)



Figure 1.3: A metropolitan area network like cable TV

1.3.4 WAN

Wide Area Networks are bigger than MANs and LANs. WAN spans over a

large geographical area like country or continent. A collection of MANs and

LANs can form a WAN intended or used for running user programs.

Machines which are used for running user program are called as hosts.

Hosts are connected by communication subnet (collection of communication

devices other than host). Hosts are owned, operated and maintained by a

telephone company or internet service provider.

WAN consists of two components:

1. Transmission lines.

2. Switching elements.

Transmission lines are responsible for moving bits between machines and

they are made up of copper wire, or co-oxial cable, or optical fiber or radio

frequency links (Wireless links).

Switching elements are small and specialized computers that connect three

or more transmission lines. When data arrives on incoming line, the

switching element must choose appropriate outgoing line. The best

examples for switching elements are routers.

In the figure, each host (computer) has connected to its LAN on which router

is present. The collection of transmission lines and routers (excluding host)

from a subnet where Subnet is responsible for moving packets from one end

of host to destination host.



Figure 1.4: WAN with host on LANs and Routers

A subnet is organized to route the packet from one source to another via

one or more intermediate routers. The packet is stored in intermediate

router until the required output line is free and then packet is forwarded. This

principle is known as a store and forward or packet switched subnet.

Almost all WANs have store and forward subnet when the packets are small

and of same size, they are called as cells. The messages on WANs are

transmitted from one host to another in the form of packets. The message to

be sent to a process on other host is divided into packets. Each packet has

sequence number and packets are then injected into the network one at a

time in quick session. These packets are reassembled or collected on other

end to get the original message.

1.3.5 Wireless

Wireless networks provide connection via wireless communication links

such as infrared, radio frequency, microwaves links etc. these types of

networks allow mobile device to move freely which are completely wireless.

Wireless networks can be divided into three categories:

1. System interconnection (wireless)

2. Wireless LANs

3. Wireless WANs

LAN - 1

Bus

LAN-2

LAN-3

LAN-4

Routers

Ring



System interconnection is interconnecting the various components and

mobile device using short range wireless links. The different components of

computer such as monitor, keyboard, mouse and printer are connected

together and form a short range of wireless network without wires called

blue tooth which do not use cables, no drivers installation.

In wireless LANs, every computer has radio modem and antenna through which it can communicate with other computers.

Figure 1.5: Wireless LANs

WLANs are popular in small offices, home networking and conferences. The

standard used for WLAN is IEEE 802. 11x, where x represents different

versions.

Wireless WAN bigger than wireless LANs and has wide coverage area. The

radio network used for cellular telephones is an example of low bandwidth

wireless system.

The low bandwidth wireless has already evolved into three generations. In

first generation only voice and analog were there. The second generation

was digital and voice only. The third generation is digital and is for both

voice and data. Wireless LAN can operate up to 50 Mbps over distance of

tens of meters. The wireless network can be integrated with wired network

to provide access to files, database and internet.

1.3.6 Home Networks:

IN the present world each home can have more than one computer. In order

to share the programs, files, printers and other peripheral devices these

computers are interconnected using LAN networks. Such a residential LAN

network is called Home Networks.



There are five different types of Home Networks. They are:

1. Direct cable connection: This is a feature that shares the files and

transfers the data to another computer in the home.

2. Traditional Ethernet: Traditional Ethernet supports data transfer at the

rate of 10 Mbps.

3. AC network: An AC (alternating current) network is a possibility when

computers are in different locations in your house. It doesnt need any

drilling of holes or no wirings is required in the rooms. It just need to

simply plug one end of an adapter into the parallel port of your computer

and plug the other end into an outlet. The data is transmitted through the

power lines.

4. Phone line network: Phone line network is one of the ways to connect

two or more computers in the same home but in different rooms. It is

also referred to as HomePNA. This technology uses existing phone

wiring to connect multiple computers.

5. Radio Free (RF) network: Radio Free Network exists to help facilitate the

use of Open Source broadcast and communications technologies by

community organizations and development initiatives.

Self Assessment Questions: II

State whether the following statements are True or False:

1. A MAN is a network with a size between a LAN and a WAN.

2. The standard used for wireless LANS is IEEE 802.10.

3. Latest LANs operate at very high speeds up to 1000 Gbps.

4. The combination of LANs and MANs is known as WAN.

1.4 Networks Software

Network software is highly structured. The software has well defined

boundaries of layer. Each layer has its own function.

1.4.1 Protocol hierarchy

The network functionality can be expressed in the form of levels or layers.

These layers are formed to reduce the design complexity of the networks

and are organized as a stack of layers. Each layer is built upon another

layer. One network will differ from the other in terms of number of layers in

each layer, the contents of each layer and the function of each layer.



The main purpose of each layer is to give services to upper layer. The idea

is that a particular piece of software (or hardware) provides a service to its

user. The details of internet state and algorithm are hidden from them, and

hence the users will be aware only, the services available, not the internal

details.

Figure 1.6: Layers + Protocols +interfaces = network architecture

Each computer or host can have n layers. Layers n on computer 1 carries

on a conversation with layer n of another machine. For having this

conversation between them, certain mechanisms and conventions are used.

The rules and conventions used between the communicating parties on how

communication is to proceed are known as Protocol. The rules and

convention used in conversation of n layer on two machines are collectively

known as the layer n protocol.

In figure, n layer network is illustrated. Any active element (such as process,

hardware device or human) is called and Entity. Entities on one layer on a

machine will have corresponding entities on a different machine which are

known as Peers. For example, a simple hardware on layer 1 on computer-1

is peer for the simple hardware on layer-1 on computer-2. in reality, no data

is transmitted directly from layer-1 on machine-1 to layer-1 on machine-2.

here each layer passes data and control information to layer immediately

Layer n Layer n

Layer 3

Layer 2

Layer 1

Layer 3

Layer 2

Layer 1

Transmission Medium

Layer 1 Protocol

Layer 2 Protocol

Layer 3 Protocol

Layer n Protocol

Computer - 1 Computer - 2

Layer 1/2 interface

Layer 2/3 interface

Layer 3/4 interface



below it, until the lowest layer is reached. After reaching the last layer i.e

layer 1, data is transmitted to transmission medium (physical or wireless)

through which actual data communication happens.

Interfaces are used to get the service and primitive operations from lower

layer made available to the upper layer. Interfaces can be identified between

any two adjacent layers.

A set of layers, interfaces and protocols can be considered as Network

Architecture. The network architecture must clearly specify enough detailed

information to allow implementer (developer) to write the program or build

the hardware for each layer so, that it satisfies the network design

specifications. Interfaces do not give the inner details of the implementation

as they are hidden inside the machines.

A list of protocols is used by a system. One protocol per layer is called

protocol stack as shown in the figure. This protocol stack has five layers,

and each layer has one protocol. They are available in each layer by

defining set of rules or conventions used to communicate and provide

services to the upper layers.

Figure 1.7: Protocol Stack

For the purpose of illustration, consider a five layer network. A message M,

is produced by an application process running in layer 5 and gives to layer 4

by adding header in front of the message to identify the message and

passes the result to layer 3. Headers added by each layer contain

information such as sequences number, size, times and other control fields.

In layer 3, incoming messages are split into smaller units M1 and M2 and

adds header to each split messages. Layer 3 passes the packets to layer 2.

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Protocol 5

Protocol 4

Protocol 3

Protocol 2

Protocol 1



Layer 2 adds a header to each piece of message and also adds trailer.

Finally the information is sent on to the transmission media. At the other

enc, in receiving machine, the message moves upward, from layer to layer,

with header removed off as it moves upward.

M : Message Mn : Message produced at nth

layer

H : Header M1 : Message M is split into M1 and M2

T : Trailer H4 : Header is added at layer 4

Figure 1.8: Information flow in each layer

1.4.2 Design issues for the layers

The various key design issues are present in several layers in computer

networks. The important design issues are:

1. Addressing: Mechanism for identifying senders and receivers, on the

network need some form of addressing. There are multiple processes

running on one machine. Some means is needed for a process on one

machine to specify with whom it wants to communicate.

2. Error Control: There may be erroneous transmission due to several

problems during communication. These are due to problem in

communication circuits, physical medium, due to thermal noise and

interference. Many error detecting and error correcting codes are known,

M

H4 M

H3 H4

M1 H3

M2

H2 H3

H4 T2

M1 H2 H3

T2

M2 H2 H3

H4 T2

M1 H2 H3

T2

M

1

H3

M2 H3 H4

M1

H4 M

M

Layer 2 protocol

Layer 3 protocol

Layer 4 protocol

Layer 5 protocol

Transmission media



but both ends of the connection must agree on which one being used. In

addition, the receiver must have some mechanism of telling the sender

which messages have been received correctly and which has not.

3. Flow control: If there is a fast sender at one end sending data to a slow

receiver, then there must be flow control mechanism to control the loss

of data by slow receivers. There are several mechanisms used for flow

control such as increasing buffer size at receivers, slow down the fast

sender, and so on. Some process will not be in position to accept

arbitrarily long messages. Then, there must be some mechanism to

disassembling, transmitting and then reassembling messages.

4. Multiplexing / demultiplexing: If the data has to be transmitted on

transmission media separately, it is inconvenient or expensive to setup

separate connection for each pair of communicating processes. So,

multiplexing is needed in the physical layer at sender end and

demultiplexing is need at the receiver end.

5. Routing: When data has to be transmitted from source to destination,

there may be multiple paths between them. An optimized (shortest)

route must be chosen. This decision is made on the basis of several

routing algorithms, which chooses optimized route to the destination.

1.4.3 Connection Oriented and Connectionless Services

Layers can offer two types of services namely connection oriented service

and connectionless service.

Connection oriented service: The service user first establishes a connection,

uses the connection and then releases the connection. Once the connection

is established between source and destination, the path is fixed. The data

transmission takes place through this path established. The order of the

messages sent will be same at the receiver end. Services are reliable and

there is no loss of data. Most of the time, reliable service provides

acknowledgement is an overhead and adds delay.

Connectionless Services: In this type of services, no connection is

established between source and destination. Here there is no fixed path.

Therefore, the messages must carry full destination address and each one

of these messages are sent independent of each other. Messages sent will

not be delivered at the destination in the same order. Thus, grouping and



ordering is required at the receiver end, and the services are not reliable.

There is no acknowledgement confirmation from the receiver. Unreliable

connectionless service is often called datagram service, which does not

return an acknowledgement to the sender. In some cases, establishing a

connection to send one short messages is needed. But reliability is required,

and then acknowledgement datagram service can be used for these

applications.

Another service is the request-reply service. In this type of service, the

sender transmits a single datagram containing a request from the client

side. Then at the other end, server reply will contain the answer. Request-

reply is commonly used to implement communication in the client-server

model.

1.4.4 Service Primitives

Primitives are operations. Service is specified as a set of primitives available

to a user process. The primitives for connection oriented service are

different from those of connectionless service.

There are five service primitives for connection service. They are:

1. LISTEN Block waiting for an incoming connection.

2. CONNECT Establish connection with peer on other side.

3. RECEIVE Block waiting for an incoming message.

4. SEND Send a message to the peer.

5. DISCONNECT Terminate a connection from the peer.



The above primitives are used for illustrating connection oriented services

interactions.

Figure 1.9: connection oriented services interactions

Self Assessment Questions: III

1 ________is a rule to communicate between the two machines.

2 A set of layers, interface and protocols can be considered as

___________________.

3 The service primitive to establish a connection between server and client

is ____________.

Server Client

LISTEN CONNECT

RECEIVE SEND

SEND

RECEIVE

DISCONNECT

DISCONNECT

To establish a connection

Acknowledgement is send

Connection established

Request for data

Send a reply

Request for terminate a connection

Acknowledging the connection and

releasing connection



Self Assessment Questions: IV


1. The main purpose of each layer is to give services to lower layer.

2. In connection oriented service, the source and destination have a fixed

path.

3. Unreliable connectionless service is often called datagram service.

4. Demultiplexing is needed in the physical layer at sender end and

multiplexing is need at the receiver end.

1.5 Summary

Computer networks can be used for numerous services, both for companies

and for individuals. For companies, networks of personal computers using

shared often provide to corporate information. For individuals, networks

offers access to a variety of information and entertainment resources. An

up-coming area in wireless networking with new application such as mobile

e-mail access and m-commerce. Networks can be divided into LANS,

MANs, WANs, and internetworking. LANs cover a building and operate at

high speeds. MANs cover a city. Example cable television system, which is

now used by many people to access the Internet. WANs cover a country or

continent. Wireless networks are becoming extremely popular, especially

wireless LANs. Networks can be interconnected to form internetworks.

Networks software consists of protocols, which are rules by which process

communicate. Protocols are either connectionless or connection-oriented.

Most networks support protocol hierarchies, with each layer providing

services to layer above it and insulating them from the details of the

protocols used in the lower layers and some of the design issues should be

considered in each layer specially to provide the error control and flow

control in transmission medium.


1. Why computer network is needed for people and companies?

2. Distinguish between connection-oriented and connectionless.

3. Explain the classification of networks?

4. Write a note on design issues of the layers?

5. List out the service the primitives?



1.7 Answers to SAQs and TQs:

SAQ I

1. Server

2. Electronic mail

3. Computer network

SAQ II

1. True

2. False

3. False

4. True

SAQ III

1. Protocol

2. Network architecture

3. Connect

SAQ IV

1. False

2. True

3. True

4. False

Answers to Terminal Questions:

1. Refer to Section 1.2

2. Refer to Section 1.4.3






Unit 2 Reference Models

Structure:

2.1 Introduction

Objectives

2.2 Reference Models

The OSI Reference Model

The TCP/IP Reference Model

Comparison of the OSI & the TCP/IP Reference Models

2.3 Example networks

The Internet

Ethernet

Wireless LANs 802:11

2.4 Network Standardization

Whos who in the telecommunication world?

Whos who in the standards world?

Whos who in the Internet standards world?

2.5 Summary



2.1 Introduction

In this chapter you will learn the two important architecture reference models

i.e. OSI Reference Model and The TCP/IP Reference model, the layer by

layer explanation and differences of these architectures and the various

examples network. The last we discuss about the standards which is

essential in creating and maintaining an open and competitive market for

equipment manufacturers and in guaranteeing national and international

interoperability of data and telecommunications technology and processes.

Standards provide guidelines to manufacturers, vendors, government

agencies, and other service providers to ensure the kind of interconnectivity

necessary in todays marketplace and in international communications.

Objectives:


(a) Describe the communication though reference models



(b) Explain Different types of networks

(c) Explain why standardization is required.

(d) Analyze which committees for which standards.

2.2 Reference Models

In this section we will discuss about two important network architectures:

1. The OSI reference model

2. The TCP/IP reference model.

2.2.1 The OSI Reference Model

The OSI model is based on a proposal developed by the International

Standards Organization as a first step towards international standardization

of the protocols used in the various layers. The model is called the ISO

(International Standard Organization Open Systems Interconnection)

Reference Model because it deals with connecting open systems that is,

systems that follow the standard are open for communication with other

systems, irrespective of a manufacturer.

Its main objectives were to:

Allow manufacturers of different systems to interconnect equipment

through a standard interfaces.

Allow software and hardware to integrate well and be portable on

different systems.

The OSI model has seven layers shown in Figure. The principles that were

applied to arrive at the seven layers are as follows:

1. Each layer should perform a well-defined function.

2. The function of each layer should be chosen with an eye toward defining

internationally standardized protocols.

3. The layer boundaries should be chosen to minimize the information flow

across the interfaces.

The set of rules for communication between entities in a layer is called

protocol for that layer.



Figure 2.1: The OSI Reference Model

The Physical Layer

The Physical layer coordinates the function required to carry a bit (0s and

1s) stream over a physical medium. It defines electrical and mechanical

specifications of cables, connectors and signaling options that physically link

two nodes on a network.

The Data Link Layer

The main task of the data link layer is to provide error free transmission. It

accomplishes this task by having the sender configure the input data into

Presentation

Physical Physical

Session

Transport

Network

Data Link

Network

Data Link

Application

Physical

Network

Data Link

Presentation

Physical

Session

Transport

Network

Data Link

Application

Communication subnet boundary

Transmission medium

(Coaxial cable, Fiber

optics etc.)

Application Layer Protocol

Presentation Layer Protocol

Session Layer Protocol

Transport Layer Protocol



data frames, transmit the frames sequentially, between network devices and

process the acknowledgement frames sent back by the intermediate

receiver. The data link layer creates and recognizes frame boundaries. This

can be accomplished by attaching special bit pattern to the beginning and

end of the frame. Since these bit patterns can accidentally occur in the data,

special care must be taken to make sure these patterns are not incorrectly

interpreted as frame boundaries.

The Network Layer

Whereas the data link layer is responsible for delivery on a hop, the network

layer ensures that each packet travels from its sources and destination

successfully and efficiently. A key design issue is determining how packets

are routed from source to destination. Routes can be based on static tables

that are wired into the network and rarely changed. They can also be

determined at the start of each conversation, for example, a terminal

session. Finally, they can be highly dynamic, being determined a new for

each packet, to reflect the current network load. When a packet has to travel

from one network to another to get its destination, many problems can arise.

The addressing used by the second network may be different from the first

one. The second network may not accept the packet at all because it is too

large. The protocols may differ, and so on. It is up to the network layer to

overcome all these problems to allow heterogeneous networks to be

interconnected.

The Transport Layer

The basic function of the transport layer is to accept data from the session

layer, split it up into smaller units if need be, pass these to the network layer,

and ensure that the pieces all arrive correctly at other end. Furthermore, all

this must be done efficiently, and in a way that isolates the upper layers

from the inevitable changes in the hardware technology. Transport layer

provides location and media independent end-to-end data transfer service to

session and upper layers.

The Session Layer

The session layer allows users on different machines to establish sessions

between them. A session allows ordinary data transport, as does the

transport layer, but it also provides enhanced services useful in some



applications. A session might by used to allow a user to log into a remote

timesharing systems or to transfer a file between two machines.

One of the services of the session layer is to manage dialogue control.

Sessions can allow traffic to go in both directions at the same time, or in

only one direction at a time. If traffic cans only way at a time (analogous to a

single railroad track), the session layer can help keep track of whose turn it

is.

A related session service is token management. For some protocols, it is

essential that both sides do not attempt the same operation at the same

time. To manage these activities, the session layer provides tokens that can

be exchanged. Only the side holding the token may perform the desired

operation.

Another session service is synchronization. Consider the problem that

might occur when trying to do a 2 hour file transfer between two machines

with an one hour mean time between crashes. After each transfer was

aborted, the whole transfer would have to start over again and would

probably fail again the next time as well. To eliminate this problem, the

session layer provides a way to insert markers after the appropriate

checkpoints.

The Presentation Layer

Unlike all the lower layers, which are just interested in moving bits reliably

from here to there, the presentation layer is concerned with the syntax and

semantics of the information transmitted.

A typical example of a presentation service is encoding data in standard

agreed upon way. Most user programs do not exchange random binary bit

strings, they exchange things such as peoples names, dates, amounts of

money and invoices. These items are represented as character strings,

integers, floating-point number, and data structures composed of several

simpler items. Different computers have different codes for representing

character strings (e.g., ASCII and Unicode), integers (e.g., ones

complement and twos complement), and so on. In order to make it possible

for computers with different representations to communicate, the data

structure to be exchanged can be defined in an abstract way, along with a

standard encoding to be used on the wire. The presentation layer



manages these abstract data structures and converts from the

representation used inside the computer to the network standard

representation and back.

The Application Layer

Application layer supports functions that control and supervise OSI

application processes such as start/maintain/stop application, allocate/

deallocate OSI resources, accounting, and check point and recovering. It

also supports remote job execution, file transfer protocol, message transfer

and virtual terminal.

2.2.2 The TCP/IP Reference Model

The TCP/IP model is quite different from the OSI model which is a

conceptual model. The TCP/IP network architecture is a set of protocols that

allow communication across multiple diverse networks. The architecture is

an outcome of research that had the original objective of transferring

packets across three different packets switched networks: the ARPANET

packet-switching network, a packet radio network, and a packet satellite

network. Due to military application, the research focused on robustness

and flexibility in operating over diverse networks and led to a set of protocols

that ate highly effective in having communications among the many different

types of computer systems and networks. Today, the internet has become

the primary fabric for interconnecting the worlds computers and the TCP/IP

is the main protocol for carrying information.

The TCP/IP network architecture consists of four layers. The application

layer provides services that can be used by other applications such as

remote login, e-mail, file transfer, and network management operations.

Figure 2.2: TCP/IP Network Architecture

Application Layer

Transport Layer

Internet Layer

Network Interface Layer



The application layer programs are intended to run directly over the

transport layer. Two basic types of services offered in the transport layer are

reliable connection oriented transfer of a byte stream, provided by the

Transmission Control Protocol (TCP) and best-effort connectionless

transfer of individual messages, provided by the User Datagram Protocol

(UDP). This service provides no mechanisms for error recovery or flow

control. UDP is used for applications that require quick without guaranteeing

reliable delivery. The TCP/IP model does not require strict layering which is

not there in the OSI model. The application layer may run directly over the

Internet layer.

The Internet layer is similar to the part of the OSI network layer that is

concerned with the transfer of packets between machines that are

connected to different networks through the use of gateways and routers. It

must therefore deal with the routing of packets across these networks as

well as with the control of congestion. The Internet layer also defines

globally unique addresses for machines that are attached to the Internet.

Internet Packet (IP) is a main protocol at this layer which provides a single-

service, namely, best-effort connectionless packet transfer. IP packets are

exchanged between routers without a connection setup; the packets are

routed independently, and so they may traverse different paths for the same

destination. For this reason, IP packets are also called datagrams. The

connectionless approach makes the system robust; which means that in the

case of failures, there is no need to set up the connections. The gateways

that interconnect the intermediate networks may discard packets when

congestion occurs. The responsibility for recovery from these losses done

by the transport layer.

Finally, the network interface layer is concerned with the network-specific

aspects of the transfer of packets. As such, it must deal with parts

equivalent to OSI network layer and data link layer. Various interfaces are

available for connecting end computer systems to specific networks such as

x.25, frame relay, Ethernet, and token ring.

The network interface layer is particularly concerned with the protocols that

access the intermediate networks. At each gateway (the term is described in

the second block) the network access protocol encapsulates the IP packet

into a packet of the underlying network or link. The IP packet is recovered at



the exit gateway of the given network. This gateway must then encapsulate

IP packet into new packet of the type of the next network or links. This layer

provides a clear separation of the internet layer from the technology-

dependent network interface layer. This approach also allows the internet

layer to provide a data transfer service that is transparent in the sense of not

depending on the details of the underlying networks.

The figure shows some of the protocols of the TCP/IP protocol suite. It is

shown in the figure, two application layer protocols namely, HTTP and

SMTP operating over TCP whereas two other protocols DNS and Real-Time

Protocol (RTP) operate over UDP. The transport layer protocols TCP and

UDP, on the other hand, operate over IP. Many networks interfaces are

defined to support IP. The salient feature of figure is that all higher-layer

protocols access over multiple networks. The basic IP protocol is

complemented by many additional protocols (ICMP, IGMP, ARP, and

RARP) that are required to operate an internet.

Figure 2.3: The TCP/IP Protocol suite

The operations of the single IP protocol over various networks provide

independence from the underlying network technologies. The

communication services of TCP and UDP provide a network independent

platform on which applications can be developed. By allowing multiple

network technologies to coexist, the internet is able to provide wide

connectivity and to achieve economies of scale.

HTTP SMTP DNS RTP

TCP UDP

IP

Network

Interface 1

Application Layer

Network

Interface 1

Network

Interface 1

Transport Layer

Network Layer



2.2.3 Comparison of the OSI & the TCP/IP Reference Models

OSI Reference Model TCP/IP Reference Model

1. Seven layers

2. It distinguishes between service,

interface, and protocol.

3. Firstly description of model and

protocol came next.

4. Both have Network.

5. Supports connectionless and

connection oriented

communication in network layer

and only connection-oriented

communication in transport layer.

6. Protocol in OSI model are better

hidden and can be replaced

relatively easily (No Transparency).

1. Four layer

2. Does not clearly distinguish between

service, interface and protocol.

3. Protocol comes first and description

of model later.

4. Transport and Application layer.

5. TCP/IP has only one mode in

Network layer (connection less) but

supports both modes in Transport

layer.

6. Protocols in TCP/IP are not hidden

and thus cannot be replaced easily

(Transparency).


1. There are seven layers in OSI model but in TCP/IP ___________ layers.

2. The _____________ layer will do the synchronization.

3. The main task of the ____________ layer is to provide error free

transmission.

2.3 Example Networks

The computer network covers many kinds of networks, large and small.

Each of them has different goals, scales, size and technologies.

2.3.1 The Internet

Internet is collection of different types of networks. The interconnection of

variety of network with different domain, size, scalability and technology. In

the following section more about history, development and popularity of

internet are discussed.

ARPANET (Advanced Research Project Agency Network)

The main reason behind this development of ARPANET is security. Military

communication used public telephone network which was more insecure.

Anybody could easily hack these networks. Then they created a single

defense research organization, ARPA, the advanced research projects



agency. ARPANET consists of subnet with mini computers called IMPs

(Inter Message Processors) connected by 56 Kbps leased line.

For high reliability, each IMP would be connected to at least two other IMPs.

The subnet was developed to support datagram subnet. If some lines and

IMPs were destroyed, messages could be still automatically rerouted along

alternative path.

Figure 2.4: The original ARPANET design

The software was split into two parts: subnet and host. The subnet software

consists of the Host-IMP protocol, IMP connection, the IMP-IMP protocol

and a source IMP to destination IMP protocol is designed to improve

reliability.

NSFNET (National Science Foundation Network)

NSF was developed to design a successor to the ARPANET that would be

open to all university research groups. NSF decided to build a backbone

network to connect its six supercomputers centers. Each super computer

was given with microcomputer called fuzz ball. The fuzz balls form the

subnet, using the same hardware technology as that of ARPANET used.

Subnet

Host IMP protocol

Hosts



NSF funded some regional networks that connected to the backbone to

allow users at thousands of universities, research labs, libraries and

museum to access any resources and to communicate with one another.

The complete network i.e., backbone and regional forms a NSFNET.

INTERNET Usage:

The number of networks, machines and users connected to ARPANET grew

rapidly after TCP/IP became the official protocol. When NSFNET and the

ARPANET interconnected, the growth becomes exponential. Internet can be

viewed as collection of networks and it is the TCP/IP model and TCP/IP

protocol stack keeping together. Internet can be defined as, any machine

run on TCP/IP protocol stack, has an IP address, and can send IP packets

to all other machines on the internet. Internet and its predecessor

(ARPANET and NSFNET) had 4 main applications: E-mail, News, Remote

Login, and File Transfer.

Architecture of the Internet:

To explain the architecture of the internet, let us consider the figure,

overview of the

internet.

Figure 2.5: Overview of the Internet

Router

Server



Here, client will try to connect ISP (Internet Service Provider) over a dial-up

telephone line as shown in figure. The built in modem within the PC

converts digital signals to analog signals and passes over the telephone

system. These signals are transmitted to ISPs POP (Point of Presence),

where they are removed from telephone line and given to ISP and regional

network consists of interconnected routers in various cities through ISP

servers. If the packet is destined for a host served directly by the ISP, the

packet is delivered to the host. Otherwise, it is handed over to the ISPs

backbone operator. If packets are allowed to hop between back bones, all

the major backbones connect at NAP (Network Access Point). NAP is a

room full of routers, at least one per backbone. A LAN in the room connects

all the routers, so that packet can be forwarded from any backbone to any

other backbone.

Connection-Oriented Networks

In this section, different types of connection oriented networks are explained

X.25 and Frame Relay

X.25 was the first public data networks. X.25 uses computers to establish

connection to the remote computer. This connection will be identified by a

connection number, which is used to transfer data packets. Each data

packet contains 3 byte header and up to 128 bytes of data.

Figure 2.6: X.25 Packet Number Format

These X.25 network were largely replaced by a new kind of network called

frame relay. Frame relay does not support flow control and error control.

Packets are delivered in order, because frame relay is a connection oriented

network. Frame relay is used in wide area LAN; it is used for interconnecting

LANs at different locations.

Asynchronous Transfer Mode (ATM)

ATM was formed by two standard committees. The ATM Forum (ATM 2002)

and International Telecommunication Union (ITU 2002) used for broadband

digital service networks. ATM switches forwarded data at very high rates

and are deployed in internet backbone networks.

Connection Number

Packet Reg. No. + Acknowledgment No.



ATM Virtual Circuits

Connection oriented network sends data required for first connection setup.

Virtual circuits are analogous connections, reassembles physical circuits

used within the telephone system. Most ATM networks support permanent

virtual circuits (PVC), which support permanent connections between two

distant hosts. Each connection has a unique connection identifier.

Figure 2.7: Virtual Circuit Path

First connection path is established between distinct host. After establishing

the connection, data will be transmitted on the virtual circuit path. The basic

idea behind ATM is to transmit all data in small fixed size packets called

cells. ATM cell consists of 53 bytes of which 5 bytes are header and 48

bytes are payload.

Figure 2.8: an ATM Cell Format

Part of ATM cell header is used for connection identifier. This helps sending

and receiving host and all the intermediate routers can identify which cell

belongs to which connection. This information also helps to identify route for

each incoming cell. Cell routing is done in hardware, at high speed.

ATM hardware can be set up to copy one incoming cell to multiple output

lines. Due to small cells, these wont block any line for very long time, and

also improves QOS (quality of service). ATM provides speed between

155Mbps and 622 Mbps or even more.

The ATM Reference Model

1. ATM reference model consists of three layers:

2. Physical Layers

Header User data (payload)



3. ATM Layers

4. TM adaptation Layers (AAL)

Figure 2.9: ATM Reference Model

Physical layers of ATM deals with physical medium such as voltage, bit

timing and other related issues. ATM is designed to be independent of the

transmission medium.

ATM layer deals with cells and cell transport. It defines the layout of a cell

and what the header field contains. It also deals with establishment and

releases of virtual circuits. Congestion control is also handled here.

ATM Adaptation Layer is also used to send packets larger than a cell. An

ATM interface segment into packets transmits the cells individually and

reassembles them at the other end.

ATM model is defined as three dimensional, user plane dealing with data

transport, flow control, error correction and other user functions. Control

plane deals with connection management.

The physical and AAL layers are each divided into two sublayers. The

physical layer has PMD (Physical Medium Dependent) and TC

(Transmission Convergence). The PMD sublayer have interface to the

actual cable. It moves the bits and also handles the bit timing.

CS: Convergence Sub layer SAR: Segmentation and Reassembly sub-layer TC: Transmission Convergence sub-layer PMD: Physical Medium

Dependent sub-layer



The other sublayer in physical layer is TC sub-layer. When cells are

transmitted, the TC sub-layer sends them as a sting of bits to the PMD

layer.

At the other end (receiver), the TC sub-layer gets a pure incoming bit stream

from the PMD sub-layer. Then it converts bit stream into cells stream and

gives them to ATM layer. TC handles all the issues related to telling where

cells begin and end in the bit stream.

The AAL Layer is split into SAR (Segmentation and Reassembly) sublayer

and CS (Convergence Sub layer). SAR breaks up packets into cells on the

transmission side and putts them back together again at the destination. CS

makes it possible to have ATM system giving different kinds of service to

different applications.

2.3.2 Ethernet

Ether (co-axial) network was formed with a thick coaxial cable upto 2.5 km

long ( with repeaters every 500 meters). The network can be formed upto

256 machines with transceiver screwed on the cable. A cable with multiple

machines attached parallel to it is called multidrop cable. The network speed

will be 2.94 Mbps. Intel, DFC and Xerox developed standard for 10 Mbps

Ethernet called DIX standard. DIX standard with two minor changes become

the IEEE 802.3 standard.

Figure 2.10: Ethernet

New version of Ethernet gives speed upto 10 Mbps, 1000 Mbps and still

higher. The cabling and switches have been improved. Additional features

are also added.



2.3.3 Wireless LANs 802:11

When there is question of wireless, the problem was with compatible.

Because the computer equipped with a brand X radio would not work in a

room equipped with a brand Y base station. Finally, IEEE committee

standardized wireless LAN, named as 802.11. A common slag name used is

WiFi. The 802.11 standard had to work in two modes:

1. In the presence of base station (with infrastructure support).

2. In the absence of base station (infrastructureless, adhoc).

In the first mode, all communication will take place through the base station.

There exist problem of finding a suitable frequency band for wireless. This is

avoided using radio signals which have a finite range.

Figure 2.11: Wireless LANs 802.11 Standard

The main problem in WLAN is multipath fading. This is due to improved

coverage of radio range. The radio signal can be reflected off solid objects,

so that it may be received multiple times.

Another problem is with the software available for mobile devices. WLAN

supports 1 Mbps or 2 Mbps data rate. Now, there are new standards which

provide speed up to 54 Mbps. We have 802.11x different standard which

uses different modulation technique.



Standard (802.11x) Data Rate

1. 802.11 a 54 Mbps uses wider frequency band

2. 802.11 b 11 Mbps uses same frequency band as 802.11 uses

different modulation scheme.

3. 802.11 g Uses modulation technique of 802.11a, but frequency

band of 802.11 b.

Self Assessment Questions: II


1. The TCP/IP consists of five layers.

2. Token management comes in the presentation layer.

3. IMP means Internet mean Processor.

4. The main problem in WLAN is multipath fading.

2.4 Network Standardization

In the world there are many network vendors and suppliers with there own

ideas of how things should be done. All of them should have coordination,

otherwise they will no use for the users and every thing will become chaos.

and also increase the market for products adhering to the standard. A large

market leads to mass production, economies of scale in manufacturing.

Standards fall into two categories: (1) de facto and (2) de jure.

De facto in Latin (from the fact) standards that those just happened,

without any formal plan. Example: The IBM PC and its successors are de

facto standards for small-office and home computers. UNIX is the de facto

operating systems in university computer science department.

De jure in Latin (by law) standards, in contrast, are formal, legal standards

adopted by some authorized standardization body. International

standardization authorities are generally divided into two classes:

1. Treaty organization national governments.

2. Nontreaty organization voluntary of different companies.

2.4.1 Whos who in the telecommunication world?

In the worlds telephone companies varies from country to country. In United

States only there were 1500 separate privately owned telephone

companies. But AT&T spread though out geographical area and has 80



percent of Americas telephones. The companies are in different services,

merging and breakup, so the industry is in a constant state of flux.

Companies in the United States that provide communication services to the

public are called common carriers. Their offerings and prices are described

by a document called a tariff, approved by the Federal Communications

Commission for the interstate and international traffic and by the state public

utilities commissions for interstate.

The national government in a country has a complete monopoly on all

communication, including the mail, telegraph, telephone, radio and

television. Most of the countries fall in this category. In other countries there

is a branch of the government known as the PTT Post, Telegraph &

Telephone administration.

In the worldwide, there is a trend towards liberalization, competition and

compatibility to ensure that people in one country can call their counterparts

in another one. In 1865, representatives from many European governments

met to form ITU International Telecommunication Union. ITU job was

standardizing international telecommunication. In 1947, ITU became the

agency of the United Nations.

ITU has three main sectors:

1. Radiocommunications Sector (ITU R).

2. Telecommunications Standardization Sector (ITU T).

3. Development Sector (ITU D).

ITU T has four classes of members:

1. National governments: ITU T has about 200 governmental member

of the United Nations.

2. Sector members: There are approximately 500 sectors members.

Some of them are telephone companies (AT&T, Vodafone, WorldCom),

telecom equipment manufacturers (Cisco, Nokia, Nortel), computer

vendors (Compaq, Sun, Toshiba), chip manufacturers (Intel, Motorola,

TI), media companies (AOL Time Warner, CBS, Sony), and other

interested companies (Boeing, Samsung, Xerox). Various nonprofit

organization and industry are also sector members.

3. Associate members: Are small organizations that are interested in a

particular Study Group.



4. Regulatory agencies: Are the folks who watch over the telecom

business, such as the U.S. Federal Communications Commission.

2.4.2 Whos who in the standards world

A voluntary nontreaty organization founded in 1946 called ISO (International

Standards Organization) will produce and publish the International

Standards with national standards organizations of different countries. ISO

issues standards on a truly vast number of subjects. Over 13,000 standards

have been issued, including the OSI standards. ISO has almost 200

Technical Committees, numbered in the order of their creation, each dealing

with a specific subject. TC1 deals with the nuts and bolts standardizing

screw thread pitches. TC97 deals with computers and information

processing. Each TC has subcommittees (SCs) divided into working groups

(WGs). The real work is done largely in the WGs by over 100,000

volunteers world wide. Academic experts also are active in many of the

WGs.

The process begins when one of the national standards organizations feels

the need for an international standard in some area. A working group is then

formed to come up with a CD (Committee Draft). The CD is then circulated

to all the member bodies, which get 6 months to criticize it. If a substantial

majority approves, a revised document, called a DIS (Draft International

Standards) is produced and circulated for comments and voting. Based on

the results of this round, the final text of the IS (International Standard) is

prepared, approved, and published. The other members are ANSI

(American National Standards Institute), NIST (National Institute of

Standards and Technology), and IEEE (Institute of Electrical and

Electronics Engineers).

2.4.3 Whos who in the Internet standards world?

When the ARPANET was set up, DOD created an informal committee to

oversee it. In 1983, the committee was renamed the IAB (Internet Activities

Board) to keep the researchers involved with the ARPANET and the

Internet. The meaning of acronym IAB was later changed to Internet

Architecture Board. Each of the approximately ten members of the IAB

headed a task force on some issue of importance. The IAB met several

times a year to discuss results and to give feedback to the DoD and NSF.

Communication was done by a series of technical reports called RFCs



(Request for Comments). RFCs are stored on-line and can be fetched by

anyone interested in them from www.ietf.org/rfc. They are numbered in

chronological order of creation.

In 1989, the IAB reorganized and had two subsidiaries i.e. IRTF (Internet

Research Task Force) and IETF (Internet Engineering Task Force). The

idea of this spilt was to have the IRTF concentrate on long-term research

while the IETF dealt with short-term engineering issues. The IETF was

divided into working groups, each with a specific problem to solve. The

chairmen of these working groups initially met as a steering committee to

direct the engineering effort. The working group topics include new

application, user information, OSI integration, routing and addressing,

security, network management and standards.

Self Assessment Questions: III

1 ________ allow different computer to communication.

2 Data communication standards fall into ______ and _____ categories.

3 _________ is produced and circulated for comments and voting.

2.5 Summary

The International Standards Organization created a model called the Open

Systems Interconnections, which allows diverse systems to communicate.

The seven-layer OSI model provides guidelines for the development of

universally compatible networking protocols. The physical, data link and

network layers are the network support layers. The Session, presentation,

and application layers are the user support layers. The transport layer links

the network support layers and the user support layers. The TCP/IP

application layer is equivalent to the combined session, presentation, and

application layers of the OSI model. Well-known networks include the

Internet, ATM networks, Ethernet, and the IEEE 802.11 wireless LAN. The

Internet evolved from the ARPANET, to which other networks were added of

many thousands of networks, rather than a single network. Standards are

necessary to ensure that products from different manufacturers can work

together as expected. The ISO, TU-T, ANSI, IEEE, and EIA are some of the

organization involved in standards creation. A Request for Comment is an

idea or concept that is a precursor to an Internet standard.




1. With neat diagram, explain the OSI reference model.

2. Distinguish between TCP/IP and OSI.

3. Explain ATM reference model with a neat diagram.

4. Briefly explain about ISO.

5. Write short notes on Ethernet.


SAQ I

1. Four

2. Session

3. Data link

SAQ II

1. False

2. False

3. False

4. True

SAQ III

1. Standards

2. De facto, de jure

3. Draft International Standards

Answers to Terminal Questions:








Unit 3 Data Communication

Structure:

3.1 Introduction

Objectives

3.2 Theoretical basis for Communication

Fourier analysis

Band limited signals

Maximum data rate of a channel

3.3 Data Transmission Modes

Serial and Parallel

Simplex, Half duplex and Full duplex

Synchronous and Asynchronous transmission

3.4 Switching

Circuit switching

Message switching

Packet switching

Comparison of switching techniques

3.5 Multiplexing

Frequency division multiplexing [FDM]

Wavelength division multiplexing [WDM]

Time division multiplexing [TDM}

3.6 Summary



3.1 Introduction

We will begin with a theoretical analysis of data transmission, with Fourier

analysis; define the bandwidth, frequency and how to find the harmonics

though the frequency and other parameters. The maximum data rate of a

channel can be send from one machine to the other. Different types of

transmission modes like serial and parallel, where data can be passed

continuously from one end to the other. A connection that allows traffic only

one way is called simplex, and either way but only at a time half-duplex,

simultaneously called full-duplex. Data transmission can be done through

synchronous and asynchronous with different types of switching and their

comparisons, why we need multiplexing and different types of multiplexing.



Objectives:


(a) Define the bandwidth and data rate of a channel.

(b) Different types of transmission modes.

(c) Switching techniques and their comparison.

(d) Define multiplexing and discuss the various types of multiplexing.

3.2 Theoretical basis for Communication

Information can be transmitted on wires by varying some physical property

such as voltage or current. By representing the value of this voltage or

current as a single-valued function of time, f(t). We can model the behavior

of the signal and analyze it mathematically.

3.2.1 Fourier analysis

The French mathematician Jean-Baptiste Fourier proved that any

reasonably behaved periodic function, g(t) with period T can be constructed

as the sum of a number of sines and cosines:

g (t) = c + an sin(2 nft) + bn cos (2 nft) (1) n=1 n=1

Where f = 1/T is the fundamental frequency, an and bn are the sine and

cosine amplitudes of the nth harmonics (terms), and c is a constant. Such

decomposition is called a Fourier series. From the Fourier series, the

function can be reconstructed; that is, if the period, T, is known and

amplitudes are given, the original function of time can be found by

performing the sums of Eq. (1)

A data signal that has a finite duration (which all of them do) can be handled

by just imagining that it repeats the entire pattern over and over forever (i.e.,

the interval from T to 2T is the same as from 0 to T, etc.).

The an amplitudes can be computed for any given g (t) by multiplying both

sides of Eq. (1) by sin(2kft) and then integrating from 0 to T. since

T

0

Only one term of the summation survives: an. The bn summation vanishes

completely. Similarly, by multiplying Eq. (3.1) by cos (2 kft) and integrating



between 0 and T, we can derive bn. By just integrating both sides of the

equation as it stands, we can find c. the results of performing these

operations are as follows:

T T T

an =2/T g(t) sin(2nft) dt bn =2/T g(t) cos(2nft) dt c = 2/T g(t) dt 0 0 0

3.2.2 Band limited signals

To see what all this has to do with data communication, let us consider a

specific example: the transmission of the ASCII character b encoded in an

8-bit byte. The bit pattern that is to be transmitted is 01100010. The

following figure shows the voltage output by the transmitting computer.

Figure 3.1: A binary signal and its root-mean-square Fourier amplitudes

The root-mean-square amplitudes, a2n + b2n, for the first few terms are

shown in the figure. These values are of interest because their squares are

proportional to the energy transmitted at the corresponding frequency.

The Fourier analysis of this signal yields the coefficients:

an = 1/n [cos(n/4) cos(3n/4) + cos(6n/4) cos(7n/4)]

bn = 1/n [sin(3n/4) sin(n/4) + sin(7n/4) sin(6n/4)]

c =

No transmission facility can transmit signals without losing some power in

the process. If all the Fourier components were equally diminished, the

resulting signal would be reduced in amplitude but not distorted.

Unfortunately, all transmission facilities diminish different Fourier

components by different amounts, thus introducing distortion. The amplitude

are transmitted undiminished from 0 up to some frequency fc [measured in

0.50

0.25

Harmonic number T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0

1

0 1 1 0 0 0 1 0

Time



cycles/sec or Hertz (Hz)] with all frequencies above this cutoff frequency

attenuated. The range of frequencies transmitted without being strongly

attenuated is called bandwidth.

The bandwidth is a physical property of the transmission medium and

usually depends on the construction, thickness, and length of the medium.

In some cases a filter is introduced into the circuit to limit the amount of

bandwidth available to each customer. Example for short distance telephone

wires have 1 MHz, but they add filters to limit each customer for 3100 Hz for

intelligible speech and improves system-wide efficiency.

Given a bit rate of b bits/sec, the time required to send 8 bits i.e., 1 bit at a

time is 8/b sec, so the frequency of the first harmonic is b/8 Hz. An ordinary

telephone line, often called a voice-grade line, has an artificially cut-off

frequency just above 3000 Hz. This restriction means that the number of the

highest harmonic passed through is roughly 3000/(b/8) or 24,000/b.

Bps T(msec) First harmonic (Hz) # Harmonic sent

300 26.67 37.5 80

600 13.33 75 40

1200 6.67 150 20

2400 3.33 300 10

4800 1.67 600 5

9600 0.83 1200 2

19200 0.42 2400 1

38400 0.21 4800 0

Figure 3.2: Relation between data rate and harmonics

For some data rates, the numbers work out as shown in above table. From

these numbers, it is clear that trying to send at 9600 bps over a voice-grade

telephone line will transform accurate reception of the original binary bit. It

should be obvious that at data rates much higher than 38.4 kbps, there is no

hope at all for binary signals, even if the transmission facility is completely

noiseless. In other words, limited the bandwidth limits the data rate, even for

perfect channels. However, sophisticated coding schemes that make use of

several voltage levels do exist and can achieve higher data rates



3.2.3 Maximum data rate of a channel

Two theoretical formulas were developed to calculate the data rate: one by

Nyquist for a perfect channel (noiseless) has a finite transmission capacity,

another by Shannon for a random noisy channel.

For a noiseless channel, the Nyquist bit rate formula defines the theoretical

maximum bit rate.

Maximum bit rate = 2H log2 V bits/sec

In this formula, bandwidth H is the bandwidth of a channel, V is the number

of signal levels used to represent data. According to the formula, we might

think that, given a specific bandwidth, we can have any bit rate we want by

increasing the number of signal levels. Although the idea is theoretically

correct, practically there is a limit. When we increase the number of signal

levels, we impose a burden on the receiver. If the number of levels in signal

if just 2, the receiver can easily distinguish between a 0 and 1. if the level of

a signal is 64, the receiver must be very supplicated to distinguish between

64 different levels. In other words, increasing the levels of a signal reduces

the reliability of the system.

In reality, we cannot have a noiseless channel; the channel is always noisy.

And there is always random thermal noise present due to the motion of the

molecules in the system. The amount of thermal noise present is measured

by the ratio of the signal power to the noise power, called the signal-to-noise

ratio. If we denote the signal power by S and the noise power by N, the

signal-to-noise ratio is S/N. These units are called decibels (dB).

So the Shannon maximum data rate channel is given by

Maximum number of bits/sec = H log2 (1 + S/N)

Note that in the Shannon formula there is no indication of the signal level,

which means that no matter how many levels we have, we cannot achieve a

data rate higher than the capacity of the channel. In other words, the

formula defines a characteristic of the channel, no the method of

transmission.


1. For noiseless channel,_______________ formula defines the maximum

data rate of channel



2. The range of frequencies transmitted without being strongly attenuated

is called _____________.

3. The signal-to-noise ratio measured in __________.

3.3 Data Transmission Modes

The transmission of binary data across a link can be accomplished in either

parallel or serial mode. In parallel mode, multiple bits are sent with each

clock tick. In serial mode, 1 bit is sent with each clock tick. While there is

one way to send parallel data, there are three subclasses of serial

transmission: asynchronous, synchronous, and isochronous.

Figure 3.3: Data transmission and modes

3.3.1 Serial and Parallel

Serial Transmission

In serial transmission one bit follows another, so we need only one

communication channel rather than n to transmit data between two

communicating devices.

The advantage of serial over parallel transmission is that with only one

communication channel, serial transmission reduces cost of transmission

over parallel by roughly a factor of n.

Since communication within devices is parallel, conversion devices are

required at the interface between the sender and the line (parallel-to-serial)

and between the line and the receiver (serial-to-parallel). Serial transmission

occurs in one of three ways: asynchronous, synchronous, and isochronous.

Data transmission

Parallel Serial

Asynchronous Synchronous Isochronous



Parallel Transmission

Binary data, consisting of 1 s and 0 s, may be organized into groups of n

bits each. Computers produce and consume data in groups of bits much as

we conceive of and use spoken language in the form of words rather than

letters. By grouping, we can send data n bits at a time instead of 1. This is

called parallel transmission.

The mechanism for parallel transmission is a simple one: Use n wires to

send n bits at one time. That way each bit has its own wire, and all n bits of

one group can be transmitted with each clock tick from one device to

another.

The advantage of parallel transmission is speed. All else being equal,

parallel transmission can increase the transfer speed by a factor on n over

serial transmission.

But there is a significant disadvantage: cost. Parallel transmission requires n

communication lines just to transmit the data stream. Because this is

expensive, parallel transmission is usually limited to short distances.

3.3.2 Simplex, Half duplex and Full duplex

There are three modes of data transmission that correspond to the three

types of circuits available. These are:

a) Simplex

b) Half-duplex

c) Full-duplex



Figure 3.4: Different Modes of Data Transmission

Simplex

Simplex communications imply a simple method of communicating, which

they are. In simplex communication mode, there is a one-way

communication transmission. Television transmission is a good example of

simplex communications. The main transmitter sends out a signal

(broadcast), but it does not expect a reply as the receiving units cannot

issue a reply back to the transmitter. A data collection terminal on a factory

floor or a line printer (receive only). Another example of simplex

communication is a keyboard attached to a computer because the keyboard

can only send data to the comput

Documents

MI0026