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MI 0026 Computer Networks (MBA – IS) Contents BKID – B1041 27 th June 2009 Revised Edition: Fall 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) SUBJECT INTRODUCTION Sikkim Manipal University Page No. 1 1.1 Introduction Computer Networks Unit 1
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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.
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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.
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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.
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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.
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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
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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
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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)
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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.
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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
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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.
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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.
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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
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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
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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
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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
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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.
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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
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Self Assessment Questions: IV
State whether the following statements are True or False:
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.
1.6 Terminal Questions
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?
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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
3. Refer to Section 1.3.1
4. Refer to Section 1.4.2
5. Refer to Section 1.4.4
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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
2.6 Terminal Questions
2.7 Answers to SAQs and TQs
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:
After studying this unit, you will be able to:
(a) Describe the communication though reference models
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(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.
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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
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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
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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
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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
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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
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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
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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).
Self Assessment Questions: I
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
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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
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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
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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.
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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)
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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
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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.
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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.
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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
State whether the following statements are True or False:
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
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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.
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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
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(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.
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2.6 Terminal Questions
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.
2.7 Answers to SAQs and TQs
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:
1. Refer to Section 1.2.1
2. Refer to Section 1.2.3
3. Refer to Section 1.3.2
4. Refer to Section 1.4.2
5. Refer to Section 1.3.3
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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
3.7 Terminal Questions
3.8 Answers to SAQs and TQs
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.
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Objectives:
After studying this unit, you will be able to:
(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
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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
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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
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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.
Self Assessment Questions: I
1. For noiseless channel,_______________ formula defines the maximum
data rate of channel
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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
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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
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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