MC0075 (A)-Unit-01

Computer Networks Unit 1

Sikkim Manipal University Page No. 1

Unit 1 Introduction To Computer Networks

Structure

1.1 Introduction

Objective

1.2 Definition of a Computer Network

1.2.1 What is a Network?

1.2.2 Components of a computer network

Self Assessment Questions

1.3 Use of Computer networks

1.3.1 Networks for companies

1.3.2 Networks for people

1.3.3 Social Issues


1.4 Classification of networks

1.4.1 Based on transmission technology

1.4.2 Based on the their scale

1.4.3 Local area networks

1.4.4 Metropolitan area networks

1.4.5 Wide area networks

1.4.6 Wireless networks


1.5 Summary

1.6 Terminal Questions

1.7 Answer to Self Assessment Questions

1.8 Answer to Terminal questions

1.1 Introduction

The 18th century was the time of the great mechanical systems

accompanying the industrial revolution. The 19th century was the age of the



steam engine. During 20th century, the key technology is been gathering,

processing, and distribution.

The merging of computers and communication has had a profound influence

on the way the computer systems are organized. The old model of a single

computer serving all of the organization‟s computational needs has been

replaced by one in which a large number of separate but interconnected

computers do the job. These systems are called computer networks. This

course deals with the design, organization and application of these

networks.

Objectives

By the end of Unit 1, the learners should be able to:

1. Explain what is computer network?

2. Discuss the Uses of computer networks

A) For business or company‟s

B) For common people

3. Discuss the classification of networks

1.2 Definition of a Computer Network

A network is a group of various components connected together in such a

way that, it is possible to distribute or collect information over the entire

group.

1.2.1 What is a Network?

The term “Computer Network” is used to mean an interconnected collection

of autonomous computers. Two computers are said to be interconnected, if

they are able to exchange information. The connection can be wired or

wireless. A large computer with remote printers and terminals is not a

network. Nor a system with one control and many slaves is a network. With

a network the user must explicitly log into a machine, explicitly submit jobs



remotely, explicitly move files around and generally handle all network

management personally.

Distribution or exchange of information is very important for any group

activity to work or function efficiently and effectively. This is best achieved

by networking. Computer network have become an important tool of

communication, with an added advantage of being able to store large

amount of data and also being able to deliver it as and when demanded. A

computer network finds application in almost every field of life.

A computer network can do many things, which are listed below, that a

stand-alone machine cannot do:

1. Share a single copy of a file among all the users, even simultaneously.

2. Transfer files from computer to computer or from one computer to the

central hard disk on the server.

3. Share applications over the entire network, thus reducing the cost of

purchasing the multiple copies. A multi-user license costs much less

than buying a stand alone license.

4. Share resources like printers, hard disks, CD-ROM drives, modems, fax

machines etc.

5. Use e-mail for internal communications, inert office notes etc., creating

an almost paper less office.

1.2.2 Components of a computer network

A network is a combination of different components. The most essential

components of a simple network are listed below:

Servers: Servers are faster computers that run various software‟s, store

and process information and also provide a human interface for the

users to be able to use the networked computers.

Nodes: Nodes are the computers on the network, which are provided to

the users to carry out their tasks using the network.



Workstation: A node, which is more powerful, and can handle local

information processing or graphics processing is called a workstation.

The workstation works only for the person sitting in front of it, where as a

server serves all the people on the network to share its resources. A

workstation usually has an inexpensive, small hard disk to carry out local

tasks. Some workstations, called diskless workstations, have no disk

drive of their own. Such workstations also called dumb terminals and

they rely completely on the LAN for their access. The network operating

system lets the nodes work as if all the resources at the server belong to

the node itself.

Network Operating System (NOS): The network requires some

software to control all the information transfer activity on the network,

like the traffic police to control the traffic. The software called NOS

handles these tasks. Networks, which are more complex, require

network devices like hubs, switches & routers to carry out different

network functions.

LAN Software: On the network, each computer is called a node or a

workstation unless there are certain computers designed as servers.

LAN cables connect all the nodes and servers together to form the

network. In addition to its local disk operating system, each node

requires networking software that enable the nodes to communicate with

the servers. In return, the file servers run network software that

communicates with the nodes.

LAN Cable: This is the medium or channel over which the information

travels from computer to computer. The information travels from one

computer onto the medium and then from the medium to another

computer in the form that it can be read.



Network Interface Card: Each computer contains a network interface

card. This card is used to connect the cables to the computers. These

cards help the computer to transfer the data at a faster rate and in the

form of packets. These cards are plugged into the computer

motherboard. These cards are generally called as Ethernet cards.


1. Define the term computer network

2. List the tasks a computer network can do and stand alone machine

cannot

3. List the Basic essential components of a computer network

4. NOS is a software or hardware?

1.3 Use of Computer networks

It is very much important to see why people are interested in computer

networks, in what way it is helpful.

1.3.1 Networks for companies

Many organizations have a number of computers in operations often located

far apart. For example, a company with many branches may have computer

at each location to keep track of inventories, monitor productivity, and do the

payroll. Initially these branches work in isolation with each others, but at

some point of time, the management decides to connect them to extract and

correlate information about the entire company for the benefit of the

company.

A basic diagram is given in figure 1.1, where two or more computers are

connected with the server through the network for business application.

Error!



Figure 1.1: Network with two client and one server

Benefit for the company or organization may be due to many factors like

listed in the following section.

1. Resource sharing

The goal is to make all programs, equipments, especially data available

to anyone on the network without regard to the physical location of the

resource and the user. It is an attempt to end the “tyranny of

geography”. For example: Printers, scanners which are expensive

resources can be shared among many computers. That is printer is

installed on a single machine which is networked and used by all

machine connected to that network.

2. High reliability

A second goal is to have alternative sources of supply. For example: All

files could be replicated on two or three machines, so if one of them is

unavailable due to say hardware failures, the other copies could be

used. Also many CPU‟s means if one goes down the others may be able

to can take over its work, although at reduced performance. It finds



extensively useful in military, banking, air traffic control, nuclear reactor

safety, and many other applications, where the ability to continue

operating in the face of hardware problems is of utmost importance.

3. Saving money

Small computers have mush better price/performance ratio than larger

ones. Main frames are roughly a factor of ten times faster than

personnel computers, but their cost is thousand times more than

personnel computers. This is the fact that many system designers go for

building systems where many personal computers are interconnected

with a larger machine. A system with a model consisting of personal

computer one per user, with the sharable data kept on one or more

shared file server. Here the users are called clients and the whole

arrangements is called the client-server model illustrated in figure 1.2.

Figure 1.2: Client server model

In the client server model communication takes the form of a request

message from the client to the server asking for some work to be done.

The server replies back after doing the work.

4. Scalability

It is the ability to increase system performance gradually as the workload

grows just by adding processors. With main frames as server at some

point of time the system is full, and it must be replaced by a larger one

usually at great expense and even greater disruption to the users. With

the client server model, new client and new servers can be added as



needed without any disturbance to the users. That is the network can be

expanded easily.

5. Communication medium among employees

A computer networks provides a powerful communication among the

widely separated employees. Using a network it is easy for two or more

people who live far apart write a report together. When ever one worker

makes changes to an online document, the other can see the change

immediately, instead of waiting for several days. This speed makes the

co-operation among far-flung workers easy. In the long run the use of

computer networks can be used and prove probably important to

enhance human to human communication in an organization.

1.3.1 Networks for people

In 1990‟s, computer networks began to start delivering services to private

individuals at home. These services are quite different from the corporate

efficiency discussed in the above section. Below are few exciting uses for

individuals:

1. Access to remote information

There are many areas where access to remote information is necessary in

our daily life. Here applications involve interactions between a person and

remote database. One area seen is financial institutions like banks, power

supply department etc. Many people pay their bills, manage bank accounts,

and handle investments electronically.

Now a day we have banking from anywhere. That is a person can withdraw

money, check the balance of his account from anyplace no matter in which

place he has his account.

News paper goes online and can be personalized. The magazines, scientific

journals etc or rather digital library are all available online. Depending on the



cost, size and weight of book sized notebook computers, printed books may

become obsolete.

The most exciting application is access to information like current world wide

web, which contains information about arts, business, cooking, government,

health, history, hobbies, science, sports, travel etc.

2. Person-to-person communication

The facility like instant messaging derived from UNIX talk program allows

two people to type messages at each other in real time. The multi-person

version of this idea is chat room in which a group of people can type

messages for all to see. Another application is a peer to peer system and is

illustrated in figure 1.3. To distinguish it from client server model, there are

no fixed clients and servers. That is there is no fixed division into clients and

servers. Here individuals can form a group and can communicate with

others in a group. Every user can interact with one or more other people.

Figure 1.3: A peer to peer system

Electronic mail or e-mail is widely used by almost every person. It is one of

the most popular internet application that contains audio and video as well

as text. Real time e-mail will allow remote users to communicate with



no delay, possibly seeing and hearing each other. This technology makes it

possible to have virtual meetings, called videoconference among far flung

people. This videoconference technology finds extensively helpful for

remote school, getting medical opinions from distant specialists, and

numerous other applications.

The discussions in which one person posts a message and all other

subscribers can read the message or one subscriber may put a query and

anyone who knows can answer the query by posting so that all subscribers

can read.

3. Interactive entertainment

The most common application is entertainment. It is a huge and a growing

industry. The major and widely used application is video on demand. It is

possible to select any movie or television program in any country and have it

displayed on individual screen instantly. New films becomes more

interactive, game playing, live television also becomes interactive, with

audience participating in quiz shows, choosing among contestants, and so

on.

4. Electronic commerce

Home shopping is already popular and enables users to inspect the online

catalogs of thousands of companies. Some of these catalogs provide the

ability to get an instant video on any product by just clicking on the products

name. A customer can buy a product online. The customer can consult for

any technical support online. Another area in which e-commerce is already

being used is financial institutions. Many people already pay their bills;

manage bank accounts, etc online.

Now a day, we have electronic flea markets. That is online auctions of

second hand goods have become a massive industry. Online auctions are



of more of a peer to peer system, sort of customer to customer. Some forms

of e-commerce are illustrated in table 1.1. Here „2‟ used in a tag means „to‟.

Table 1.1: Some forms of e-commerce

Computer networks have become hugely important to people. Universities

may use Tele-learning national or international. Tele-medicine that is remote

patient monitoring is also now started.

1.3.3 Social Issues

The widespread introduction of networking will introduce social, political

problems. As long as like minded people have interactions with respect to

technical topics or hobbies there will not be any problems. The trouble

comes when the discussion is about politics, religion etc. views may be

posted and messages is not limited to text, high resolution color

photographs and even short video clips can be easily transmitted using

computer networks, which can lead to problems.

Another issue is employee versus employer rights. Many people read and

write mails at work. Some employers have claimed the right to read and

possibly censor employees mails including those that come from home

machines.

Another issue is anonymous messages. It allows ordinary person to

distribute their views in different ways and to different audiences. This might

bring many unsolved social, political, and moral issues.




1. Explain two benefits of computer network for an organization

2. Explain how computer networks helps for a common people.

3. Discuss different form of e-commerce

1.4 Classification of Computer Networks

There is no generally taxonomy accepted into which all computer networks

fit. The computer networks are classified depending on transmission

technology and scale.

Computer networks may be classified according to the network layer at

which they operate according to some basic reference models that are

considered to be standards in the industry such as the seven layer OSI

reference model and the four layers TCP/IP model. Few ways of

classification are listed below.

By transmission techniques: Computer networks may be classified as

broadcast or point to point networks.

By scale: to the scale or extent of reach of the network, for example as

a Personal area network (PAN), Local area network (LAN), Campus

area network (CAN), Metropolitan area network (MAN), or Wide area

network (WAN).

By connection method: Computer networks may be classified

according to the technology that is used to connect the individual

devices in the network such as HomePNA, Power line communication,

Ethernet, or Wireless LAN.

By functional relationship: Computer networks may be classified

according to the functional relationships which exist between the

elements of the network, for example Active Networking, Client-server

and Peer-to-peer (workgroup) architectures.

http://en.wikipedia.org/wiki/Personal_area_network

http://en.wikipedia.org/wiki/Local_area_network

http://en.wikipedia.org/wiki/Campus_area_network

http://en.wikipedia.org/wiki/Campus_area_network

http://en.wikipedia.org/wiki/Metropolitan_area_network

http://en.wikipedia.org/wiki/Wide_area_network

http://en.wikipedia.org/wiki/Wide_area_network



By network topology: Computer networks may be classified according

to the network topology upon which the network is based, such as Bus

network, Star network, Ring network, Mesh network, Star-bus network,

Tree or Hierarchical topology network, etc. This topic is covered in detail

in unit 4.

By services provided: Computer networks may be classified according

to the services which they provide, such as Storage area networks,

Server farms, Process control networks, Value-added network, Wireless

community network, etc.

By Protocol: Computer networks may be classified according to the

communications protocol that is being used on the network. Read the

articles on List of network protocols and protocol stacks for more

information.

The most common way of classifications is by transmission techniques,

by scale and by the way the computers are connected. The first two

ways are discussed in the following section where as the classification

by topology is discussed in unit 4.

1.4.1 Based on transmission technology

Broadcast links

Point-to-point links

Broadcast networks have a single communication channel that is shared by

all the users on the network. Short messages are commonly called as

packets or frames (in certain context). The user on the network sends

packets. All other machines receive these packets. An address field within

the packet or frame specifies the address of the destination machine. So

upon receiving the packet, all machines check the address field. Only

intended user uses or processes the packet or frame and others neglect and

discard it. As an example in a class of 50 students, the teacher puts

http://en.wikipedia.org/wiki/Bus_network

http://en.wikipedia.org/wiki/Bus_network

http://en.wikipedia.org/wiki/Star_network

http://en.wikipedia.org/wiki/Ring_network



question to say „X‟ student (where X is the name of the student). All the

students hear to the question but will not answer as the question is intended

to X only. Hence only X will analyze the question and others will not

respond.

Broadcast system generally allows the possibility of addressing a packet to

all the destinations by using a special code in the address field. When this

code is transmitted, it is received and processed by every machine on the

network. Again considering the above example: A teacher put forth the

question in a class to all students. That is the teacher does not ask to a

specific student by any unique name. Then, all are supposed to analyze the

question and answer. This mode of operation is referred to broadcasting.

Some broadcasting systems also support transmission to a subset of the

users, which is a group of users. This mode is called as multicasting.

In contrast the point-to-point network consists of many connections between

individual pairs of machines. A packet to be sent from source to destination

may have to first visit one or more intermediate machines. Usually different

routes of different length are possible. So finding the best path or route is

important in point-to-point networks. This type of transmission with one

sender and one receiver is also referred to as unicasting.

Geographically localized networks or smaller networks tend to use

broadcasting where as larger networks usually are point-to-point networks.

1.4.2 Based on the their scale

We classify multiple processor system based on physical size and is

illustrated in figure 1.4.

Error!



Figure 1.4: Classifications of processors by scale

At the top we have personal area networks (PAN), networks meant for a

single person. For example a wireless network connecting a computer with

its mouse, keyboard and a printer can constitute a personal area network.

Beyond the personal area network we have longer-range networks which

are broadly classified networks as

LAN

MAN

WAN

We will see these three networks in detail later. Finally the connection of two

or more networks is called an inter-network. The world wide Internet is a

well known example of inter-network. Distance is important as a

classification metric as different techniques are used at different scales.



1.4.3 Local Area Networks

Figure 1.5: LANs using (a) Bus topology (b) Ring topology

Local Area Networks are generally called LANs. They are privately owned

networks within a single building or campus of up to few kilometers in size.

Most of LAN‟s use Bus or ring topology for connection and is illustrated as

shown in fig. 1.5. They are used to connect personal computers and

workstations in company offices and factories to share resources and

exchange information. Traditional LANs run at speeds of 10Mbps to

100Mbps, have low delay (microseconds and nanoseconds) and make very

few errors. Newer LANs operate at 10Gbps. Various topologies are possible

for broadcast LANs.

1.4.4 Metropolitan Area Networks

Figure 1.6: MAN based on cable TV



A Metropolitan Area Networks, referred as MANs covers a city. The best

known example is cable television network available in many cities. Earlier

these were used for TV reception only but with changes a two way internet

service could be provided. A MAN might look something like the system

shown in figure 1.6. In this system both television signals and internet being

fed into centralized head end for distribution to people‟s home.

Cable television is not the only MAN. Recent developments in high speed

wireless internet access also resulted in MAN.

1.4.5 Wide Area Network

Figure 1.7: WAN system

A wide area network is referred as WAN. WAN spans a large geographical

area often a continent or country. WAN contains a collection of machines,

traditionally called as hosts. As illustrated in figure 1.7, these hosts can be

on LANs and are connected by a subnet or also called communication

subnet. The hosts are owned by customers or are personal computers. The

communication subnets are owned by a telephone company or internet

service provider. The subnet carries the messages from hosts to hosts, just

as telephone system carries words from speaker to listener. Each host is

connected to a LAN on which a router is present. Sometimes a host may be

connected directly to a router. The collection of communication lines and

routers is called a communication subnet.



Figure 1.8

In most WANs, the network contains many transmission lines each

connecting a pair of routers. As illustrated in figure 1.8, a packet is sent from

one router to another via one or more intermediate routers. The packet is

received at each intermediate router in its entirety. That is store the packet

in full until the required output line is free, and then forwards it. A subnet that

works according to this principle is called store and forward or packet

switched subnet. Not all WANs are packet switched. A second possibility for

a WAN is a satellite system. Satellite networks are inherently broadcast

networks.

1.4.6 Wireless Networks

The Italian physicist Guglielmo Marconi in 1901, demonstrated a ship-to-

shore telegraph, using Morse code. Morse code is a collection of binary

digits called dots and dashes. Modern digital wireless systems have better

performance, but the basic idea is the same. Wireless networks are divided

into three main categories. These are discussed below.

1. System interconnection

It is all about interconnecting the components of a computer using short

range radio. Every computer has a monitor, keyboard, mouse, printer

connected to the main unit by cables. Bluetooth configuration is as shown in

figure 1.9(a). Some companies got together to design a short range wireless

network called Bluetooth to connect these components without wires.



Figure 1.9: (a) Bluetooth (b) Wireless LAN

Blue tooth allows digital cameras, headsets, scanners, and other devices

like even computers to connect to a computer by merely being brought

within range. No cables, no driver installation, just put them on and turn

them on they work.

2. Wireless LANs

These are systems in which every computer has a radio modem and a

antenna with which it can communicate with other systems. Often there is

an antenna on the ceiling that the machines talk to as shown in figure 1.9(b).

Wireless LANs are becoming common in small offices and homes, where

installing Ethernet is considered too much trouble. Also used in older

buildings, company cafeterias, conference rooms etc. IEEE 802.11 is the

standard for wireless LANs.

3. Wireless WANs

This is also wireless network but is a wide area system.




1. Discuss different ways of classification of computer networks

2. Discuss how longer range networks are classified?

3. Discuss wireless networks

1.5 Summary

This unit introduces the subject computer networks. We have seen the

classification of networks depending on size or scale of the network, or

topology, protocol etc. We have taken a look at the uses of computer

networks for a business in resource sharing, reliability and scalability, as

well as for people in using e-mail facility, games- entertainments.

1.6 Terminal Questions

1 Explain in what way computer networks is useful for the people with

examples

2 Explain video conferencing and its uses.

3 Discuss different forms of e-commerce

4 Classify the networks based on their scale

1.7 Answers to Self Assessment Questions

Section 1.2

1. The term “Computer Network” is used to mean an interconnected

collection of autonomous computers

2. Task like share a copy of a file among all the users, even simultaneously

or transfer files from computer or even share resources like printers etc.

(Refer section 1.2.1)

3. Servers, nodes, workstation, NOS, LAN cable and software, and NIC

(explain each of them in 2-3 lines)

4. A software



Section 1.3

1. Resource sharing, reliability, scalability etc

(Discuss at least in 2-3 lines each)

2. (Refer 1.3.2) Any two points can be discussed.

3. B2C, B2B, G2C, C2C, P2P

(Refer table 1.1 and explain with examples)

Section 1.4

1. Several ways are there 6-7 ways are given (Refer section 1.4)

2. LAN, MAN, and WAN explain each in 2-3 lines

3. Refer section 1.4..6 different categories are discussed.

1.7 Answer to Terminal questions

1. Access to remote information, person to person communication etc.

For more details Refer 1.3.2

2. Videoconference technology finds extensively helpful for remote school,

getting medical opinions from distant specialists, and numerous other

applications.

Explanation: Refer 1.3.2

3. B2C, B2B, G2C, C2C, P2P

Refer table 1.1

4. Refer section 1.4.2, Figure 1.4

MC0075 (A)-Unit-01

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