CHAPTER 2

COMMUNICATION

2.1 DATA COMMUNICATION

We all are acquainted with some sorts of communication in our day-

to-day life. For communication of messages we use telephone and

postal communication systems. Similarly data and information from

one computer system can be transmitted to other systems across

wide geographical areas.

Definition: data communication means transfer of data between

two or more devices via a transmission medium.

Communication devices must be a part of communication system,

made up with software and hardware. A good communication

system must have three properties.

Delivery: system must transfer data to correct destination.

Accuracy: system must deliver data correctly with any

disturbance.

Timeliness: system must deliver data in time. Late data

delivery is useless.

2.2 COMPONENTS OF DATA COMMUNICATION SYSTEM

1. Message - It is the information or data to be communicated.

Popular forms of information include text, pictures, audio,

video etc. Mostly text data or information is transferred and

text data is called code.

Codes are used to represent characters. Two main codes,

which are widely used in the world of communication are :

American Standard Code for Information

Interchange -ASCII – 7 bit code represents 128

characters whereas ASCII –8 bit code represents 256

characters.

Extended Binary-Coded Decimal Interchange Code

- EBCDIC – 8-bit code developed by IBM represents 256

characters.

2. Sender - It is the device which sends the data messages. It can

be a computer, workstation, telephone handset etc.

3. Receiver - It is the device which receives the data messages. It

can be a computer, workstation, telephone handset etc.

4. Transmission Medium - It is the physical path by which a

message travels from sender to receiver. Some examples include

twisted-pair wire, coaxial cable, radio waves etc.

5. Protocol - It is a set of rules that governs the data

communications. It represents an agreement between the

communicating devices. Without a protocol, two devices may be

connected but not communicating.

You may be wondering how computers send and receive data across

communication links. The answer is data communication software.

It is this software that enables us to communicate with other

systems. The data communication software instructs computer

systems and devices as to how exactly data is to be transferred

from one place to another. The procedure of data transformation in

the form of software is commonly known as protocol.

The data transmission software or protocols perform the following

functions for the efficient and error free transmission of data.

1. Data sequencing: A long message to be transmitted is

broken into smaller packets of fixed size for error free data

transmission.

2. Data Routing: It is the process of finding the most

efficient route between source and destination before sending

the data.

3. Flow control: All machines are not equally efficient in

terms of speed. Hence the flow control regulates the process

of sending data between fast sender and slow receiver.

4. Error Control: Error detecting and recovering is the one

of the main functions of communication software. It ensures

that data are transmitted without any error.

Example of data communication Protocols are TCP/IP, OSI,

SAN, etc.

2.3 DATA TRANSMISSION MODES

There are three ways for transmitting data from one point to

another.

1.Simplex: In simplex mode the communication can take place

in one direction always. The receiver receives the signal

from the transmitting device. In this mode the flow of

information is Unidirectional. Hence it is rarely used for data

communication.

2.Half duplex: In half-duplex mode the communication

channel is used in both directions, but only in one

direction at a time. Thus a half-duplex line can alternately

send and receive data.

3.Full duplex: In full duplex the communication channel is

used in both directions at the same time. Use of full-duplex

line improves the efficiency as the line turnaround time

required in half-duplex arrangement is eliminated. Example of

this mode of transmission is the telephone line.

2.4 TYPES OF COMMUNICATION MEDIA

Following are the major communication devices, which are

frequently used:

1. Twisted Pairs: Wire pairs are commonly used in local

telephone communication and for short distance data

communication. They are usually made up of copper and

the pair of wires is twisted together. It is cheap and widely

available for installation. It radiates good amount of

energy and susceptible to outside noise.

2. Co-axial Cable: Coaxial cable is groups of specially wrapped

and insulated wires that are able to transfer data at higher

rate. They consist of a central copper wire surrounded by an

insulation over which copper mesh is placed. They are used

for long distance telephone lines and local area

network for their noise immunity and faster data

transfer.

3. Fibre optics: Fibre-optic technology is used to transmit the

signal using glass fibre. It radiates light rather than

electricity. It is made of glass and plastic and the cladding

part has a different refractive index from the core, which

minimizes the light loss through the sides of the cable and

promotes internal reflections down the length of the core. It is

very good transmission media in terms of speed and capacity

and less hazards. Fibre-optic cables has made great

contribution in reduction of communication media in terms of

size and weight but enhanced the speed of

communication. A single glass fibre can carry more than

50,000 telephone calls simultaneously compared to 5,500

calls on a standard coaxial cable line. Speed of optical fibre in

laboratory is six trillion bits per sec.

4. Microwave: Microwave system uses very high frequency

radio signals to transmit data through space. The

transmitter and receiver of a microwave system should be in

line-of-sight because the radio signal cannot bend. With

microwave very long distance transmission is not

possible. In order to overcome the problems of line of sight

and power amplification of weak signal, repeaters are used

at intervals of 25 to 30 kilometers between the

transmitting and receiving end.

5. Communication Satellite: The problem of line-sight and

repeaters are overcome by using satellites, which are the

most widely used data transmission media in modern days. A

communication satellite is a microwave relay station placed in

outer space. INSAT-1 B is such a satellite that can be

accessible from anywhere in India. In satellite

communication, microwave signal is transmitted from a

transmitter on earth to the satellite at space. The satellite

amplifies the weak signal and transmits it back to the

receiver. The main advantage of satellite communication is

that it is a single microwave relay station visible from any

point of a very large area. In microwave the data

transmission rate is 16 giga bits per second.

6.5 INTERNET, INTRANET, AND EXTRANET

Internet, intranet, and extranet are three terms that describe

“Internet-type” applications that are used by an organization, but

how do you know if a web application is part of your intranet or part

of the Internet?

Internet: If you wish to expose information to everyone in the

world, then you would build an Internet-type application. An

Internet-type application uses Internet protocols such as HTTP, FTP,

or SMTP and is available to persons anywhere on the Internet. We

use the Internet and web applications as ways to extend who the

application can reach

Intranet: An application is considered to be on the company’s

intranet if it is using Internet-type protocols such as HTTP or FTP but

the application is available only within the company. The

information on a company’s intranet would not be accessible to

persons on the Internet because it is not for public use.

Extranet: From time to time, an application that has been built for

the company’s intranet and used by internal employees will need to

be extended to select business partners or customers. If you extend

your intranet out to select business partners or customers, you have

created an extranet. An extranet cannot be used by anyone else

external to the company except for those selected individuals.

2.6 NETWORK TOPOLOGIES

Network architecture is made up of a topology, a cable type, and an

access method. A network topology is the physical layout of

computers, cables, and other components on a network. There are a

number of different network topologies, and a network may be built

using multiple topologies. The different types of network layouts are

:

1. Bus topology

2. Star topology

3. Mesh topology

4. Ring topology

1. Bus topologies

A bus topology uses one cable as a main trunk to connect all of

the systems together. A bus topology is very easy to set up and

requires no additional hardware such as a hub. The cable is also

called a trunk, a backbone, or a segment.

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Advantages of a bus topology

One advantage of a bus topology is low cost. A bus

topology uses less cable than a star topology or a mesh

topology, and you do not need to purchase any additional

devices such as hubs. Another advantage of a bus topology

is the ease of installation. With a bus topology, you

simply connect the workstation to the cable segment or

backbone.

You need only the amount of cable to connect the

workstation to the backbone. The most economical choice

for a network topology is a bus topology, because it is easy

to work with and a minimal amount of additional devices

are required. Most importantly, if a computer fails, the

network stays functional.

Disadvantages of a bus topology

The main disadvantage of a bus topology is the difficulty of

troubleshooting it. When the network goes down, it is

usually due to a break in the cable segment. With a

large network, this problem can be tough to isolate.

Scalability is an important consideration in the dynamic

world of networking. Being able to make changes easily

within the size and layout of your network can be important

in future productivity or downtime. The bus topology is not

very scalable.

2. Star topologies

In a star topology, all computers are connected through one

central device known as a hub or a switch. Each workstation

has a cable that goes from the network card to the hub device. One

of the major benefits of a star topology is that a break in the cable

causes only the workstation that is connected to the cable to go

down, not the entire network, as with a bus topology.

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Advantages of a star topology

One advantage of a star topology is scalability and ease

of adding another system to the network. If you need

to add another workstation to the network with a star

topology, you simply connect that system to an unused

port on the hub.

Another benefit is the fact that if there is a break in the

cable it affects only the system that is connected to that

cable but a hub with a few ports available.

Centralizing network components can make an

administrator’s life much easier in the long run.

Centralized management and monitoring of network

traffic can be vital to network success. With a star

configuration, it is also easy to add or change

configurations because all of the connections come to a

central point.

Disadvantages of a star topology

On the flip side, if the hub fails in a star topology, the

entire network comes down, so we still have a central

point of failure.

Another disadvantage of a star topology is cost. To

connect each workstation to the network, you will need to

ensure that there is a hub with an available port, and you

will need to ensure you have a cable to go from the

workstation to the hub.

3. Mesh topologies

In a mesh topology, every workstation has a connection to every

other component of the network.

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Advantages of a Mesh topology

The biggest advantage of a mesh topology is fault tolerance,

meaning that, if there is a break in a cable segment, traffic can be

rerouted through a different pathway because there are multiple

pathways to send data from one system to another. This fault

tolerance means that it is almost impossible for the network to go

down due to a cable fault.

Disadvantages of a Mesh topology

A disadvantage of a mesh topology is the cost of the additional

cabling and network interfaces to create the multiple pathways

between each system. A mesh topology is very hard to administer

and manage because of the numerous connections.

4. Ring topologies

In a ring topology, all computers are connected via a cable that

loops in a ring or circle. A ring topology is a circle that has no

start and no end. Because there are no ends, terminators are not

necessary in a ring topology.

Signals travel in one direction on a ring while they are passed from

one computer to the next, with each computer regenerating the

signal so that it may travel the distance required.

Advantages of a ring topology

A major advantage of a ring topology is that signal degeneration is

low because each workstation is responsible for regenerating or

boosting the signal. With the other topologies, as the signal

travels the wire, it gets weaker and weaker as a result of outside

interference: eventually, it becomes unreadable if the destination

system is too far away. Because each workstation in a ring topology

regenerates the signal, the signal is stronger when it reaches its

destination and seldom needs to be retransmitted.

Disadvantages of a ring topology

The biggest problem with ring topologies is that if one computer

fails or the cable link is broken, the entire network could go

down. With newer technology, however, this isn’t always the case.

The concept of a ring topology today is that the ring will not be

broken when a system is disconnected; only that system is dropped

from the ring.

2.7 TYPES OF NETWORKS

Organizations of different sizes, structures, and budgets need

different types of networks. Networks can be divided into one of two

categories: peer-to-peer or server-based networks.

1. Peer-to-Peer network

A peer-to-peer network has no dedicated servers; instead, a

number of workstations are connected together for the purpose of

sharing information or devices. When there is no dedicated server,

all workstations are considered equal; any one of them can

participate as the client or the server.

Peer-to-peer networks are designed to satisfy the networking needs

of home networks or of small companies that do not want to

spend a lot of money on a dedicated server but still want to

have the capability to share information or devices.

For example, a small accounting firm with three employees that

needs to access customer data from any of the three systems or

print to one printer from any of the three systems may not want to

spend a lot of money on a dedicated server. A small peer-to-peer

network will allow these three computers to share the

printer and the customer information with one another. The

extra cost of a server was not incurred because the existing client

systems were networked together to create the peer-to-peer

network.

Most of the modern operating systems such as Windows XP and

Windows Vista already have built-in peer-to-peer a

networking capability, which is why building a peer-to-peer

network, would be a “cheap” network solution.

The disadvantage of a peer-to-peer network is the lack of

centralized administration—with peer-to-peer networks, you

need to build user accounts and configure security on each system.

It is important to note that peer-to-peer networks are designed for

fewer than 10 systems, and with Microsoft client operating systems

such as Windows XP Professional, only 10 concurrent network

connections to those clients are allowed. This means that if you

have 15 or 20 employees, you eventually will need to implement a

server-based network.

2. Server-based networks

The advantage of a server-based network is that the data files that

will be used by all of the users are stored on the one server. This will

help you by giving you a central point to set up permissions on the

data files, and it will give you a central point from which to back up

all of the data in case data loss should occur. With a server-based

network, the network server stores a list of users who may use

network resources and usually holds the resources as well. The

server in a server-based network may provide a number of different

services. The services it will offer to the network usually are decided

by the server’s role.

2.8 LOCAL AREA NETWORKS

The next step up is the LAN (Local Area Network). A LAN is a

privately owned network that operates within and nearby a

single building like a home, office or factory. LANs are widely

used to connect personal computers and consumer electronics to let

them share resources (e.g., printers) and exchange information.

A LAN is a network that is used for communicating among

computer devices, usually within an office building or home.

LAN’s enable the sharing of resources such as files or

hardware devices that may be needed by multiple

users.

Is limited in size, typically spanning a few hundred meters,

and no more than a mile.

Is fast, with speeds from 10 Mbps to 10 Gbps.

Requires little wiring, typically a single cable connecting to

each device.

Has lower cost compared to MAN’s or WAN’s.

LAN’s can be either wired or wireless. Twisted pair, coaxial

cabel or fibre optic cable can be used in wired LAN’s.

Every LAN uses a protocol – a set of rules that governs how

packets are configured and transmitted.

Nodes in a LAN are linked together with a certain topology.

These topologies include:

Bus

Ring

Star

Mesh

Advantages of LAN

• Speed

• Cost

• Security

• E-mail

• Resource Sharing

Disadvantages of LAN

• Expensive To Install

• Requires Administrative Time

• File Server May Fail

• Cables May Break

6.9 METROPOLITAN AREA NETWORKS

• A metropolitan area network (MAN) is a large computer

network that usually spans a city or a large campus.

• A MAN is optimized for a larger geographical area than a LAN,

ranging from several blocks of buildings to entire cities.

• A MAN might be owned and operated by a single organization, but

it usually will be used by many individuals and organizations. A MAN

often acts as a high speed network to allow sharing of regional

resources.

• A MAN typically covers an area of between 5 and 50 km

diameter.

• Examples of MAN: Telephone company network that

provides a high speed DSL to customers and cable TV

network.

6.10 WIDE AREA NETWORKS

WAN covers a large geographic area such as country,

continent or even whole of the world.

• A WAN is two or more LANs connected together. The LANs can be

many miles apart.

• To cover great distances, WANs may transmit data over leased

high-speed phone lines or wireless links such as satellites.

• Multiple LANs can be connected together using devices such as

bridges, routers, or gateways, which enable them to share data.

• The world's most popular WAN is the Internet.

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6.11 IMPORTANT TERMS USED IN NETWORKING

(a) Internet

The newest type of network to be used within an organisation is an

Internet or Internet web. Such networks enable computers (or

network) of any type to communicate easily. The hardware and

software needs are the same as for the Internet, specifically TCP/IP,

server and browser software used for the World Wide Web. With the

help of Internet, all computers of an organisation can work as stand-

alone systems, connected to a mainframe, or part of a LAN or WAN.

(b) E-Mail

E-mail stands for electronic mail. Mails are regularly used today

where without the help of postage stamp we can transfer mails

anywhere in the world with electronic mail service. Here data is

transmitted through Internet and therefore within minutes the

message reaches the destination may it be anywhere in the world.

Therefore the mailing system through e-mail is excessively fast and

is being used widely for mail transfer.

(c) Voice Messaging

It is a new communication approach, which is similar to electronic

mail except that it is audio message rather than text messages that

are processed. A sender speaks into a telephone rather than typing,

giving the name of the recipient and the message. The main

advantage of voice mail over electronic mail is that the sender does

not have to type. Voice mail also makes it easy to include people in

the firm's environment in an communication network.

(d) E-Commerce

Electronic commerce or e-commerce as it is popularly known refers

to the paperless exchange of business information using

Electronic Data Interchange, Electronic mail, Electronic

Bulletin Boards, Electronic Fund Transfer and other network

based technologies. Electronic Commerce (EC) not only

automates manual process and papers transactions, but it also

helps organisations to move into a fully electronic environment and

change the way they usually operate. EC is associated with buying

and selling of products and services over computer communication

networks.

(e) Electronic Data Interchange (EDI)

EDI is the computer-to-computer exchange of business documents

in a standard format. These formats look much like standard forms

and are highly structured.

(f) Browser: To access the content of a Web site, users must have

a browser that can help them locate a Web site on the Internet and

then view the content of a Web page. Users can also use the

browser to download and upload files on the Internet. A few popular

browsers that are used worldwide are Internet Explorer, Netscape

Navigator, and Opera. To access a Web site, a user needs to specify

an address that helps the browser track a particular Web site.

(g) URL

A unique resource locator (URL) is a link that is used to access a

Web site on the Internet. A URL is unique to each site, and is

typically based on the nature of the site. Without specifying a valid

URL, users cannot access a particular site

(h) ISP

Internet service providers (ISPs) are companies that help users

connect to the Internet for a monthly fee. In return, they provide a

username, a password, and telephone number. The username and

the password are used to authenticate the user on the Internet. The

telephone number is used to establish connection with the dial-up

server of the ISP. However, to access the Internet a user (client

computer) requires a modem.

(i) Web Page

Any page that is hosted on the Internet is a Web page. A Web page

is viewed by using a browser. The basic framework of Web pages

can be designed using a language called HyperText Markup

Language (HTML).

(j) HTML (short for “Hypertext Markup Language”): The

authoring language used to create documents on the World Wide

Web. HTML is a mark-up language (versus a programming

language) that uses tags to structure text into headings,

paragraphs, lists, and links. The tags tell a Web browser how to

display text and images.

(k) Hyperlink: An element in an electronic document that links to

another place in the same document or to an entirely different

document. Typically, you click on the hyperlink to follow the link.

Hyperlinks are the most essential ingredient of all hypertext

systems, including the World Wide Web.

(l) Web Site: A place on the Internet or World Wide Web. It refers

to a body of information as a whole, for a particular domain name. A

Web site is a place made up of Web pages.

(m) World Wide Web: A system of Internet servers that

support specially formatted documents. The documents are

formatted in a markup language called HTML (HyperText Markup

Language) that supports links to other documents, as well as

graphics, audio, and video files. This means you can jump from one

document to another simply by clicking on hot spots. Not all Internet

servers are part of the World Wide Web.

(n) Type of Internet

Analog (up to 56k): Also called dial-up access, it is both

economical and slow. Using a modem connected to your PC, users

connect to the Internet when the computer dials a phone number

(which is provided by your ISP) and connects to the network. Dial-

up is an analog connection because data is sent over an

analog, public telephone network. The modem converts

received analog data to digital and vise versa. Because dial-up

access uses normal telephone lines the quality of the connection is

not always good and data rates are limited. Typical Dial-up

connection speeds range from 2400 bps to 56 Kbps.

ISDN (Integrated services digital network) is an international

communications standard for sending voice, video, and data

over digital telephone lines or normal telephone wires.

Typical ISDN speeds range from 64 Kbps to 128 Kbps.

DSL (Digital Subscriber Lines) is also called an always on

connection because it uses existing 2-wire copper telephone line

connected to the premise and will not tie up your phone as a dial-up

connection does. There is no need to dial-in to your ISP as DSL is

always on. ADSL supports data rates of from 1.5 to 9 Mbps when

receiving data (known as the downstream rate) and from 16 to 640

Kbps when sending data (known as the upstream rate). The main

categories of DSL for home subscribers are called ADSL

(Asymmetric Digital Subscriber Line), SDSL (Symmetric

Digital Subscriber Line), VDSL (Very High DSL) and HDSL

(High-data-rate Digital Subscriber Line).

Cable: Through the use of a cable modem you can have a

broadband Internet connection that is designed to operate over

cable TV lines. Cable Internet works by using TV channel space for

data transmission, with certain channels used for downstream

transmission, and other channels for upstream transmission.

Because the coaxial cable used by cable TV provides much greater

bandwidth than telephone lines, a cable modem can be used to

achieve extremely fast access. Cable speeds range from 512

Kbps to 20 Mbps.

Wireless Internet Connections

Wireless Internet, or wireless broadband is one of the newest

Internet connection types. Instead of using telephone or cable

networks for your Internet connection, you use radio frequency

bands. Wireless Internet provides an always-on connection

which can be accessed from anywhere — as long as you

geographically within a network coverage area. Wireless access is

still considered to be relatively new, and it may be difficult to find a

wireless service provider in some areas. It is typically more

expensive and mainly available in metropolitan areas.

T-1 Lines: they are a popular leased line option for businesses

connecting to the Internet and for Internet Service Providers (ISPs)

connecting to the Internet backbone. It is a dedicated phone

connection supporting data rates of 1.544Mbps.  A T-1 line

actually consists of 24 individual channels, each of which supports

64Kbits per second. Each 64Kbit/second channel can be configured

to carry voice or data traffic. Most telephone companies allow you to

buy just one or some of these individual channels. This is known as

as fractional T-1 access.

Bonded T-1

A bonded T-1 is two or more T-1 lines that have been joined

(bonded) together to increase bandwidth.

T-1 Lines support speeds of 1.544 Mbps

Fractional T-1 speeds are 64 Kbps per channel (up to 1.544

Mbps), depending on number of leased channels.

Typical Bonded T-1 (two bonded T-1 lines) speed is around 3

Mbps.

Internet over Satellite (IoS) allows a user to access the Internet

via a satellite that orbits the earth. A satellite is placed at a static

point above the earth's surface, in a fixed position. Because of the

enormous distances signals must travel from the earth up to the

satellite and back again, IoS is slightly slower than high-speed

terrestrial connections over copper or fiber optic cables.

Typical Internet over Satellite connection speeds (standard

IP services) average around 492 up to 512 Kbps.