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C H A P T E R 1

Introduction

1.1 WHAT IS DATA COMMUNICATION?

Communication, in the layman language means to convey a message, an idea, a picture or speech

that is received and understood clearly and correctly by the person for whom it is conveyed. There

could be several methods of conveying the message. If the sender and the receiver are close by,

you may send it by speaking loudly. If the distance is not very far, you may convey it by writing it

on a piece of paper and sending it through a peon. If the distance is large, you may send it by post.

If the message is not very long, and sender as well as the person for whom the message is being

conveyed are having telephones with them, you may pass it on telephone. But all these methods

have an inherent factor and that is the speed of conveying the message. That is how soon it reaches

and the cost of sending the message. Postal method is probably the cheapest but there is an

inherent delay of few days if the letter is sent from one city to other city. There is no surety that it

will reach at the door step of the receiver. So there is every chance that the receiver may reply that

he never got the letter.

Telephonic message is more reliable because the person on the other side of the phone can

immediately confirm that he got the message and understood it. However, he/she may forget after

a little while unless reminded. Telephonic communication is popular because it is cheap and

instantaneous. You can talk to a person and convey a lot of message on telephone. But picture can

not be sent on telephone. Large amount of data or messages cannot be remembered if conveyed on

telephone. It is in this context, that data communication containing messages, pictures and voice

has taken the importance. Basic factors that need to be considered in the data communications are:

(a) The cost of conveying message, picture, or voice over a large distance should be small.

(b) The transmission should take place without an iota of doubt/confusion in the mind of the

receiver. It means, there should be no interference like noise etc.

(c) The message should reach within a reasonable time before it becomes obsolete. For exam-ple, the varying rates of shares/equity in the stock exchange needs to be conveyed imme-

diately so that buyer/sellers of shares can decide the action to be taken by them. If the

results of an entrance examination are declared today, it must reach to all the schools the

same day.

(d) The message should be safe and secured. It should only reach to the person for whom it is

meant. Postal method of conveying a message is not secured at all. Telephonic conversa-

tion can be heard if some one taps the telephone line.

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2 Data Communication

We shall therefore try to study Data Communications keeping in mind the above mentioned

four very important factors.

1.2 ELECTRONIC METHODS OF COMMUNICATIONS

With the invention of telephone instrument and the communication satellites, the means of elec-

tronic communication has become very popular in India even though the cost of installation and

maintenance of telephones is still very high and beyond the means of a common man. Western

countries and some developed Eastern nations adopted this method of communications long ago

and improved their capability and competitiveness in business, administration and education. This

method improved the general conditions of the people more cost effectively. In India it is in the

later part of the 20th Century, that telephone has become a necessity rather than a luxury. STD and

Local Call booths are now visible all over the country. Citizens of this country do have access to

telephonic communication within their means. However, telephonic communication method has

the following limitations:

(a) Both the sender of the message and the receiver of the message should be available at the

same time and should speak the same language to understand each others message.

(b) Telephonic communication is not a secured means of communication because any one can

overhear the message or tap the telephone line.

(c) It is not suitable to send picture, or any other type of message except a spoken message.

(d) It is affected by the electrical interference or by the people digging roads etc.

(e) It is still quite costly to make a telephone call outside the city or the country.

1.3 COMPUTERISED COMMUNICATIONSSince the time there had been a fall in the prices of PCs, computers have started playing an

important role in the communication field. The main reason for this is that computers can send

data extremely fast. They can even pass pictures, sound in a very secured manner. Further, PCs

can send information on the existing telephone line. So, users do not have to spend any extra

money except for the purchase of a PC machine and its connection to the telephone line using an

instrument called modem.

Computer based communication system has the following advantages:

(a) Telephonic calls can be made to any part of the world with the same expenses as a local

telephone call made within the city.

(b) Pictures, sound and written data can be sent within minutes and a confirmation about it

reaching at the destination can be obtained immediately.(c) Messages can be coded so that it is not understood by any body else except the person who

is sending and the person who is receiving the message. Even if the message is intercepted

on the way, it can not be decoded.

(d) Messages can be sent in any language including Hindi, regional languages or European

languages from any part of the world to any other part of the world.

(e) Users need not take highly specialized training for sending or receiving messages.

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Introduction 3

(f) In addition to conveying a message, the same PC can also be used as an instrument to get

information from varied sources such as railways, stock exchange and weather depart-

ments. Even goods can be purchased using computers and in return the payments can be

sent electronically.

(g) PCs connected with modem can also be used for education, entertainment, etc.

It is in this context, that the study of Data Communications and Networking becomes very

essential for us because there is a vast scope of employment in this direction.

1.4 DATA COMMUNICATION SYSTEM MODEL

The purpose of a data communications system is the exchange of data between two parties. Figure

1.1 shows the basic diagram of a communication model between two PCs. A similar picture can bedrawn for analog communication system where the two people speak through thier telephones.

These days, the data communication system is of more general in nature where a system can pass

digital data as well as analog speech signals.

Figure 1.1(a)

A simple

Communicati

on model

The key elements of a simple model are the following:

(a) Source and its accompanied transmitter

(b) Transmission system

(c) Receiver and its destination

1.4.1 Source and its Accompanied Transmitter

This device generates the data to be transmitted. These devices could be PCs, Fax machine or a

simple telephone instrument. Sometimes the data generated by the source system are sent to the

transmission system using a modulator/demodulator that converts the data (if in digital form) to its

equivalent analog form so that it can be sent on the telephone wire before it reaches to the terminal

exchange where it may be used for reconnection to the exchange for which this data is to be sent.

Endoffice office

EndofficeToll

(telephonecompany

Digital

(localAnalog

loop)

Computer

Modem Modem

Computer

Digital

(short cable) (short cable)

Digital

(localAnalog

loop)truks)

Codec Codec

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1.4.2 Transmission System

The transmission system will depend on the type of data to be sent and the amount of data. Also it

will depend on the speed of transmission required. The system could be simple telephone wire or it

can be coaxial cable, or fiber optic cables.

1.4.3 Receiver and Destination

The receiver accepts the signal coming from the transmission system and converts it into a form

that can be handled by the destination device. It can be a receiving modem that will convert the

incoming analog signal to a form say digital so that it can be received and accepted by the PC on

the receiving side.

1.5 KEY TASKS PERFORMED BY COMMUNICATIONS SYSTEM

Some of the key tasks performed by the communication system are the following:

(a) Transmission system utilization

(b) Interfacing

(c) Signal generation

(d) Exchange management

(e) Error detection and correction

(f) Addressing and routing

(g) Security

(h) Network Management

1.5.1 Transmission system utilizationThis term means the need to make full use of the transmission facilities that may be shared among

various users. One of the technique used for sharing the facilities among various users is known as

multiplexing. In this method, total capacity of transmission medium is distributed among a number

of users.

1.5.2 Interfacing

Interface is a device that connects the source to the output. All communication system must have

the interface to propagate properly over the transmission medium.

In addition, there must be some form of synchronization between transmitter and receiver so

that the receiver can understand when the signal begins to arrive and when it ends. It must also

know the duration of each signal element.

1.5.3 Signal generation

After an interface is created, the signal needs to be generated. The signal should be of sufficient

intensity so that it is :

(a) Capable of being propagated through the transmission system

(b) Interpretable as data at the receiver end.

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Introduction 5

1.5.4 Exchange management

When data need to be transferred between two parties over a period of time, the two parties must

cooperate. For example, if two people want to converse on telephone, one must deal the telephone

number of the other person and when the connection is made, the second person must lift the

phone to make the connection through. In a data g system, there will be much more need. Some

convention are decided namely, whether the transmission to take place simultaneously or one after

the other. The amount of data that can be sent at one time and how the error is to be checked.

These all conventions form the exchange management.

1.5.5 Error detection and correction

In every communication system, there is a possibility of occurrence of error while signal travelsthrough the transmission medium. There is also likely the distortion of the signal as it reaches the

destination. Under these circumstances, the error detection and correction is very much required.

This is particularly important in the case of data processing systems. For example, in transferring a

file from one computer to another, it is simply not acceptable for the contents of the file to be

accidentally altered.

1.5.6 Addressing and routing

When a transmission facility is shared by more than two devices, a source system must somewhat

indicate the identity of the intended destination. The transmission system must assure that the

destination system, and only that system receives the data. It may so happen, that the transmission

system may be a network through which various paths may be taken . In such a case, a specific

route through the network must be selected.

1.5.7 Security

Security means preventing the communication system falling in the hands of unauthorized people.

The sender of data may wish to be assured that only the intended party actually receives the data.

The receiver of data must authenticate itself that it is one who is authorized to receive the data and

thereafter the exchange of data takes place.

1.5.8 Network management

Modern data communications systems are complex and can not run by itself. They need to be

incorporated with the network management capability to operate the system effectively. This net-

work management would monitor the status of the traffic at various points, point out the failures

and over loading before actual occurrence and also plan or route the traffic intelligently for

efficiently working.

1.6 GOALS OF NETWORKING

Networking has the following main goals:

(a) To share computer files

(b) To share computer equipment

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(c) To enable unlike computer equipment to communicate

(d) To improve communiation speed and accuracy

(e) Cost effectiveness

(f) To enable group working

(g) Possiblility of Integrated sytem

1.6.1 To Share Computer Files

Networks enable users to share files with others. This advantage is invaluable, for example, to

word-processing personnel who need access to filed documents or to managers using electronic

mail. Different departments of an organization may be separated physically, being at distant

places, but their data could be stored on a central computer. This data is accessed by computers

located in different departments. The data at the central computer may be updated from time totime and accessed by all users. This prevents any bottlenecks in the smooth functioning of the

organization. In this way, the latest data can be easily made available at all times to all the users.

1.6.2 To Share Computer Equipment

Laser printers and large hard-disk drives can be expensive. Networks enable users to share such

equipment by networking microcomputers or workstations together. For example, a company with

twenty computer users, each requiring only limited hard disk space and occasional printing, might

save many thousands of rupees in equipment costs by buying less expensive terminals or disk-less

microcomputers.

1.6.3 To Enable unlike Computer Equipment to Communicate

A company having computers of different makes using different operating systems such as MS

DOS, UNIX and Apple DOS cannot transfer files from one computer to another unless they are

networked using a Networking operating system such as Netware 4.1 or Windows NT 4.0. Since

most computers have standard RS 232C output ports, these can be used to network unlike

machines together. It is also possible to share files and send messages across the machines con-

nected through a network.

1.6.4 To Improve Communiation Speed and Accuracy

It is not uncommon for the inter-office mail of even fairly small companies to take several days for

delivery. In contrast, sending messages through local area networks is virtually instantaneous, and

there is also less chance of a message being lost. Communications will also be more accurate

because word processors can be used to compose messages and no ambiguous messages are takenby others.

1.6.5 Cost Effectiveness

Computers can send data at a very fast speed over long distances using satellite and microwave

links. Therefore, the cost of transfer of documents using computers connected on netwoks is

cheaper than other conventional means like telegrams.

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Introduction 7

1.6.6 To Enable Group Working

The time taken to transmit the messages is very small. Hence different computers on a network

can be connected together and the users can work together as a group. Software packages have

been developed for group working in Data Base Management systems (DBMS) and graphic

works.

1.6.7 Possiblility of Integrated Sytem

Computers can have fax systems integrated with netwoking. This allows them to send pictures

along with the text. Thus the newspaper reports can be prepared and sent all over the world at a

very high speed by composing and publishing them from different centres.

1.7 COMPLETE COMMUNICATION MODEL

Figure 1.2 shows the complete communication model.

Figure 1.2

Complete

communica-

tion system

Central Office

PersonalComputer

ComputerPersonal

TimeDivision

Multiplexers

Multiplexers

Division

Time

Central Office

Modem

Modem

Local Loop

Analog

Analog

Local Loop

T1CarrierTrunk

Binary Signal Modulated Analog Signal Digital Signal

Modulated Analog SignalBinary Signal Digital Signal

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The fiugre gives the data communication taking between two PCs. Although digital communica-

tions offer many advantages over analog, one may have to dial with many analog components and

systems in a total communication system design. Figure 1.2 shows how a complete

communication system looks from signal perspective.

A good understanding of the differences between these digital and analog signals and how to

bring the two together is the key to connectivity. The communications channels provide the link

for data communications.

You will note in Figure 1.2 that the link between the PC to the central office or telephone

exchange is by modulated analog signal created by the modem. The communication between

exchange or central office to another trunk exchange or central office is by digital signal using

time division multiplexers (codecs). The final communication between the trunk exchange and the

PC is again by using modulated analog signal. Modem at the other end converts this analog signalto digital signal to be detected by the PC.

The difference between the two basic forms of electronic communication, namely, analog and

digital is a little like the difference between water streaming from a hose and bullets firing from a

a machine gun. An analog signal is a continuous electro magnetic wave, whose pattern varies to

represent the message being transmitted. A digital signal, by contrast is a series of discrete elec-

tronic burst and the bursts indicate the message.

Figure 1.3 (a) shows analog signal. The key characteristics of an analog signal include

strength, or amplitude (vertical distance between a wave trough and crest) and frequency (the

number of times per second the wave cycle repeats).

Figure 1.3 (a)

Analog signal showing a

sine wave,

represen-

tative of a

pure tone

Figure 1.3 (b)

Typical

sound signalin Analog

form

Electronic Analogs for Sound

One Cycle

0 hertz

3,400 hertz

250 hertz

4,000 hertz

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Introduction 9

Figure 1.3(b) shows a typical analog signal, the many variations in amplitude and frequency

convey the gradations of loudness and pitch in speech or music. Similar signals are used to trans-

mit television pictures, but at much higher frequencies. Telephone equipment allows the voice a

band width of 4,000 hertz, which includes a guard band at top and bottom to prevent interference.

T.V. Signals a band width of four million hertz (4 MHZ).

Figure 1.3(c) shows the binary digital form of electronic signal. Computer messages are com-

posed of bytes, or groups of binary digits (group of eight bits) conveys the presence or absence of

voltage in an electronic signal. For the byte shown in Figure 1.3(c) the voltage goes successively

off, on, off, off, off, off, off, on.

Figure 1.3 (c)

Electronicpulses for

Binary Digits

Figure 1.3 (d)

Two digital

signals with

different bits

per second

rate

Figure 1.3(d) shows two digital signals, each made up of 0s (zero volts, white space) and 1s

(five-volt pulses), are depicted as transmitted at different rates over a 1/50- second interval. The

top signal carries 300 bits per second (bps) the bottom one 2,400 bps.

Byte

0 1 0 0 0 0 0 1Bit

Off On Off Off Off Off Off On

0 Volts

5 Volts

5 Volts

0 Volts

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