Computer Networks UNIT-3

8/7/2019 Computer Networks UNIT-3

1/19

C o n f i d e n t i a l

Course : BCA

Semester : IV

Subject Code : BC 0048

Subject Name : Computer Networks

Unit number : 3

Unit Title : Data Communications

HOME NEXT


2/19


Unit-3 Data Communications

Data Communications

Objectives

After going through the presentation, you should be able to:

Describe Theoretical basis for communication

Discuss Transmission impairments

Describe different Data transmission modes

HOME NEXTPREVIOUS


3/19



Lecture outline

Introduction

Theoretical Basis for Data Communication

Transmission impairments

Data transmission modes

Summary

Data Communications

NEXTPREVIOUS


4/19



Introduction

We have two types of signals digital and analog.

Digital data are discrete and analog data is continuous data.

The signal is never received as it was transmitted even if a channel is

directly connected between a transmitter and receiver. That is there is

some change in the signal due to various reasons, which we refer as

Transmission impairments.

We will be studying various impairments throughout this presentation

HOME NEXTPREVIOUS


5/19


6/19




Maximum data rate of a channel :

In 1924, H. Nyquist realized the existence of the fundamental limit and

derived the equation expressing the maximum data for a finite bandwidth

noiseless channel.

In 1948, Claude Shannon carried Nyquist work further and extended it to

the case of a channel subject to random noise.

It is the ratio of signal to noise power that is important, rather than the

noise power alone. This Signal--to-Noise Ratio (SNR), usually expressed

in decibel (dB), is one of the most important specifications of any

communication system.

HOME NEXTPREVIOUS


7/19




Maximum data rate of a channel :(Continued..)

If the signal consists of V discrete levels, then Nyquist theorem states

that, for a noiseless channel

Maximum data rate = 2H.log2 (V) bits per second.

For a noisy channel with bandwidth is again H, knowing signal to noiseratio S/N, the maximum data rate according to Shannon is given as

Maximum data rate = H.log2 (1+S/N) bits per second.

HOME NEXTPREVIOUS


8/19




Transmission lines suffers from three major problems :

1. Attenuation distortion

2. Delay distortion

3. Noise

1. Attenuation distortion:

It is the loss of energy as the signal propagates outward. The amount of

energy depends on the frequency.

If the attenuation is too much, the receiver may not be able to detect the

signal at all, or the signal may fall below the noise level.

For reliable communication, the attenuation and delay over the range of

frequencies of transmission should be constant.

HOME NEXTPREVIOUS


9/19




2. Delay distortion :

The second transmission impairment is delay distortion.

It is caused by the fact that different Fourier components travel at

different speed.

It is due to velocity of propagation that varies with frequency.

3. Noise :

Noise is a third impairment. It can be define as unwanted energy from

sources other than the transmitter.

Thermal noise is caused by the random motion of the electrons in a wire

and is unavoidable.

Source of Noise are Thermal, Intermodulation, Cross talk, Impulse

HOME NEXTPREVIOUS


10/19




Serial & Parallel :

Serial Communication :

Serial communication is the sequential transmission of the signal

elements of a group representing a character or other entity of data.

The characters are transmitted in a sequence over a single line, rather

than simultaneously over two or more lines.

HOMEPREVIOUS NEXT


11/19


12/19




The comparisons of the serial and parallel transmissionmodes :

HOMEPREVIOUS

Serial mode Parallel mode

Cost Less costly (only one wire) More costly (many wires)

Speed Low ( only 1 bit at a time) High (more bits at a time)

Throughput Low High

Used in Longer distance comm. Shorter distance comm..

NEXT


13/19




Simplex, Half duplex & Full duplex Communication :

Simplex Communication :

Simplex refers to communications in only one direction from thetransmitter to the receiver.

There is no acknowledgement of reception from the receiver, so errors

cannot be conveyed to the transmitter.

Ex : A radio is a simplex device.

Fig : simplex Communication

HOMEPREVIOUS

A B

NEXT


14/19




Half duplex Communication :

Half-duplex refers to two-way communications but in only one direction

at a time.

Ex : A walkie-talkie

Fig : Half duplex Communication

HOMEPREVIOUS

A B

NEXT


15/19




Full duplex Communication :

Full duplex refers to simultaneous two-way transmission.

Ex : Certain computer video cards are full-duplex devices.

HOMEPREVIOUS

A B

NEXT


16/19




Synchronous & Asynchronous transmission :

Synchronous Transmission

Synchronous is any type of communication in which the parties

communicating are "live" or present in the same space and time.

Ex : A Chat room.

Advantage:

This reduces overhead bits

It overcomes the two main deficiencies of the asynchronous method,

that of inefficiency and lack of error detection.

Disadvantage:

For correct operation the receiver must start to sample the line at

the correct instant

HOMEPREVIOUS NEXT


17/19




Asynchronous transmission :

Asynchronous refers to processes that proceed independently of each

other until one process needs to "interrupt" the other process with a

request.

E-mail is an example of an asynchronous mode of communication where

one party can send a note to another person and the recipient need not

be online to receive the e-mail.

HOMEPREVIOUS NEXT


18/19




Advantages and disadvantages Of Asynchronous transmission:

Advantage:

The character is self contained & Transmitter and receiver need not

be synchronized

Transmitting and receiving clocks are independent of each other

Disadvantage:

Overhead of start and stop bits

False recognition of these bits due to noise on the channel

HOMEPREVIOUS NEXT


19/19



Summary

We have two types of signals digital and analog.

Digital data are discrete and analog data is continuous data

For the noiseless channel Maximum data rate = 2H.log2 (V) bits per

second

For the noisy channel Maximum data rate =H.log2 (1+S/N) bits per

second .

Simplex means communication in one direction, Half-duplex refers to

two-way communications , Full duplex refers to simultaneous two-

way transmission.

HOMEPREVIOUS

Documents

Computer Networks UNIT-3