Chapter No 04

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Communication Techniques (9116)

Compiled By:- Mr. Joshi R.R. Page 1

Chapter No:-04

DATA ENCODING AND TRANSMISSION

Introduction:-

Information must be transformed into signal before it can be

transported across a communication channel. However information which

is transformed depends on its original format and on the format used by

the communication hardware.

Information must be translated into agreed upon patterns pf 0s and

1s (e.g. ASCII) before transmission.

Types Of Conversion/Classification:-

1) Data stored in a computer are in the form of 0s and 1s to be carried

from one place to another data are usually converted in the digital form

(or signal).This is called as encoding digital data into digital signal.2) Sometimes we need to convert an analog signal into digital signal for

several reasons, such as to minimize effect of noise .This is called as a

Analog To-Digital Conversion.

3) Sometimes we want to send a digital signal coming out of a computer

through a medium designed for an analog signal. In this case signal

from computer should be converted to an analog signal. This is called

as Digital To Analog Conversion.

4) Analog signal is sent over long distance using analog media. However,frequency of these media is not capable of this kind of transmission

through air medium.So,it should be carried off by a HF signal and the

process is called Analog to AnalogConversion.


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Digital-To-Digital Conversion:-

Encoding:-Encoding is a process of converting a signal into a coded form. The

different types of encoding techniques are as given below.

Encoding Techniques

Source Coding Line coding

Source Coding:-These techniques are used to convert the analog data signal into

equivalent digital data. Sampling and source coding is done in every digital

transmission system.

Line Coding:-

This is the process of converting binary data into a digital data .The

data text images stored in computer memory are in format sequence ofbits. Line coding converts the sequence into digital signal.

Line coding

Definitions:-

1) Pulse Rate:-Pulse Rate is defined as the number of pulses per second and a pulseis defined as the minimum amount of time required to transmit a symbol.


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2) Bit Rate :-Bit Rate is defined as the number of bits per second. If one pulse

corresponds to one bit, then the pulse rate is equal to the bit rate. But ifa pulse carries more than one bit then the pulse rate is lower than bit

rate. The relation between bit rate and pulse rate is

Bit Rate = Pulse Rate * log2L

Where, L= No. of levels (2-levels,4-levels,etc.)

Encoding Techniques:-Classification of Line Codes (Encoding Techniques)

Unipolar Polar Bipolar

RZ NRZ RZ NRZ Manchester Diff. Manchester AMI

B8ZS

HDB3

Line codes are basically divided into three categories:

1) Unipolar2) Polar3) Bipolar


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1) Unipolar Codes:-

Unipolar codes use only one voltage level other than zero.So,

the encoded signal will have either +A volts value or 0 v. These codesare very simple and primitive and are almost obsolete now days.

2) Polar codes:-Polar codes uses two voltage levels other than zero, such as

+A/2 and A/2 volts. This will bring the DC level for some codes to

zero which is a desired character.

3) Bipolar Codes :-Bipolar coding uses three voltage levels i)Positive ,ii)Negative

and iii)Zero ,which is similar to polar codes .But here the zero level is

always used for representing 0 of data stream at input.

Important Properties of Line Codes:-1) All cable systems and other communication system do not allow

transmission of a DC signal .Therefore the line signal must have a zero

average (DC value). NRZ bipolar format usually satisfy this requirement

.For this reason long string of element sequences having same polarity

should not be transmitted.

2) As the code adds redundancy, the code efficiency should be as high as

possible.

3) To ensure synchronization at the receiver the line signal should undergo

a sufficient number of zero crossings, that means transmitted signal

should always undergo transitions.

4) The crosstalk between channels should be minimized.


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Unipolar Line Codes:-

A] Unipolar RZ format:-

Data 0 1 1 0

A

Unipolar

RZ Signal

t

Tb

In this format each 0 is represented by an off pulse(0v) and each

one pulse for 1 with amplitude +A and duration of Tb/2 followed by

return to zero value .Thats why this format is called as return to zero

format. As voltage level is either +A or zero ,this is a unipolar format. Due

to the unipolar nature, the unipolar RZ format has a non-zero DC value i.e.

does not contain any information.

B] Unipolar NRZ Format:-

Data 1 0 1 1 0

Unipolar A +A

NRZ Signal t

In this format a logic 1 is represented by a pulse of full bit duration

Tb and an amplitude +A while a logic 0is represented by on off pulse or

zero amplitude .During the on time ,the pulse does not return to zero after

half bit period. Therefore, the name NRZ format. As pulse have either +A or

0v amplitude .It is called as unipolar format. E.g. internal computer W/Fs


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are usually of this type. Due to longer pulse duration, the NRZ pulse carries

more energy than the RZ pulses. But they need synchronization at receiver

as there is no separation between pulses.

Polar Line Codes :-

A] Polar RZ format :-

1 0 1 1

+A/2

t

Tb/2

-A/2

Tb

The advantage of the two unipolar formats is that they results in a

DC component that does not carry any information and waste power.

The polar RZ format as shown in fig. has opposite polarity pulses of

amplitude +-A/2 are used to represent logic 1 and logic 0.Therefore it is

called as a polar format. As the pulses return to zero after half the bit

duration Tb/ . This format is a RZ format.


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B] Polar NRZ format:-

Data 1 0 1 1 0

Polar +A/2

NRZ Signal t

-A/2

In polar NRZ format, a pulse of amplitude +A/2 of duration Tb is used

to represent a logic 1 and pulse of amplitude -A/2 of duration Tb

represents a logic 0.

Unlike the unipolar w/f, a polar w/f has no DC component if the 0s

and 1s in the data occurs in equal propagation.

C] Split Phase Manchester Format :-

Data 1 0 1 1+A/2

Manchester

Signal t

-A/2


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In split Manchester format symbol 1 is represented by transmitting

positive pulse of +A/2 amplitude for one half of the symbol is followed by

negative pulse of amplitude A/2 for remaining half of the symbol dur . For

symbol 0 these two pulses are transmitted in reverse order .The w/f does

not have any DC component.

D] Differential Manchester code:-

Data 0 0 1 0 1

Tb

Differential Manchester

t

Tb/2

In this code there is always a transition in the middle of a bit interval.

The binary no. 0(zero) has an additional transition at the beginning of the

bit interval.

Bipolar Line Codes:-

A) Bipolar NRZ Format (AMI) :-

Data 1 0 1 1 0

Bipolar NRZ Format

+A


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t

-A

In bipolar NRZ format successive 1 is represented by pulse with

alternating polarity and no pulse is transmitted for a logic 0.Note that in

this representation there are three levels +A,0,-A.Therfore this is also

known as Pseudo Ternary or alternate mark division format.

The most important feature of this formation is the absence of a DC

component eventhough the input binary data may contain long strings of0s sand 1s. Moreover the bipolar format eliminates ambiguity that may

arise because of the polarity inversion during the course of transmission,

this is the reason why the bipolar NRZ format is used in the PCM-TDM T1

system for digital telephony.

The absence of the DC component allows the use of transformation

for coupliry.

B) Bipolar 8-Zero Substitution(B8ZS) :-

It is necessary in order to have sync. Between the transmitter and

receiver the line code needs to cross the zero line frequency. As per U.S.

T1 standards, not more than 15 0s can be sent in succession to ensure

proper synchronization.

In order to solve the problem related to synchronization a new line

code called B8ZS was developed, whenever eight successive 0s aredetected, the implementation of this line code will automatically insert a

special 8-bit sequence containing a bipolar violation. This can be easily

detected and corrected by CSU/DSU(Channel/Digital Service Unit).


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e.g. Data Sent 11000000001

+ve 1 BVP 1

t

-ve 1 BPV

8 zeros

+ 0 0 0 0 0 0 0 0 Will change to

+ 0 0 0 + - 0 - +v

Possible bits

Polarity

The voltage levels in the violating byte have zero DC value.

C) High Density Bipolar Signaling(HDBS):-

In case of bipolar NRZ or AMI signal the transmitted signal is equal

to zero when a binary 0 is to be transmitted ,this is true even for the

unipolar RZ and unipolar NRZ signal .The absence of transmitted signal


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can cause problem in synchronization at the receiver if long sequence of

binary 0 are being transmitted.

This problem can be solved by adding pulses when long strings of0s exceeding a non are being transmitted. This type of coding is called as

high density bipolar coding .It is denoted by HDBN where N=1, 2,3.The

most widely used HDB format is N=3 i.e. HDB3.

In the string of message bits when (N+1) number of zeros occur

,they are replaced by special binary sequence of (N+1) length ,this

sequence contain some binary 1s which are necessary for Sync. at the

receiver. The HDB3 format is the converted into the bipolar NRZ (AM1)

format.

Message bits 1 0 1 1 0 0 0 0

HDB3 coded 1 0 1 1 0 0 0 v

signal

Bipolar NRZ t

equivalent

Some Other Line Code :-

1) 2B1Q(2 Binary 1 Quarternary) :-


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2)Polar Quarternary :-

The polar quarternary NRZ format delivered by grouping the

message bits in the blocks of two and using four amplitude levels

to represent the four possible combinations 00,01,10,11.

These four combination and four amplitude levels assigned

are shown in following table.

Message Combination X(t)= ak

0 0 -3A/2

0 1 -A/2

1 0 +A/2

1 1 +3A/2

Thus here for a message of two bits only one pulse of

duration D=2Tb is transmitted

D = 2Tb

Signaling rate = r=1/2Tb message/sec.

1 1

3A/21 0

A/2

t


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0 1

- A/2

-3A/2 0 0

3) NRZ level ,NRZ-mark ,NRZ-Space formats :-

Clock Clock Signal

Data 0 0 1 0 Data Stream

NRZ -L

High till next 1

data bit

NRZ -M

NRZ-S

In NRZ-L coding a bit 0 or 1 is represented by a voltage level which

remains constant during the bit duration. A binary one is represented by a

voltage high level and zero by voltage low level.


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In NRZ- Mthe w/f changes .its level when the binary digit is 1. The w/f

does not change its level when the binary digit is 0.

In NRZ Slevel w/f changes its level when binary digit is 0.The w/f

does not change its level when digit is 1.

Important Definitions:-

1) Bit Interval(Tb):-

Bit interval is time required to send one single bit.

+v

t

Tb

2) Bit Rate:-Bit rate is the number of bits transmitted in one second .It is

expressed in one second. It is expressed in bits per second (bps).

Relation between bit rate and bit interval:-

Bit Interval=1/Bit Interval

Bit rate is also called as signaling rate and it should be as high as

possible. With increase in bit rate the bandwidth of transmission medium

must be increased in order to transmit the signal without distortion.

3) Data Rate:-Data rate is the number of bits transmitted per second

.Increasing the number of bits per sample increases the bit rate which is

given as

D= nFs


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Where, D = Data Rate in bits /sec

Fs= Sample rate in samples per sec.

n = No. of bits per sample.

But extra bits are often included to detect and correct the errors .A

few bits called framing bits are also needed to ensure that transmitter

and receiver are in sync. The actual bit rate is therefore always greater

than theoretical rate.

e.g. The minimum data rate needed to transmit audio with a sampling

rate of 40 KHz and 14 bits per sample is,

D = nFs= 14 * 40 * 10^3

Data rate = 560 kps

4) Baud Rate or Bauds:-

Baud is the unit of signaling speed or modulation rate or the rate of

symbol transmission. It indicates the rate at which a signal level changes

over a given period of time. When binary bits are transmitted as an

electrical signal with two values 0 and 1,the bit rate and the modulation

rate i.e. baud rate are same.

Two level signals, bit rate and bauds are equal.

i/p bits 0 1 0 0 0 1 1

Tb

Bit Rate = 8 bits/sec

Baud Rate = 8 bauds


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If here four different levels are used to represent data.

10 1 0

4 11

levels 01

00 0 1 0 0 0 1 T= 1 sec

Bit Rate=8bits/sec, Baud Rate=4 level/sec. Bit rate is therefore not

equal to the baud rate.

5) Modulation Rate:-

The ratio of bit rate to transmission bandwidth is called modulation

rate or spectral efficiency.

6) Channel Capacity:-

The Shannon Hartley theorem gives a formula for the capacity of

channel when its bandwidth and noise level are known.


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C= B log2 (1+S/N) (*log2= log to the base 2)

Where, C = channel capacity in bps

B = Bandwidth in Hz

S/N= Ratio of total signal power to total

Random noise power at the

Inputs to the receiver within the

Frequency limits.

The channel capacity is denoted by C and is defined as themaximum possible bit rate supported by the channel without introducing

errors. The unit of channel capacity is bits/sec. This theorem shows a limit

that cannot be exceeded by the signaling speed in a channel in which noise

is purely random.

7) Channel Bandwidth:-

The channel bandwidth is defined as the range of signal

frequencies (say f1 to f2) over which the channel allows a distortion free

transmission. If the signal frequencies distortion while travelling over the

channel bandwidth is measured in Hz, Khz, Mhz, etc.

8) S/N Ratio:-

Signal to noise ratio is defined as the ratio of the signal power tothe noise power at the same point. Thus in S/N ratio denotes signal power

and N denotes the noise power.


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