Data Encoding 2

8/8/2019 Data Encoding 2

1/53

DATA ENCODINGDATA ENCODING

Data Encoding refers the various techniques of impressingdata (0,1) or information on an electrical, electromagnetic oroptical signal that would propagate through the physicalmedium making up the communication link between the twodevices.


2/53

ANALOG &DIGITALANALOG &DIGITALDifferencesDifferences

Analog Data &Digital Data Analog Data Take on continuous values on some

interval Eg. Voice, Video are continuously varying pattern

of intensity

Digital Data take on discrete value

Eg. Text and integers

Analog Signal & Digital Signal An analog signal is a continuous varying

electromagnetic wave propagate over a medium

Adigital signal is a sequence of voltage pulses

that may be transmitted over a wire medium


3/53


4/53

ADVANTAGE OF ANALOGADVANTAGE OF ANALOG

SIGNALSIGNAL

Most mediums support analog

transmission - used for wirelesscommunication

The telephone infrastructure provides a

relatively cheap individual point-to-point transmission


5/53

Information, Data and Signals

Information Data Signal

001011101


6/53

Information, Data and Signals Data - A representation of facts,

concepts, or instructions in a formalizedmanner suitable for communication,interpretation, or processing by humanbeings or by automatic means

Information - The meaning that iscurrently assigned to data by means ofthe conventions applied to those data


7/53

Computers Use Signals for

Communcation Computers transmit data using digital

signals, sequences of specified voltagelevels. Graphically they are oftenrepresented as a square wave.

Computers sometimes communicate

over telephone line using analogsignals, which are formed bycontinuously varying voltage levels.


8/53

Figure 5-1

Different Conversion Schemes


9/53

Digital DataDigital Data -- DigitalDigital

SignalSignal Signal changes value as the data

changes value from 0 to 1 and 1 to 0

Several line encoding schemes arepossible. Each has pros and cons

Digital-to digital encoding fall underthree broad categories: unipolar, polarand bipolar


10/53

Figure 5-2

Digital to Digital Encoding


11/53

Encoding SchemesEncoding Schemes Unipolar encoding Polar encoding

Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Manchester Differential Manchester

Bipolar AMI Pseudoternary B8ZS HDB3


12/53

Encoding SchemesEncoding Schemes


13/53

Figure 5-3

Types of Digital to Digital Encoding


14/53

BASIC TERMSBASIC TERMS To Understand different Encoding schemes

first we will have to understand following

terminologies Unipolar -All signal elements have same sign

Polar -One logic state represented bypositive voltage the other by negative voltage

Data rate- Rate of data transmission in bitsper second

Duration or length of a bit-Time taken fortransmitter to emit the bit


15/53

BASIC TERMSBASIC TERMS Modulation rate-Rate at which the signal

level changes

Measured in baud = signal elements per second Mark and Space- Binary 1 and Binary 0

respectively

Differential encoding Signal is decoded

by comparing the polarity of adjacent signalelements

Multilevel Binary-Use more than two levels


16/53

Unipolar encoding Unipolar encoding is very simple and

primitive. It uses only one polarity

This polarity is assigned to one of thetwo binary states, usually the 1.

The other state, usually the 0, is

represented by the zero voltage.


17/53

Figure 5-4

Unipolar Encoding


18/53

Figure 5-5

Types of Polar Encoding


19/53

Polar encoding Polar encoding uses two voltage levels;

one positive and one negative.

In Polar encoding there are three mostpopular variations:

Nonreturn to zero (NRZ)

Return to zero (RZ)

Biphase


20/53

Polar encoding NRZ refers to Nonreturn to zero, invert (NRZ-I) and nonreturn to zero, level (NRZ-L).

Biphase also refers to two methods, the firstis the Manchester method used by EthernetLANs.

And the second is Differential Manchestermethod used by Token Ring LANs.


21/53

Nonreturn to zero encoding In NRZ-I, an inversion of the voltage level

represents a 1 bit, it is the transition between

a negative and a positive voltages. A0 bit represented by no change.

Note: In NRZ-I the signal is inverted if a 1 isencounted.

In NRZ-L sequence, positive and negativehave specific meanings; positive for 0 andnegative for 1.


22/53

Figure 5-6

NRZ-L and NRZ-I Encoding


23/53

RZ Encoding RZ encoding uses three values; positive,

negative and zero.

In RZ, signal changes not between bitsbut during each bit.

A1 bit is actually represented by

positive-to-zero and a 0 bit by negative-to-zero rather than negative andpositive alone.


24/53

Figure 5-7

RZ Encoding


25/53

Biphase encoding In Biphase method the signal changes

at the middle of bit interval but does

not return to zero. Instead it continuesto the opposite pole.

The two types of Biphase encoding in

use on networks today are Manchesterand Differential Manchester.


26/53

Manchester encoding Manchester encoding uses the inversion

at the middle of each bit interval for

both synchronization and bitrepresentation

Anegative to positive transition

represents binary 1 and positive tonegative transition represents thebinary 0.


27/53

Differential Manchester encoding Differential Manchester, the inversion at

the middle of the bit interval is used for

synchronization. And the presence or absence of an

additional transition at the beginning ofthe interval is used to identify the bit.

A transition means binary 0 and notransition means binary 1.


28/53

Figure 5-8Manchester and Diff. Manchester Encoding


29/53

Bipolar encoding Bipolar encoding, uses three voltage

levels; positive, negative and zero.

However the zero level in bipolar encodingused to represent the binary 0.

The 1s are represented by alternating

positive and negative voltages.


30/53

Figure 5-9

Types of Bipolar Encoding


31/53

Bipolar Alternate Mark Inversion

(AMI) Bipolar Alternate Mark Inversion (AMI)

is the simplest type of the bipolar

encoding. AMI means alternate 1inversion.

A variation of bipolar AMI is called

pseudoternary in which binary 0alternates between positive andnegative voltage.


32/53

Figure 5-10

Bipolar AMI Encoding


33/53

Bipolar 8-zero Substitution

(B8ZS) B8ZS is the conversion to provide

synchronization of a long string of 0s.

The B8ZS function identically to bipolar AMI,their different occurs whenever eight or moreconsecutive 0s are encountered in the datastream.

B8ZS provide the artificial signal changes,called the violations between the 0 string.


34/53

Bipolar 8-zero Substitution

(B8ZS) Any time eight 0s occur in succession, B8ZS

introduces changes in the pattern based on

the polarity of the previous 1. If the previous 1 was positive, the eight 0s

will be encoded as zero, zero, zero, positive,negative, zero, negative, positive.

If the polarity of the previous 1 is negative,the pattern of violation is the same but withinverted polarities.


35/53

Figure 5-11

B8ZS Encoding


36/53

High Density Bipolar 3 (HDB3) The HDB3 convention, introduces

changes into the bipolar AMI pattern

every time four consecutive 0s areencountered instead of waiting for theeight expected by B8ZS.


37/53

Figure 5-12

HDB3 Encoding


38/53

Example 5.1Using B8ZS encode the bit stream

10000000000100. Assume that the

polarity of the first 1 is positive.


39/53

Figure 5-13

Solution to Example 5.1


40/53

Example 5.2Using HDB3, encode the bit stream

10000000000100. Assume that the

number of 1s so far is odd and the first1is positive.


41/53

Figure 5-14

Solution to Example 5.2


42/53

Analog Encoding of DigitalAnalog Encoding of Digital

DataData Data encoding and decoding technique

to represent data using the properties

of analog waves

Modulation: combining signal wave withcarrier wave

Demodulation: splitting signal wavefrom carrier wave


43/53

Digital DataDigital Data -- AnalogAnalog

SignalsSignals Transmitting digital data through PSTN

300Hz to 3400Hz

Use modem (modulator-demodulator) Modem is the device used to convert

digital data to analog signals and viceversa

Uses a constant-frequency signal known as a carriersignal

Converts a series of binary voltage pulses into ananalog signal by modulating the carrier signal

The receiving modem translates the analog signal

back into digital data


44/53

Analog Encoding of DigitalAnalog Encoding of Digital

DataDataThere are three techniques to convert

Digital Data into Analog Signal

Amplitude shift keying (ASK)

Frequency shift keying (FSK)

Phase shift keying (PK)


45/53

Modulation TechniquesModulation Techniques


46/53

Amplitude Shift KeyingAmplitude Shift Keying Values represented by different amplitudes of

carrier

Usually, one amplitude is zero i.e. presence and absence of carrier is used

Susceptible to sudden gain changes

Inefficient

Used over optical fiber using LED or LaserTransmitter


47/53

Amplitude Shift Keying


48/53

Amplitude Shift KeyingAmplitude Shift Keying

(ASK)(ASK) In radio transmission, known as amplitude

modulation (AM)

The amplitude (or height) of the sine wavevaries to transmit the ones and zeros

Major disadvantage - telephone lines are verysusceptible to variations in transmissionquality that affect amplitude


49/53

Frequency Shift KeyingFrequency Shift Keying

(FSK)(FSK) In radio transmission, known as frequency

modulation (FM)

The frequency of the carrier wave varies inaccordance with the signal to be sent

Signal is transmitted at constant amplitude

More immune to noise than

ASK

Requires more analog bandwidth than ASK


50/53

Frequency Shift KeyingFrequency Shift KeyingValues represented by different

frequencies (near carrier)

Less susceptible to error than ASK High frequency radio transmission 3 to

30 MHz


51/53

Frequency Shift KeyingFrequency Shift Keying


52/53

Phase Shift Keying (PSK)Phase Shift Keying (PSK) Also known as phase modulation (PM)

Frequency and amplitude of the carrier signal

are kept constant The carrier is shifted in phase according to

the input data stream

Each phase can have a constant value, or

value can be based on whether or not phasechanges (differential keying) Phase shifted relative to previous transmission

rather than some reference signal


53/53

Data Encoding 2

Documents

Transcript of Data Encoding 2