DATA ENCODINGDATA ENCODING

Data Encoding refers the various techniques of impressing data (0,1) or information on an electrical, electromagnetic or optical signal that would propagate through the physical medium making up the communication link between the two devices.

ANALOG &DIGITAL ANALOG &DIGITAL DifferencesDifferences

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

A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium

ADVANTAGE OF ADVANTAGE OF DIGITAL SIGNALDIGITAL SIGNAL The signal is exact Signals can be checked for errors Noise/interference are easily filtered

out A variety of services can be offered

over one line Higher bandwidth is possible with

data compression

ADVANTAGE OF ADVANTAGE OF ANALOG SIGNALANALOG SIGNAL

Most media support analog transmission - used for wireless communication

The telephone infrastructure provides a relatively cheap “individual point-to-point” transmission

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

concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means

Information - The meaning that is currently assigned to data by means of the conventions applied to those data

Information, Data and Signals

Information Data Signal

001011101

Computers Use Signals for Communication Computers transmit data using digital

signals, sequences of specified voltage levels. Graphically they are often represented as a square wave.

Computers sometimes communicate over telephone line using analog signals, which are formed by continuously varying voltage levels.

Figure 5-1

Different Conversion Schemes

Digital Data - Digital Digital Data - Digital SignalSignal Signal changes value as the data

changes value from 0 to 1 and 1 to 0

Several line encoding schemes are possible. Each has pros and cons

Digital-to-digital encoding fall under three broad categories: unipolar, polar and bipolar

Figure 5-2

Digital to Digital Encoding

Encoding SchemesEncoding Schemes Unipolar encoding Polar encoding

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

Bipolar –AMI Pseudoternary B8ZS HDB3

Encoding SchemesEncoding Schemes

Figure 5-3

Types of Digital to Digital Encoding

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 by positive

voltage the other by negative voltage Bipolar- uses three voltage levels; positive,

negative, and zero. Data rate- Rate of data transmission in bits per

second Duration or length of a bit-Time taken for

transmitter to emit the bit.

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 signal elements

Multilevel Binary-Use more than two levels

Unipolar encoding Unipolar encoding is very simple

and primitive. It uses only one polarity

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

The other state, usually the 0, is represented by the zero voltage.

Figure 5-4

Unipolar Encoding

Figure 5-5

Types of Polar Encoding

Polar encoding Polar encoding uses two voltage

levels; one positive and one negative.

In Polar encoding there are three most popular variations: Nonreturn to zero (NRZ) Return to zero (RZ) Biphase

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 first is the Manchester method used by Ethernet LANs.

And the second is Differential Manchester method used by Token Ring LANs.

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.

A 0 bit represented by no change.Note: In NRZ-I the signal is inverted if a 1 is encounted.

In NRZ-L sequence, positive and negative have specific meanings; positive for 0 and negative for 1.

Figure 5-6

NRZ-L and NRZ-I Encoding

RZ Encoding RZ encoding uses three values;

positive, negative and zero. In RZ, signal changes not between

bits but during each bit. A 1 bit is actually represented by

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

Figure 5-7

RZ Encoding

Biphase encoding In Biphase method the signal

changes at the middle of bit interval but does not return to zero. Instead it continues to the opposite pole.

The two types of Biphase encoding in use on networks today are Manchester and Differential Manchester.

Manchester encoding Manchester encoding uses the

inversion at the middle of each bit interval for both synchronization and bit representation

A negative to positive transition represents binary 1 and positive to negative transition represents the binary 0.

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 of the interval is used to identify the bit.

A transition means binary 0 and no transition means binary 1.

Figure 5-8Manchester and Diff. Manchester Encoding

Bipolar encoding Bipolar encoding, uses three voltage

levels; positive, negative and zero. However the zero level in bipolar

encoding used to represent the binary 0.

The 1s are represented by alternating positive and negative voltages.

Figure 5-9

Types of Bipolar Encoding

Bipolar Alternate Mark Inversion (AMI)

Bipolar Alternate Mark Inversion (AMI) is the simplest type of the bipolar encoding. AMI means alternate 1 inversion.

A variation of bipolar AMI is called pseudoternary in which binary 0 alternates between positive and negative voltage.

Figure 5-10

Bipolar AMI Encoding

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 more consecutive 0s are encountered in the data stream.

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

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 with inverted polarities.

Figure 5-11

B8ZS Encoding

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

changes into the bipolar AMI pattern every time four consecutive 0s are encountered instead of waiting for the eight expected by B8ZS.

Figure 5-12

HDB3 Encoding

Example 5.1

Using B8ZS encode the bit stream 10000000000100. Assume that the polarity of the first 1 is positive.

Figure 5-13

Solution to Example 5.1

Example 5.2

Using HDB3, encode the bit stream 10000000000100. Assume that the number of 1s so far is odd and the first 1 is positive.

Figure 5-14

Solution to Example 5.2

END