Data Communication and Computer Networks

[EEE 314]

Mubashir Husain RehmaniMubashir Husain Rehmani

Assistant Professor

Department of Electrical Engineering

COMSATS Institute of Information Technology,

Wah Cantt, Pakistan

Digital

Transmission

Conversion Methods

Digital

to

Digital Digital

Encoding

Line Line coding is the process of coding is the process of

converting digital data to digital converting digital data to digital

signalssignalssignalssignals

Line Coding

Signal Level vs Data Level

Lack of Synchronization

Line Coding

Unipolar encoding uses only one

voltage level.

In a unipolar scheme, all the

signal levels are on one side of

the time axis, either above

or below.

UniPolar Encoding

Polar encoding uses two voltage Polar encoding uses two voltage

levels (positive and negative).levels (positive and negative).

Types of Polar Encoding

� NRZ � Non Return to Zero

� NRZ-L � Non Return to Zero - Level

� NRZ-I � Non Return to Zero – Inverted

In NRZIn NRZ--L the level of the signal is L the level of the signal is

dependent upon the state of the dependent upon the state of the

bit.bit.bit.bit.

In NRZIn NRZ--I the signal is inverted if a I the signal is inverted if a

1 is encountered.1 is encountered.

NRZ-L and NRZ-I

RZ Encoding

Return-to-zero (RZ) scheme uses three values: positive,

negative, and zero. In RZ, the signal changes not

between bits but during the bit. The signal goes to 0 in the

middle of each bit. It remains there until the beginning of

the next bit.

Manchester Encoding

In Manchester encoding, the duration of the bit is divided into

two halves. The voltage remains at one level during the first

half and moves to the other level in the second half.

In Manchester encoding, the In Manchester encoding, the

transition at the middle of the bit transition at the middle of the bit

is used for both synchronization is used for both synchronization is used for both synchronization is used for both synchronization

and bit representation.and bit representation.

Differential Manchester

Encoding

In differential Manchester In differential Manchester

encoding, the transition at the encoding, the transition at the

middle of the bit is used only for middle of the bit is used only for

synchronization. synchronization. synchronization. synchronization.

The bit representation is defined The bit representation is defined

by the inversion or by the inversion or noninversionnoninversion

at the beginning of the bit.at the beginning of the bit.

In bipolar encoding, we use three In bipolar encoding, we use three

levels: positive, zero, levels: positive, zero,

and negative.and negative.and negative.and negative.

Types of Bipolar Encoding

BiPolar AMI Encoding

B8ZS Encoding

Analog

to

Digital Digital

Encoding

Analog to Digital Encoding

Pulse Code Modulation (PCM)

� The Analog signal is Sampled

� The Sampled signal is Quantized

� The Quantized values are encoded as stream

of bitsof bits

Sampling: PAM

Quantized PAM Signal

Digital

to

AnalogAnalog

Encoding

Digital to Analog Encoding

Bit rate is the number of bits per

second. Baud rate is the number of

signal units per second. Baud rate is signal units per second. Baud rate is

less than or equal to the bit rate.

ASK

In amplitude shift keying, the amplitude of the carrier signal is

varied to create signal elements. Both frequency and phase

remain constant while the amplitude changes.

FSK

In frequency shift keying, the frequency of the carrier

signal is varied to represent data.

PSK

In phase shift keying, the phase of the carrier is varied to

represent two or more different signal elements. Both peak

amplitude and frequency remain constant as the phase

changes.

PSK Consetllation

A constellation diagram can help us define the amplitude

and phase of a signal element

4-PSK Method

4-PSK Constellation

8-PSK

QAM

Quadrature amplitude modulation is a

combination of ASK and PSK so that a

maximum contrast between each signal maximum contrast between each signal

unit (bit, dibit, tribit, and so on) is

achieved.

Table 5.1 Bit and baud rate comparison

ModulationModulation UnitsUnits Bits/BaudBits/Baud Baud rateBaud rate Bit Rate

ASK, FSK, 2ASK, FSK, 2--PSKPSK Bit 1 N N

44--PSK, 4PSK, 4--QAMQAM Dibit 2 N 2N

88--PSK, 8PSK, 8--QAMQAM Tribit 3 N 3N

1616--QAMQAM Quadbit 4 N 4N

3232--QAMQAM Pentabit 5 N 5N

6464--QAMQAM Hexabit 6 N 6N

128128--QAMQAM Septabit 7 N 7N

256256--QAMQAM Octabit 8 N 8N

Transmission Mode

Parallel Transmission

Serial Transmission

In asynchronous transmission, In asynchronous transmission,

we send 1 start bit (0) at the we send 1 start bit (0) at the

beginning and 1 or more stop beginning and 1 or more stop beginning and 1 or more stop beginning and 1 or more stop

bits (1s) at the end of each byte. bits (1s) at the end of each byte.

There may be a gap between There may be a gap between

each byte.each byte.

Asynchronous here means Asynchronous here means

“asynchronous at the byte level,” “asynchronous at the byte level,”

but the bits are still but the bits are still but the bits are still but the bits are still

synchronized; their durations are synchronized; their durations are

the same.the same.

Asynchronous Tranmission

In synchronous transmission, In synchronous transmission,

we send bits one after another we send bits one after another

without start/stop bits or gaps. without start/stop bits or gaps. without start/stop bits or gaps. without start/stop bits or gaps.

It is the responsibility of the It is the responsibility of the

receiver to group the bits.receiver to group the bits.

Synchronous Tranmission

Modulation of Analog Signals

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation (PM)

Analog to Analog Modulation

Types of Analog Modulation

AM

FM