DATA COMMUNICATIONS & NETWORKING

LECTURE-08

Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

LECTURE OVERVIEW Conversion methods Line coding Characteristics of line coding Signal level & Data Level Pulse Rate & Bit Rate DC Component Self Synchronization Line coding Schemes Unipolar Polar NRZ RZ Manchester and Differential Manchester Bipolar

CONVERSION METHODS OR ENCODING & MODULATION

How information is transformed depends on its original format and on the format used by the communication hardware.

Conversion methods Digital-to-digital conversion or Line coding Analog-to-digital conversion or Sampling Digital -to- analog modulation Analog -to -analog modulation

LINE CODING

Line coding is the process of converting binary data, a sequence of bits to a digital signal.

In other words, line coding converts a sequence of bits to a digital signal.

At the sender, digital data are encoded in to digital signal.

At the receiver, the digital data are recreated by decoding the digital signal back in to digital data.

LINE CODING & DECODING

SIGNAL LEVEL & DATA LEVEL The number of values allowed in a particular signal is

known as the number of signal levels. The number of values used to represent data is known

as the number of data levels.

DATA ELEMENT VERSUS SIGNAL ELEMENT

A data Element is the smallest entity that can represent a piece of information this is the bit.

A signal element is the shortest unit time wise of a digital signal.

In other words data elements are what we need to send.

Signal elements are what we can send. Data elements are being carried, signal

elements are the carriers. r is defined as the ratio which is the number of

data elements carried by each signal element.

DATA ELEMENT VERSUS SIGNAL ELEMENT

DATA RATE VERSUS SIGNAL RATE

The data rate is defined as the number of data elements sent in one second usually expressed in bps.

Data rate is also called Bit rate. The signal rate is defined as the number of signal

elements sent in one second. A signal rate is also called a pulse rate. A signal element is also called a symbol or pulse. One pulse or signal element can carry more than one bit. When a pulse or signal element carry only one bit then

the bit rate and pulse rate/signal rate are same. The signal rate is sometimes called modulation rate or

baud rate.

FORMULA FOR BIT RATE

Bit Rate=Pulse Rate x x log2 L

Where L is the number of data Levels.Where L is the number of data Levels.

Example 1Example 1

A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:

Pulse Rate = 1/ 10Pulse Rate = 1/ 10-3-3= 1000 pulses/s= 1000 pulses/s

Bit Rate = Pulse Rate x logBit Rate = Pulse Rate x log22 L = 1000 x log L = 1000 x log22 2 = 1000 bps 2 = 1000 bps

SolutionSolution

Example 2Example 2

A signal has four data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:

Pulse Rate = = 1000 pulses/sPulse Rate = = 1000 pulses/s

Bit Rate = Pulse Rate x logBit Rate = Pulse Rate x log22 L = 1000 x log L = 1000 x log22 4 = 2000 bps 4 = 2000 bps

SolutionSolution

DC COMPONENT

When the voltage level is constant for a while very low frequencies are often created.

These frequencies around zero are called DC or Direct Current components.

This component is undesirable for two reasons. First if a signal has to pass through a system (e.g. a

telephone line which does not allow frequencies below 200 Hz)

which does not allow low frequencies to pass the signal is distorted and may create errors in the output.

Second this component is extra energy residing on the line and is useless.

DC COMPONENT

SELF-SYNCHRONIZATION To correctly interpret the signals received from the

sender, the receiver’s bit intervals must correspond exactly to the sender’s bit intervals.

If the receiver’s clock is faster or slower, the bit intervals are not matched and the receiver might interpret the signals differently then the sender intended.

A self synchronizing digital signal includes timing information in the data being transmitted.

This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle or end of the pulse.

If the receiver clock is out of synchronization, these alerting points can reset the clock.

LACK OF SYNCHRONIZATION

Example 3Example 3

In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps?

SolutionSolution

At 1 Kbps:1000 bits sent 1001 bits received1 extra bpsAt 1 Mbps: 1,000,000 bits sent 1,001,000 bits received1000 extra bps

LINE CODING SCHEMES

UNIPOLAR SCHEME Digital transmission systems work by sending voltage

pulses along a medium link, usually a wire or cable. In many types of encoding, one voltage level stands for

binary 0,another level stands for binary 1. The polarity of a pulse refers to whether it is positive or

negative. Unipolar encoding is so named because it uses only one

polarity. The polarity is assigned to one of the two binary states,

usually the 1. The other state usually 0 is represented by zero voltage.

UNIPOLAR SCHEME

PROBLEMS USING UNI POLAR SCHEME

DC Component The average amplitude of a unipolar encoded signal is

non zero. This creates a DC component.

Lack of synchronization If the data contain a long sequence of 0’s and 1’s

there is no change in the signal during this duration that can alert the receiver to potential synchronization problems.

POLAR ENCODING SCHEMES

Polar encoding uses two voltage levels, one positive and one negative.

By using the two Levels, in most polar encoding methods the average voltage level on the line is reduced and the dc component problem seen in unipolar encoding is alleviated.

NRZ(NON RETURN TO ZERO) SCHEMES

In NRZ encoding the value of the signal is always positive or negative.

Two popular forms of NRZ are: NRZ-L(Non Return to Zero-Level) In NRZ-L the level of the signal depends on the type of

bit that it represent. The positive voltage usually means a 0 while a negative

voltage means the bit is a 1. Problem: Long stream of 0’s or 1’s may create

synchronization problem.

NRZ(NON RETURN TO ZERO) SCHEMES

NRZ-I(NRZ-Invert) In NRZ-I ,an inversion of the voltage level represents a 1 bit. It is the transition between a positive and negative voltage, not the

voltage itself, that represents a 1 bit. A 0 bit is represented by no change. NRZ-I is superior to NRZ-L due to the synchronization provided by

the signal change each time a 1 bit is encountered. The existence of 1’s in the data stream allows the receiver to

synchronize its timer to the actual arrival of transmission. A string of 0’s can still cause problems but because they are not as

Likely,they are less of a problem.

NRZ(NON RETURN TO ZERO) SCHEMES

THANKS!! !