Lecture 08

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.

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 log2 L
Where L is the number of data Levels.

Example 1

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

Solution
Pulse Rate = 1/ 10-3= 1000 pulses/s
Bit Rate = Pulse Rate x log2 L = 1000 x log2 2 = 1000 bps

Example 2
A signal has four data levels with a pulse duration of 1
ms. We calculate the pulse rate and bit rate as follows:
Solution
Pulse Rate = = 1000 pulses/s
Bit Rate = Pulse Rate x log2 L = 1000 x log2 4 = 2000 bps

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.

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.

Example 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?

Solution
At 1 Kbps:
1000 bits sent 1001 bits received1 extra bps
At 1 Mbps:
1,000,000 bits sent 1,001,000 bits received1000 extra bps

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.

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.

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.

SCHEMES

Lecture 08

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Lecture 08