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Welcome
Chapter- 2
Analog Communication
System
Data Communication & Computer
Network-1
What is communication?
Communication :
 Communication is about sending
and receiving information or the
transmission of information and
meaning from one party to
another through using shared
symbols or messages.
Communication process model
Sender
Message
Channel
Medium
Receiver
Feedback
© PhotoDisc
Elements of a Communication System
 Communication involves the transfer of information or
intelligence from a source to a recipient via a channel
or medium.
 Basic block diagram of a communication system:
Source
Transmitter
Receiver
Recipient
7
Brief Description
 Source: analogue or digital
 Transmitter: transducer, amplifier, modulator,
oscillator, power amp., antenna
 Channel: e.g. cable, optical fibre, free space
 Receiver: antenna, amplifier, demodulator, oscillator,
power amplifier, transducer
 Recipient: e.g. person, speaker, computer
8
Elements of a Communication System
Output
message
Input
message
Input
Transducer
Transmitter
Channel
Receiver
Output
Transducer
Input Transducer: The message produced by a
source must be converted by a transducer to a
form suitable for the particular type of
communication system.
Example: In electrical communications, speech
waves are converted by a microphone to voltage
variation.
Transmitter: The transmitter processes the input
signal to produce a signal suits to the characteristics of
the transmission channel.
Signal processing for transmission almost always
involves modulation and may also include coding. In
addition to modulation, other functions performed by
the transmitter are amplification, filtering and
coupling the modulated signal to the channel.
Channel: The channel can have different forms: The atmosphere (or
free space), coaxial cable, fiber optic, waveguide, etc.
The signal undergoes some amount of degradation from noise,
interference and distortion
Receiver: The receiver’s function is to extract the desired signal from
the received signal at the channel output and to convert it to a form
suitable for the output transducer.
Other functions performed by the receiver: amplification (the received
signal may be extremely weak), demodulation and filtering.
Output Transducer: Converts the electric signal at its input into the
form desired by the system user.
Example: Loudspeaker, personal computer (PC), tape recorders.
Data can be analog or digital.
Analog data are continuous and take
continuous values.
Digital data have discrete states and take
discrete values.
Signals can be analog or digital.
Analog signals can have an infinite number
of values in a range; digital signals can
have only a limited
number of values.
Figure Comparison of analog and digital signals
Figure
Two signals with the same phase and frequency,
but different amplitudes
Frequency and period are the inverse of
each other.
Figure
Two signals with the same amplitude and phase,
but different frequencies
Frequency is the rate of change with
respect to time.
Change in a short span of time
means high frequency.
Change over a long span of
time means low frequency.
Phase describes the position of the
waveform relative to time 0.
Figure Three sine waves with the same amplitude and frequency,
but different phases
Bandwidth
 Width of the spectrum of frequencies that can be
transmitted
 if spectrum=300 to 3400Hz, bandwidth=3100Hz
 Greater bandwidth leads to greater costs
 Limited bandwidth leads to distortion
22
Figure 1.1 Components of a data communication system
1.23
Data flow (simplex, half-duplex, and full-duplex)
Asynchronous Transmission
 Used in serial
 Timing needed only
communication
 Data transmitted 1
character at a time
 Character format is
usually 1 start & 1+ stop
bits, plus data of 5-8 bits
 Character may include
parity bit
within each character
 Resynchronization is
accomplished with each
start bit
 Uses simple, cheap
technology
 Wastes 20-30% of
bandwidth
25
Asynchronous Character
Transmission
26
26
Asynchronous Transmission
Synchronous Transmission
 Used in parallel
 Data framed by
communication
 Large blocks of bits
transmitted without
start/stop codes
 Synchronized by a clock
signal or clocking data
preamble (opening)/
postamble (closing) bit
patterns
 More efficient than
asynchronous
 Overhead typically
below 5%
 Used at higher speeds
than asynchronous
28
Synchronous Transmission
29
29
Bytes, Blocks, and Frames
30
30
Chapter- 2
Analog Communication
System
Amplitude Modulation
 Amplitude Modulation is a process where the
amplitude of a carrier signal is altered according to
information in a message signal.
 The frequency of the carrier signal is usually much
greater than the highest frequency of the input
message signal.
Figure 5.16 Amplitude modulation
FREQUENCY MODULATION
The modulating signal changes the freq. fc of the
carrier signal
 The bandwidth for FM is high
 It is approx. 10x the signal frequency

5.34
Figure Frequency modulation
5.35
PHASE MODULATION (PM)
The modulating signal only changes the phase of
the carrier signal.
 The phase change manifests itself as a frequency
change but the instantaneous frequency change is
proportional to the derivative of the amplitude.
 The bandwidth is higher than for AM.

5.36
Figure Phase modulation
Chapter- 3
Digital Communication
System
DIGITAL-TO-DIGITAL
CONVERSION
How we can represent digital
data by using digital signals. The
conversion
involves
three
techniques: line coding, block
coding, and scrambling. Line
coding is always needed; block
coding and scrambling may or
may not be needed.
LINE CODING
The
process of converting
digital data to digital signals
 Digital
data – sequences of bits
LINE CODING AND
DECODING
CHARACTERISTICS OF LINE CODING

Signal element vs. data element
◦
◦

A signal element is the shortest time unit of a digital signal.
Data elements are what to need to send as information.
Data rate vs. signal rate
Data rate (bit rate) – the number of data elements in a unit time
(bps).
◦ Signal rate (pulse, modulation rate, baud rate) – the number of signal
element in a unit time (baud).
◦ The goal is two-fold of “high data rate” and “low signal rate”.
◦

Bandwidth
◦






Although the actual bandwidth of a digital signal is infinite, the
effective bandwidth is finite.
Baseline wandering
DC components – unable to pass a low-pass filter
Self-synchronization
Built-in error detection
Immunity to noise and interferences
Complexity
Figure
Signal element versus data element
Figure Effect of lack of synchronization
Figure Line coding schemes
UNIPOLAR
signal levels are on one side of the time
axis - either above or below
 NRZ - Non Return to Zero scheme is an
example of this code. The signal level does
not return to zero during a symbol
transmission.
 Scheme is prone to baseline wandering and
DC components. It has no synchronization
or any error detection. It is simple but costly
in power consumption.
4.48
 All
Figure
Unipolar NRZ scheme
4.49
POLAR - NRZ
voltages are on both sides of the time
axis.
 Polar NRZ scheme can be implemented with
two voltages. E.g. +V for 1 and -V for 0.
 There are two versions:


NZR - Level (NRZ-L) - positive voltage for one
symbol and negative for the other
NRZ - Inversion (NRZ-I) - the change or lack of
change in polarity determines the value of a
symbol. E.g. a “1” symbol inverts the polarity a
“0” does not.
4.50
 The
Figure
Polar NRZ-L and NRZ-I schemes
4.51
POLAR - RZ
 The
4.52
Return to Zero (RZ) scheme uses three
voltage values. +, 0, -.
 Each symbol has a transition in the middle.
Either from high to zero or from low to zero.
 This scheme has more signal transitions
(two per symbol) and therefore requires a
wider bandwidth.
 No DC components or baseline wandering.
 Self synchronization - transition indicates
symbol value.
 More complex as it uses three voltage level.
It has no error detection capability.
Figure
Polar RZ scheme
4.53
POLAR BI-PHASE: MANCHESTER
AND DIFFERENTIAL MANCHESTER
Manchester coding consists of combining the
NRZ-L and RZ schemes.
Every symbol has a level transition in the
middle: from high to low or low to high. Uses
only two voltage levels.
 Differential Manchester coding consists of

combining the NRZ-I and RZ schemes.

Every symbol has a level transition in the
middle. But the level at the beginning of the
symbol is determined by the symbol value. One
symbol causes a level change the other does not.
4.54

Figure
Polar biphase: Manchester and differential Manchester scheme
4.55
LINE CODING SCHEMES, CON’T

Bipolar. Ex: AMI and Pseudoternary
◦

use three levels: positive, zero, and negative.
Multilevel
◦
mBnL


◦

a pattern of m data elements is encoded as a pattern of n
signal elements in which 2m ≤ Ln.
Ex)
 2B1Q: 2 binary 1 quaternary (=2B4L)
 8B6T: 8 binary 6 ternary. 28 data patterns, and 36 signal
pattern
4D-PAM5 (4-D 5-level pulse amplitude mod.)
Multitransition
◦
MLT-3 : multiline transmission, three level.
4.56
Figure
Bipolar schemes: AMI and pseudoternary
4.57
Figure
Multilevel: 2B1Q scheme
4.58
Table line coding schemes
Chapter 4
Transmission
Media
Figure 4.1
Transmission medium and physical layer
Figure 4.2
Classes of transmission media
4.1 Guided Media
Twisted-Pair Cable
Coaxial Cable
Fiber-Optic Cable
Figure
Twisted-pair cable
Figure
UTP and STP
Table Categories of unshielded twisted-pair cables
Category
Bandwidth
Data Rate
Digital/Analog
Use
1
very low
< 100 kbps
Analog
Telephone
2
< 2 MHz
2 Mbps
Analog/digital
T-1 lines
3
16 MHz
10 Mbps
Digital
LANs
4
20 MHz
20 Mbps
Digital
LANs
5
100 MHz
100 Mbps
Digital
LANs
6 (draft)
200 MHz
200 Mbps
Digital
LANs
7 (draft)
600 MHz
600 Mbps
Digital
LANs
Figure
UTP connector
Figure
Coaxial cable
FIBER-OPTIC CABLE

Fiber-optic cable uses light to transmit data
signals. The core of the fiber-optic cable is
composed of one or more thin tubes of glass or
plastic. Each tube is called the optical fiber and is
as thin as the human hair. A light-emitting diode
(LED) or a laser is used to send light through the
fibers.
Figure
Optical fiber
Figure
Propagation modes
Figure
Modes
Table Fiber types
Type
Core
Cladding
Mode
50/125
50
125
Multimode, graded-index
62.5/125
62.5
125
Multimode, graded-index
100/125
100
125
Multimode, graded-index
7
125
Single-mode
7/125
Figure
Fiber construction
Figure
Fiber-optic cable connectors
Note:
Microwaves are used for unicast
communication such as cellular
telephones, satellite networks, and
wireless LANs.
Note:
Infrared signals can be used for shortrange communication in a closed area
using line-of-sight propagation.