Download Chapter 1: Basics of Communication Systems

PART I: DIGITAL
COMMUNICATION SYSTEMS
Chapter 1: Basics of Communication Systems
Contents
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Basic Telecommunication System
Types of Communication
Transmission Impairment
Analog Versus Digital Transmission
Objectives
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To study the basic building block of a
telecommunication system.
To study the various types of communication and
how the electrical signal is impaired as it travels
through the transmission medium.
To understand the difference between the analog
communication and digital communication.
Basic Telecommunication System
Basic Telecommunication System
Source
Transducer
Transducer
Transmission
Medium
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Figure 1.1 Basic telecommunication system
Sink
What is the problem of this system?
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As the electrical signal passes through the medium,
the signal gets attenuated. The attenuated signal
may not be able to drive the transducer at the
receiving end at all if the distance between the
sender and the receiver is large.
The solution...
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We can use an amplifiers, the amplifier will ensure
that the electrical signals are sufficient strength to
drive the transducer.
In an Electrical communication system, at the transmitting
side, a transducer converts the real-life information into an
electrical signal. At the receiving side, a transducer converts
the electrical signal back into real-life information.
But we still have a problem...
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The transmission medium introduces noise. The noise
cannot be eliminated at all.
Amplification alone does not solve the problem,
particularly when the system has to cover large
distance.
As the electrical signal passes through the transmission
medium, the signal gets attenuated. In addition, the
transmission medium introduces noise and as a result, the
signal gets distorted.
Design objective
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The objective of designing a communication system
is to reproduce the electrical signal at the receiving
end with minimal distortion.
Interconnecting two computers
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The serial ports of two computers can be connected
directly using a copper cable. However, due to the
signal attenuation, the distance cannot be more than
100 meters.
Channel
RS232 Port
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Figure 1.2: PC-to-PC communication.
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The modem converts the digital signals generated by
the computer into analog form for transmission over the
medium at the transmitting end and the reverse at the
receiving end.
Telephone
Network
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Figure 1.3: PC-to-PC communication over telephone
network
Generic Communication system
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Source
Baseband
Signal
Processing
Medium
Access
Processing
Decoding of
Data
Receiver
Transmitter
Baseband
Signal
Processing
Sink
Figure 1.4: Generic communication system
Radio Communication System
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The electrical signal is transformed into a highfrequency signal and sent through the air (free
space). A radio transmitter is used to do this.
A reverse of this transformation – converting the
high-frequency signal into an audio signal – is
performed at the receiving station.
Since it is a broadcasting system, many receivers
can receive the information.
One communication link
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In a communication system on which two persons
communicate with other persons located somewhere
else, but only one communication link, the voice
signals need to be combined.
It is impossible to mix the two voice signals directly
because it will not be possible to separate them at
the receiving end.
The two signals need to “multiplex”, using special
technique.
Mobile Communication System
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A radio channel has to be shared by a number of
users.
Multiple Access - Each user has to use radio channel
for a short time period during which he has to
transmit his data and then wait for his next turn.
These techniques are required in both radio
communication and cable communication.
Elements of Communication System
Multiplexer
Multiple
access
Combines the signals
from different sources
to transmit on the
channel.
Error
detection
and
correction
Switching
If the channel is noisy,
the received data will
have errors.
When two or more
users share the same
channel, each user has
to transmit his signal
only at a specified
time or using a specific
frequency band.
At the receiving end, a
demultiplexer is used
to separate the signal.
Source
coding
Detection, and if
possible correction, of
errors has to be done
at the receiving end.
This is done through a
mechanism called
channel coding
If the channel has a
lower bandwidth than
the input signal
bandwidth, the input
signal has to be
processed to reduce
its bandwidth so that it
can be accommodated
on the channel.
If the large number of
users has to be
provided with
communication
facilities, as in a
telephone network, the
users are to be
connected on the
numbers dialed. This is
done through
mechanism called
switching.
Signaling
In a telephone
network, when you
dial a particular
telephone number, you
are telling the network
whom you want to
call. This is called
signaling information.
The telephone switch
(or exchange) will
process the signaling
information to carry
out the necessary
information to carry
out the necessary
operations for
connecting to the
called party.
Types of Communication
Different types of communication
Broadcasting
Simplex
Point-toMultipoint
Point-to-Point
Half-duplex
Communication
Full-duplex
Point-to-Point Communication
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In this type, communication takes place between two
end points.
Party
1
Party
2
Point-to-Multipoint Communication
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In this type of communication, there is one sender
and multiple recipients.
Party 1
Party n
Party 2
Party 3
Broadcasting
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In broadcasting system,
there is a central
location from which
information is sent to
many recipients, as in
case of audio or video
broadcasting. In a
broadcasting system,
the listener are passive,
and there is no reverse
communications.
Receiver
1
Sender
Receiver
n
Receiver
2
Simplex Communication
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In simplex communication, communication is possible
only in one direction. There is one sender and one
receiver; the sender and receiver cannot change
roles.
Party 1
Party 2
Half-Duplex Communication
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Half-duplex communication is possible in both
directions between two entities (computers or
persons), but one at a time. These types of system is
required limited channel bandwidth, so they are low
cost systems.
Switch
Party 1
Party 2
Full-Duplex Communication
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In a full-duplex communication system, the two
parties - the caller and the called – can
communicate simultaneously, as in telephone system.
Party 1
Party 2
Convergence Technologies
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Multimedia Communication – Various types of
information (voice communication, data
communication, fax communication, and video
communications) are clubbed together.
With the advent of technology, this distinction is
slowly disappearing, and multimedia communication
is becoming the order of the day.
Transmission Impairments
3 Classification of Transmission Impairment
Attenuation
distortion
Delay
distortion
Noise
• Amplitude of the signal wave decreases as the signal travels through
the medium.
• Occurs as a result of different frequency components arriving at
different times in the guide media such as copper wire or coaxial
cable.
• Divided into four categories:
• Thermal noise
• Intermodulation
• Crosstalk
• Impulse noise
Noise
Intermodulation Noise
• When two signals of different frequencies are sent through the medium, due to nonlinearity of
the transmitter, frequency components such as f1 + f2 and f1 – f2 are produced, which are
unwanted components and need to be filtered out.
Crosstalk
• Unwanted coupling between signal paths is known as crosstalk.
• In the telephone network, this coupling is quite common. As a result of this, we hear other
conversations.
• Crosstalk needs to be eliminated by using appropriate design techniques
Impulse Noise
• This is cause by external electromagnetic disturbance such as lightning.
• This noise is unpredictable.
• When the signal is traversing the medium, impulse noise may cause sudden burst of errors. This
may cause a temporary disturbance in voice communication.
• For data communication, appropriate methods need to be devised whereby the lost data is
retransmitted.
Thermal Noise
• Noise occur due to the thermal agitation of
electrons in a conductor.
• This is distributed uniformly across the
spectrum and hence called white noise.
• This noise cannot be eliminated and hence,
when designing telecom systems, we need
to introduce some method to overcome the
ill effects of thermal noise.
Thermal Noise for a bandwidth of 1 Hz is obtained from the formula:
• No = kT
where No is noise power density, watts per Hz
• k is Boltzmann’s constant 1.3803 * 10-23 J/K
• T is temperature, K
Thermal noise for a bandwidth of B Hz is given by
• N = kTB (watts)
If N is expressed in dB (decibels)
• N = 10 log k + 10 log T + 10 logB db watts
• = -228.6 + 10 log T + log B
Thermal noise for a bandwidth B Hz is given by N = kTB (watts), where k is
Boltzmann’s constant and T is temperature. N is generally expressed in decibels.
Analog Versus Digital transmission
What is Analog Transmission?
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In analog communication, the signal, whose
amplitude varies continuously, is transmitted over the
medium.
Reproducing the analog signal at the receiving end
if very difficult due to transmission impairments.
Analog communication systems are badly affected
by noise.
What is Digital Transmission
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In a digital communication system, 1s and 0s are
transmitted as voltage pulses.
If the pulse is distorted due to noise, it is not very
difficult to detect the pulses at the receiving end.
Digital communication is much more immune to noise
as compared to analog communication.
Advantages of Digital Systems
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More reliable transmission because only
discrimination between ones and zeroes is required.
Less costly implementation because of the advances
in digital logic chips.
Ease of combining various types of signals (voice,
video, etc.)
Ease of developing secure communication systems.
Convergence Technologies
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Though a large number of analog communication
systems are still in use, digital communication
systems are now being deployed.
The old analog system are being replaced by
digital systems.
All newly developed communication systems are
digital systems. Only in broadcasting applications, is
analog communication used extensively.
End of Chapter 1