Download Communication system

King Saud University
College of Applied studies and Community Service
1301CT
1nd semester
1435-1436
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Outline
 Communication system
 Communication system model
 Mode of communication
 Data transmission mode
 Effectiveness of a communications system.
 Channel bandwidth.
 Signal power.
 Communication system resources.
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Long Distance Communication
• Communication is the transmission of information.
• Some examples of past communication methods:
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Communication System
 These methods of communication have been
superseded by electrical communication systems,
where the communication is by using electrical signal.
 Communication system is a combination of circuits
and devices put together to accomplish the
transmission of information from one point to
another.
 i.e. the purpose of a communication system is to carry
information from one point to another.
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Examples of Communications Systems
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Communications System Model
 A typical communication system can be modeled as
Input
signal
Input
Transducer
Transmitted
signal
Transmitter
Sender/
Transmitter
Received
signal
Channel
Distortion
And
Noise
Receiver
Output
signal
Output
message
Destination
Source
Input
message
Output
Transducer
Receiver
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Source
 Source: Produces an input message( voice, picture,
computer data etc ).
 There are many different types of sources and there
are different forms for messages.
 In general, input messages may be classified as analog
or digital.
 Analog messages (such as voice, music, temperature,
…) are represented by continuous-time variables while
discrete messages (such as text or numeric data) are
represented by discrete symbols
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Input Transducer
 If the input message is nonelectrical ( e.g. voice), it
must be converted by an input transducer to an
electrical signal (the Baseband signal / Message
Signal)
 A transducer, in simple terms is a device that converts
one form of energy into another.
 In the communication system, it convert the output of
a source into an electrical signal that is suitable for
transmission; e.g., a microphone and a camera.
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Transmitter
 The transmitter converts the electrical signal
(the Baseband signal/Message Signal) into a form that
is suitable for transmission through the transmission
medium or channel by a process called modulation.
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Channel
 Channel: medium used to transfer signal from
transmitter to receiver
 Channel can be wired or wireless.
 While the signal is travelling through the channel(the
medium) it is always attenuated (and the level of
attenuation increases with distance).
 Also, the signal shape may be changed during the
transmission i.e. become ‘distorted’.
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Noise
 The signal is not only distorted by a channel, but it is
also contaminated along the path by undesirable
signals lumped under the broad term noise, which are
random and unpredictable signals from causes
external ( such interference from signals transmitted
on nearby channels) and internal ( such noise resulted
from thermal motion of electrons in conductors).
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Receiver
 The function of the receiver is to recover the message
signal contained in the signal received from the
channel (received signal).
 The received signal is a corrupted version of the
transmitted signal.
 So, the receiver reconstruct a recognizable form of the
original message signal.
 It reprocess the received signal by undoing the signal
modifications ( demodulation) made at the transmitter
and the channel.
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Output Transducer
 The receiver output is fed to the output transducer,
which convert the electrical signals that are received
into a form that is suitable for the final destination;
e.g., speaker, monitor, etc.
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Mode of Communication
 There are two basic mode of communication:
1- Broadcasting  single transmitter and numerous
receiver.
2- Point to point communication  single transmitter
and single receiver.
Point to point
Broadcasting
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Data Transmission Mode
 Data transmission between two devices can be:
Simplex
Halfduplex
Fullduplex
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Data Transmission Mode
 In simplex mode, the communication is
unidirectional. Only one of the two devices on a link
can transmit; the other can only receive.
 In half-duplex mode, each device can both transmit
and receive, but not at the same time. When one device
is sending, the other can only receive, and vice versa.
 In full-duplex mode (also called duplex), both devices
can transmit and receive simultaneously.
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Effectiveness of Communications
System
 The effectiveness of a communications system
depends on four fundamental characteristics:
Delivery
Accuracy
Timeliness
Jitter
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Effectiveness
of a Communications System
 Delivery:
 The system must deliver information to the correct
receiver.
 Information must be received by the intended device or
user and only by that device or user.
 Accuracy:
 The system must deliver the information accurately.
 Information that have been altered in transmission and
left uncorrected are unusable.
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Effectiveness
of a Data Communications System
 Timeliness:
 The system must deliver information in a timely
manner.
 Information delivered late are useless.
 In the case of video and audio, timely delivery means :



delivering data in the same order as they are produced,
and without significant delay.
This kind of delivery is called real-time transmission.
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Effectiveness
of a Data Communications System
 Jitter:
 Jitter refers to the variation in the packet arrival time.
 It is the uneven delay in the delivery of audio or video
packets.
 For example, let us assume that video packets are sent
every 3 ms. If some of the packets arrive with 3 ms delay
and others with 4 ms delay, an uneven quality in the
video is the result.
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The Rate and Quality of Data
Transmission
 The fundamental parameters that control the rate and
quality of data transmission in the communication
system are:
 Channel bandwidth .
 Signal power .
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Channel Bandwidth
 The channel can transmit a range of frequencies with
reasonable fidelity.
 The bandwidth of a channel (B) is the width of the
frequency band used to transmit the data.
 It is the difference between the highest and lowest
frequencies which the channel can carry.
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Channel Bandwidth
 If a channel can carry a signal which its frequency
range from 0 to 5000 Hz (5 khz), the channel
bandwidth B is 5 khz.
 If a channel can carry frequencies between 200Hz and
4kHz, its bandwidth (the difference between those
two frequencies) is 3.8kHz.
 If a channel can carry frequencies between 10MHz and
100MHz, what is the channel bandwidth?
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Channel Bandwidth
 The rate of data transmission is directly proportional
to B.
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Signal Power
 The power of a signal is defined as the average energy
over time.
 The signal power is related to the quality of
transmission.
 Increasing the signal power ( S ), reduces the effect of
channel noise, and the information is received more
accurately.
 We measure noise relative to a signal in terms of the
signal-to-noise ratio (SNR).
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Signal Power
 The SNR is the ratio of the average signal power to the
average noise power.
 A larger SNR also allows transmission over a longer
distance.
 In any event, a certain minimum SNR is necessary for
communication.
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Communication System Resources
 So, in a communication system there are tow primary
resources :
 Channel bandwidth.
 Power
 A general system design objective is to use these
resources as efficiently as possible.
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Communication System Resources
 Based on the channel bandwidth, the communication
system can be classified as:
 Baseband communication system  channel
bandwidth 0 to f
 Bandpass communication system  channel
bandwidth f1 to f2
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Communication System Resources
 There are two types of power in a communication
system:
 Transmit power : is the average power of the transmitted
signal
 Received power : the average power of the received
signal
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Communication System Resources
 Generally, system performance will be better if there is
high transmitted power.
 Practical constraints on cost, properties of the
transmission medium, battery life imply low
transmitted power.
 The received power is a function of the transmitted
power and the channel.
 A larger distance between the transmitter and the
receiver  lower received power
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