1 nd semester 1435-1436 1 King Saud University College of Applied studies and Community Service 1301CT.

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Chapter1 : Introduction

1nd semester 1435-1436

King Saud University College of Applied studies and Community Service1301CT

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OutlineCommunication systemCommunication system modelMode of communicationData transmission modeEffectiveness 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 SystemThese 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superseded by electrical communication systems, where the communication is by using electrical signal.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 ModelA typical communication system can be

modeled as

InputTransduc

er

Transmitter Channel Receiver

OutputTransduc

er

Distortion

AndNoise

Sou

rce

Inputmessa

ge

Inputsignal

Transmittedsignal

Receivedsignal

Outputsignal

Outputmessa

ge

Dest

inati

o n

Sender/ Transmitter

Receiver

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SourceSource: 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 TransducerIf 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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TransmitterThe 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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ChannelChannel: 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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NoiseThe 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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ReceiverThe 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 TransducerThe 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 CommunicationThere are two basic mode of communication:1- Broadcasting single transmitter and

numerous receiver.2- Point to point communication single

transmitter and single receiver.

Broadcasting

Point to point

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Data Transmission ModeData transmission between two devices can

be:Simple

x

Half-duplex

Full-duplex

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Data Transmission ModeIn 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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Effectivenessof a Communications SystemDelivery:

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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Effectivenessof a Data Communications SystemTimeliness:

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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Effectivenessof a Data Communications SystemJitter:

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 TransmissionThe 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 BandwidthThe 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 BandwidthIf 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 BandwidthThe rate of data transmission is directly

proportional to B.

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Signal PowerThe 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 PowerThe 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 signalReceived 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