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Chapter1 : Introduction

King Saud University College of Applied studies and Community Service1301CTBy: Nour Alhariqi

1st semester 1434-1435

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Outline

Data communications

Communications system

Communications system resources

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DATA COMMUNICATIONS

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Outline

Data communication. Communication system. Components of a data

communications system. Effectiveness of a data

communications system. Data forms. Data transmission mode.

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Data Communication

Data communications are the exchange of data between two or more devices via some form of transmission medium such as a wire cable.

The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data.

When we communicate, we are sharing information.

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Communication System

For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs).

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Components of a Data Communications System

A data communications system has five components:

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Components of a Data Communications System

1. Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

2. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

3. Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

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Components of a Data Communications System

4. Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5. Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

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Effectivenessof a Data Communications System

The effectiveness of a data communications system depends on four fundamental characteristics:

Delivery Accuracy Timeliness Jitter

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Effectivenessof a Data Communications System

Delivery: The system must deliver data to the correct

destination. Data must be received by the intended device

or user and only by that device or user.

Accuracy: The system must deliver the data accurately. Data that have been altered in transmission

and left uncorrected are unusable.

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Effectivenessof a Data Communications System

Timeliness: The system must deliver data in a timely

manner. Data delivered late are useless. In the case of video and audio, timely

delivery means :▪ delivering data as they are produced,▪ in the same order that they are produced,▪ and without significant delay. ▪ This kind of delivery is called real-time

transmission.

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Effectivenessof 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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Data Forms

Information today comes in different forms such as

Text Number Audio Images Video

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Data Transmission Mode

Communication between two devices can be:Simple

x

Half-duplex

Full-duplex

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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 station 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 stations can transmit and receive simultaneously.

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COMMUNICATIONS SYSTEM

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Communications System

The three basic parts of the communication system:

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Examples of Communications System

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Communications System Model

A typical communication system can be modeled as :

Sourc

e

Sender Receiver

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Source

Source: Produces an input message; e.g. voice, picture, computer data etc.

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)

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Input transducer

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 video camera.

a human voice is sound wave which is a non- electrical form of energy, an input transducer will convert it into an electrical signal.

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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 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., sound waves, images, etc.

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Example

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COMMUNICATIONS SYSTEM RESOURCES

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Outline

The rate and quality of data transmission depend on what?

Channel bandwidth. Signal power. Communication system resources.

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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 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. Transmission power (the average power

of the transmitted signal ).

A general system design objective is to use these resources as efficiently as possible.

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Communication System Resources

In most communication channels, one resource may be considered more important than the other.

We may therefore classify communication channel as power limited or band limited. Telephone channel band limited. Satellite channel power limited.