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Communication System and Transmission Media

Objective

After completion of this unit students will be able

 To explain the concept of Analog and Digital Signal

 To elaborate different types of transmission media

 To distinguish between Simplex, Half-Duplex and Full Duplex

Learning process and support material

 Class demonstration with Pictures and Lecture method

 Group discussion and Questionnaire

Content’s Elaboration

Analogue Communication

An analog signal is a continuous wave denoted by a shine wave (pictured below)

and may vary in signal strength (amplitude) or frequency (time). The sine wave's

amplitude value can be seen as the higher and lower points of the wave, while the

frequency (time) value is measured in the sine wave's physical length from left to

right. There are many examples of analog signals around us. The sound from a

human voice is analog, because sound waves are continuous, as is our own vision,

because we see various shapes and colors in a continuous manner due to light

waves. Even a typical kitchen clock having its hands moving continuously can be

represented as an analog signal.

Fig: Analog Signal

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Block diagram of Analog Communication

Fig: Basic analog communication system

 The elements of basic analog communication system are input signal or

information, input transducer, transmitter, channel, noise, receiver, output

transducer.

1. Information or Input signal

 The information is transmitted from one place to another.

 This information can be in the form of a sound signal like speech, or it can be

in the form of pictures or it can be in the form of data information.

2. Input transducer

 The information in the form of sound, picture or data signals cannot be

transmitted as it is.

 First it has to be converted into a suitable electrical signal.

 The input transducer block does this job.

 The commonly used input transducers used are microphones, TV etc.

3. Transmitter

 The function of the transmitter is to convert the electrical equivalent of the

information to a suitable form so that it can transfer over long distance.

 Basic block in transmitter are: Amplifier,Oscillator, Mixer.

4. Channel

 The communication channel is the medium used for transmission of electrical

signal from one place to other.

 The communication medium can be conducting wires, cables, optical fibers

or free space.

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 Depending on the type of communication medium, two types of

communication system exist.

 Line communication: The line communication systems uses the

communication medium like the simple wires or cables or optical fibers. E.g.

Telephone, Cable TV.

 Radio communication : The radio communication systems uses the free space

as their communication medium. The transmitted signal is in the form of

electromagnetic waves. e.g. Mobile communication, satellite communication.

5. Noise

 Noise is an unwanted electrical signal which gets added to the transmitted

signal when it is travelling towards the receiver.

 Due to noise quality of information gets degraded.

 Once added the noise cannot be separated out from the information

6. Receiver

 The receiver always converts the modulated signal into original signal which

consists of Amplifier, Oscillator, Mixer.

7. Output transducer

 Output transducer converts electrical signal into the original form i.e. sound

or TV pictures etc.

e.g. loudspeaker, data and image convertor.

Digital Signal

A digital signal a must for computer processing, is described as using binary (0s

and 1s), and therefore, cannot take on any fractional values. As illustrated in the

graphic below, digital signals retain a uniform structure, providing a constant and

consistent signal. Because of the inherent reliability of the digital signal, technology

using it is rapidly replacing a large percentage of analog applications and devices.

For example, the wristwatch, showing the time of day, with its minute, hour, and

sweeping second hands, is being replaced by the digital watch, which offers the

time of day and other information using a numerical display. A typical digital signal

is represented below. Note the equally dispersed 1s and 0s.

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Fig: Digital Signal

Advantages of Digital Signals

As the signals are digitized, there are many advantages of digital communication

over analog communication, such as−

 The effect of distortion, noise, and interference is much less in digital signals

as they are less affected.

 Digital circuits are more reliable.

 Digital circuits are easy to design and cheaper than analog circuits.

 The hardware implementation in digital circuits, is more flexible than analog.

 The occurrence of cross-talk is very rare in digital communication.

 The signal is un-altered as the pulse needs a high disturbance to alter its

properties, which is very difficult.

 The probability of error occurrence is reduced by employing error detecting

and error correcting codes.

 Spread spectrum technique is used to avoid signal jamming.

 Combining digital signals using Time Division Multiplexing (TDM) is easier

than combining analog signals using Frequency Division Multiplexing

(FDM).

 The configuring process of digital signals is easier than analog signals.

 Digital signals can be saved and retrieved more conveniently than analog

signals.

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Block Diagram of Digital Communication

The elements which form a digital communication system is represented by the

following block diagram for the ease of understanding.

Following are the sections of the digital communication systems :

Source

The source can be an analog signal. Example: A sound signal

Input Transducer

This is a transducer which takes a physical input and converts it to an electrical

signal (Example: microphone). This block also consists of an analog to

digital converter where a digital signal is needed for further processes. A digital

signal is generally represented by a binary sequence.

Source Encoder

The source encoder compresses the data into minimum number of bits. This process

helps in effective utilization of the bandwidth. It removes the redundant bits

(unnecessary excess bits, i.e., zeroes).

Channel Encoder

The channel encoder, does the coding for error correction. During the transmission

of the signal, due to the noise in the channel, the signal may get altered and hence

to avoid this, the channel encoder adds some redundant bits to the transmitted data.

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These are the error correcting bits.

Digital Modulator

The signal to be transmitted is modulated here by a carrier. The signal is also

converted to analog from the digital sequence, in order to make it travel through the

channel or medium.

Channel

The channel or a medium, allows the analog signal to transmit from the transmitter

end to the receiver end.

Digital Demodulator

This is the first step at the receiver end. The received signal is demodulated as well

as converted again from analog to digital. The signal gets reconstructed here.

Channel Decoder

The channel decoder, after detecting the sequence, does some error corrections. The

distortions which might occur during the transmission, are corrected by adding

some redundant bits. This addition of bits helps in the complete recovery of the

original signal.

Source Decoder

The resultant signal is once again digitized by sampling and quantizing so that the

pure digital output is obtained without the loss of information. The source decoder

recreates the source output.

Output Transducer

This is the last block which converts the signal into the original physical form,

which was at the input of the transmitter. It converts the electrical signal into

physical output (Example: loud speaker).

Output Signal

This is the output which is produced after the whole process. Example: The sound

signal received. This unit has dealt with the introduction, the digitization of signals,

the advantages and the elements of digital communications. In the coming chapters,

we will learn about the concepts of Digital communications, in detail.

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Transmission Media:

Data is represented by computers and other telecommunication devices using

signals. Signals are transmitted in the form of electromagnetic energy from one

device to another. Electromagnetic signals travel through vacuum, air or other

transmission mediums to move from one point to another (from sender to receiver).

Electromagnetic energy (includes electrical and magnetic fields) consists of power,

voice, visible light, radio waves, ultraviolet light, gamma rays etc.

Transmission medium is the means through which we send our data from one place

to another. The first layer (physical layer) of Communication (Networks OSI Seven

layer) model is dedicated to the transmission media, we will study the OSI Model

later.

Fig: Types of transmission media

Factors to be considered while selecting a Transmission Medium

1. Transmission Rate

2. Cost and Ease of Installation

3. Resistance to Environmental Conditions

4. Distances

Bounded or Guided Transmission Media

Guided media, which are those that provide a conduit from one device to another,

include Twisted-Pair Cable, Coaxial Cableand Fiber-Optic Cable.A signal

travelling along any of these media is directed and contained by the physical limits

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of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors

that accept and transport signals in the form of electric current. Optical fiber is a

cable that accepts and transports signals in the form of light.

Twisted Pair Cable

This cable is the most commonly used and is cheaper than others. It is lightweight,

cheap, can be installed easily, and they support many different types of network.

Some important points:

 Its frequency range is 0 to 3.5 kHz.

 Typical attenuation is 0.2 dB/Km@1kHz.

 Typical delay is 50 μs/km.

 Repeater spacing is 2km.

A twisted pair consists of two conductors (normally copper), each with its own

plastic insulation, twisted together. One of these wires is used to carry signals to the

receiver, and the other is used only as ground reference. The receiver uses the

difference between the two. In addition to the signal sent by the sender on one of

the wires, interference (noise) and crosstalk may affect both wires and create

unwanted signals. If the two wires are parallel, the effect of these unwanted signals

is not the same in both wires because they are at different locations relative to the

noise or crosstalk sources. This results in a difference at the receiver.

Twisted Pair is of two types

 Unshielded Twisted Pair (UTP)

 Shielded Twisted Pair (STP)

Unshielded Twisted Pair Cable

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It is the most common type of telecommunication when compared with Shielded

Twisted Pair Cable which consists of two conductors usually copper, each with its

own color plastic insulator. Identification is the reason behind colored plastic

insulation. UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair

use RJ-11 connector and 4 pair cable use RJ-45 connector.

Advantages of Unshielded Twisted Pair Cable

 Installation is easy

 Flexible

 Cheap

 It has high speed capacity,

 100-meter limit

 Higher grades of UTP are used in LAN technologies like Ethernet.

It consists of two insulating copper wires (1mm thick). The wires are twisted

together in a helical form to reduce electrical interference from similar pair.

Disadvantages of Unshielded Twisted Pair Cable

 Bandwidth is low when compared with Coaxial Cable

 Provides less protection from interference.

Shielded Twisted Pair Cable

This cable has a metal foil or braided-mesh covering which encases each pair of

insulated conductors. Electromagnetic noise penetration is prevented by metal

casing. Shielding also eliminates crosstalk (explained in KEY TERMS Chapter).

It has same attenuation as unshielded twisted pair. It is faster unshielded and coaxial

cable. It is more expensive than coaxial and unshielded twisted pair.

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Advantages of Shielded Twisted Pair Cable

 Easy to install

 Performance is adequate

 Can be used for Analog or Digital transmission

 Increases the signaling rate

 Higher capacity than unshielded twisted pair

 Eliminates crosstalk

Disadvantages of Shielded Twisted Pair Cable

 Difficult to manufacture

 Heavy

Performance of Shielded Twisted Pair Cable

One way to measure the performance of twisted-pair cable is to compare attenuation

versus frequency and distance. As shown in the below figure, a twisted-pair cable

can pass a wide range of frequencies. However, with increasing frequency, the

attenuation, measured in decibels per kilometer (dB/km), sharply increases with

frequencies above 100kHz. Note that gauge is a measure of the thickness of the

wire.

Fig: Performance graph of Shielded Twisted Pair Cable

Applications of Shielded Twisted Pair Cable

 In telephone lines to provide voice and data channels. The DSL lines that are

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used by the telephone companies to provide high-data-rate connections also

use the high-bandwidth capability of unshielded twisted-pair cables.

 Local Area Network, such as 10Base-T and 100Base-T,also use twisted-pair

cables.

Coaxial Cable

Coaxial is called by this name because it contains two conductors that are parallel

to each other. Copper is used in this as center conductor which can be a solid wire

or a standard one. It is surrounded by PVC installation, a sheath which is encased

in an outer conductor of metal foil, barid or both. Outer metallic wrapping is used

as a shield against noise and as the second conductor which completes the circuit.

The outer conductor is also encased in an insulating sheath. The outermost part is

the plastic cover which protects the whole cable.

Here the most common coaxial standards.

 50-Ohm RG-7 or RG-11: used with thick Ethernet.

 50-Ohm RG-58: used with thin Ethernet

 75-Ohm RG-59: used with cable television

 93-Ohm RG-62: used with ARCNET.

Fig: Coaxial Cable

Coaxial Cable Connectors

To connect coaxial cable to devices, we need coaxial connectors. The most common

type of connector used today is the Bayonet Neill-Concelman (BNC) connector.

The below figure shows 3 popular types of these connectors: the BNC Connector,

the BNC T connector and the BNC terminator.

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Fig: Coaxial cable connectors

The BNC connector is used to connect the end of the cable to the device, such as a

TV set. The BNC T connector is used in Ethernet networks to branch out to a

connection to a computer or other device. The BNC terminator is used at the end of

the cable to prevent the reflection of the signal.

There are two types of Coaxial cables

1. Baseband

This is a 50 ohm (Ω) coaxial cable which is used for digital transmission. It is mostly

used for LAN's. Baseband transmits a single signal at a time with very high speed.

The major drawback is that it needs amplification after every 1000 feet.

2. Broadband

This uses analog transmission on standard cable television cabling. It transmits

several simultaneous signals using different frequencies. It covers large area when

compared with Baseband Coaxial Cable.

Advantages of Coaxial Cable

 Bandwidth is high

 Used in long distance telephone lines.

 Transmits digital signals at a very high rate of 10Mbps.

 Much higher noise immunity

 Data transmission without distortion.

 The can span to longer distance at higher speeds as they have better shielding

when compared to twisted pair cable

Disadvantages of Coaxial Cable

 Single cable failure can fail the entire network.

 Difficult to install and expensive when compared with twisted pair.

 If the shield is imperfect, it can lead to grounded loop.