What is meant by the term ‘Channel’ as applied to communication system ?

The term channel means the medium through which the message travels from the transmitter to the receiver.

In other words, we can say that the function of the channel is to provide a physical connection between the transmitter and the receiver.

Depending on the mode of transmission, we may classify the communication channels into following two categories .

1. Channels based on guided propagation
2. Channels based on free propagation

The classification of channels has been shown in fig.1.

Fig.1. Classification of Communication Channels

Some of the important characteristics of a channel are as under :

• Power required to achieve the desired S/N ratio
• Bandwidth of the channel
• Amplitude and phase response of channel
• Type of channel (Linear or non-linear)
• Effects of external interference on the channel

1. Telephone Channels

It is designed for providing service to voice signals such as telephones. The telephone channels are also used for the worldwide internet connection. Therefore, telephone channel is the best possible option for the data transmission over long distances.

Salient Features :

• Bandpass characteristics over 300 to 3400 Hz as shown in fig.2 .
• High signal to noise ratio of about 30 dB .
• Approximately linear response .

Fig.2 : Characteristics of telephone channel

The amplitude response is flat over the entire passband as shown in fig.2.

However, no particular attention is given to the phase response because human ears are not very sensitive to the phase delay variations.

But the data and images (pictures) are strongly affected by the phase delay variations. Therefore, for digital transmission over the telephone channels, it is essential to use an equalizer as shown in fig.3 (a) .

Fig. 3 (a) Use of Equalizer

As shown in fig.3 (b) , the equalizer maintains a flat amplitude response and linear phase response over the passband. Also the telephone channel is a bandwidth limited channel.

Fig. 3 (b) Amplitude and phase response of the telephone channel with equalizer

Fig.4(a) shows the variation of insertion loss with frequency and fig.4 (b) shows the variation of envelope delay with frequency, for a telephone channel .

Fig. 4 (a) Insertion loss

Insertion loss may be defined as :

Insertion loss = 10 log₁₀ (P₀ | P_L)

where P₀ = load power from a source to load via channel

P_L = load power when the load is connected directly to source

An envelope delay is defined as the negative of the phase response with respect to ω =2πf .

Fig.4 (b) Envelope delay

The telephone channels are built using the twisted pair of wires.The construction of twisted pair cables has shown in fig.5.

Fig.5 Twisted pair

This is also commonly used medium and it is quite cheaper than the co-axial cable.

A twisted pair consists of two insulated conductor twisted together in the spiral form as shown on fig.5 .

It can be shielded or un-shielded.

The un-shielded twisted pair cables are very cheap and easy to install. However, they are badly affected by the noise interference .

The noise immunity can be improved by using shielded twisted pair cable.

As shown in fig.6, a metallic braid can be used around the twisted pair.  Protective plastic coating is then provided .

Fig.6 Shielded Twisted Pairs

Transmission Rates

If we use the sophisticated modulation techniques along with equalizer, then it is possible to attain the transmission rates upto 16.8 kilobits/sec (kb/s).

2. Co-axial Cables

Fig. 7 shows the construction of co-axial cable.

Fig.7  Construction of a co-axial cable

It consists of two concentric conductors separated by a dielectric material. The external conductor is metallic braid and used for the purpose of shielding. The co-axial cable may contain one or more co-axial pairs.

These systems are operated in the range of 8.5 Mb/s to 274 Mb/s .

An important application of co-axial cable is  cable modem, with the cable modem termination system(CMTS).

One more application is Ethernet LAN using the co-axial cable.

The co-axial cable is used for its large bandwidth and high noise immunity .

Salient Features

The important characteristics of a co-axial cable are :

• Two types of cable having 75 Ω and 50 Ω impedance are available.
• Because of the shield provided, this cable has excellent noise immunity.
• It has a large bandwidth and low losses.
• This cable is suitable for point to point or point to multipoint applications. Actually, this is the most widely used medium for local area networks (LANs).
• These cables are costlier than twisted pair cables, however, they are cheaper than the optical fiber cables.
• It is essential to use closely spaced (after every 1km) repeaters to achieve that data rates of 8.5 Mb/s to 274 Mb/s.

3. Optical Fiber Cables

The construction of an optical fiber cable is shown in fig.8.

Fig.8. Construction of Optical Fiber Cable

It consists of an inner glass core surrounded by a glass cladding which has a lower refractive index.

Digital signals are transmitted in the form of intensity-modulated light signal which is trapped in the glass core.

Light is launched into the fiber using a light source such as a LED or laser.

It is detected on the other side using a photo detector such as a phototransistor.

The optical fiber cables are costlier than the other two types but they have several advantages over the other two types.

Special Characteristics

• Higher bandwidth therefore can operate at higher data rates.
• Reduced losses as the signal attenuation is low.
• Distortion is reduced hence better quality is assured.
• They are immune to electromagnetic interference.
• Small size and light weight.
• Used for point to pint communication.

Applications

• Optical fiber transmission systems are widely used in the backbone of networks. Current optical fiber systems provide transmission rates from 45 Mb/s to 9.6 Gb/s using the single wavelength transmission.
• Optical fibers are now used in the telephone system.
• It is used in the local area networks (LANs).

Advantages of Optical Fibers

(i) Small size and light weight : The size (diameter) of the optical fibers is very small  (it is comparable to the diameter of human hair). Therefore, a large number of optical fibers can fit into a cable of small diameter.

(ii) Easy availability and low cost : The material used for the manufacturing of optical fibers is silica glass. This material is easily available. Hence, the optical fibers cost lower than the cables with metallic conductors.

(iii) No electrical or Electromagnetic interference : Since the transmission takes place in the form of light rays, the signal is not affected due to any electrical or electromagnetic interference.

(iv) Large bandwidth : As the light rays have a very high frequency in the GHz range, the bandwidth of the optical fiber is extremely large. This allows transmission of more number of channels. Therefore, the information carrying capacity of an optical fiber is much higher than that of a co-axial cable.

(v) Other advantages : In addition to the above mentioned advantages, the optical fiber communication system has the following other advantages:

• No crosstalk inside the optical fiber cable.
• Signal can be transmitted upto 100 times faster.
• Intermediate amplifiers are not required as the transmission losses in the fiber are low.
• Ground loops are absent.
• Installation is very easy as the fiber optic cable are flexible.
• These cables are not affected by the drastic environmental conditions.

Disadvantages of Optical Fibers

(i) Sophisticated plants are required for manufacturing optical fiber.

(ii) The initial cost incurred is high.

(iii) Joining the optical fibers is a difficult job.

4. Wireless Broadcast Channel

These channels are used for the transmission of radio and TV signals.

The information signal which represents the speech, music etc. modulates a carrier frequency.

The carrier frequency is different for every transmitting station.

A transmitting antenna radiates the modulated signal in the form of electromagnetic radiation into the free space.

These waves are radiated in all directions or in some specified directions.

The transmitting antenna is mounted on a tower or a hall in order to reach the farther receiver.

The ground wave, sky wave and space wave are three types of propagation techniques used for the propagation of EM waves.

At the receiving end, the receiving antenna is used for picking up the transmitted signal.

The receivers are superheterodyne type.

5. RF(Radio Frequency) Link (Microwave Link)

This is actually a type of point to point wireless communication.

The radio frequencies used for RF links are in microwave range, therefore, RF links are also called as microwave links. This is shown in fig.9.

Fig.9 . Illustration of Microwave Link

Although any wire communication systems use copper wire or optical fiber, some just send the signal into the air. This happens when infrared, lasers, microwaves and radio are used for the transmission of data, as they do not need any physical medium.

For long distance communication, microwave radio transmission is widely used as an alternative to coaxial cable.

The signal transmission takes place in the form of electromagnetic waves which have wavelengths of few centimeters.

Parabolic antennas can be mounted on the towers to send a beam of waves to another antenna, tens of kilometers away.

The transmitting and receiving antennas are highly directional to enable point to point communication.

This system is widely used for both telephone and television transmission.

The higher this tower which holds the antenna, the greater is the coverage. With a 100 meter high tower, the distance of 100 km can be easily covered.

Salient Features

1. Installation of towers and associated equipment is cheaper than laying down a cable of 100 km length.
2. Less maintenance as compared to cables.
3. Repeaters can be used. Therefore, effect of noise is reduced.
4. No adverse effects such as cable breakage etc.
5. Because of the use of highly directional antenna, these links do not make any interference with other communication systems.
6. Size of transmitter and receiver reduced because of the use of high frequency.

Disadvantages

1. Signal strength at the receiving antenna reduces due to multipath reception.
2. The transmission will be affected by the thunderstorms, and other atmospheric phenomenons.

Range of Frequencies

Generally, the microwave transmission takes place at frequencies between 2 and 40 GHz. This will correspond to a wavelength of 15 cm to 0.75 cm .

6. A Mobile Radio Channel

In mobile communication, the sender and the receiver  both are allowed to move with respect to each other.

The radio propagation takes place due to scattering of EM waves from the surface of the surrounding buildings and diffraction over and around them. Hence, the transmitted energy reaches the receiver via multiple paths. This is called as multi path communication.

The signals taking different paths will have to travel different path lengths. So, they have different phase shifts when they reach the receiver.

The total signal strength at the receiver is equal to the vector sum of all the signals.

Therefore it keeps changing continuously. Hence, mobile channels are called as the linear time varying channels and it is statistical in nature.

7. Satellite Channel

Satellite microwave systems transmits signals between directional parabolic antennas.

They use low gigahertz frequencies and line of sight communication.

These systems use satellites which are in the geostationary orbit (36000 km above the earth) .

The satellites act as repeaters with receiving antennas, transponder and transmitting antenna.

Satellite microwave systems can reach the most remote places on earth and communicate with mobile devices.

This systems work in the following way : signal is sent through cable media to an antenna which beams the signal to the satellite, the satellite then transmits the signal back to another location on earth as shown in fig.10 .

Fig.1o, Illustration of Satellite System

Satellite microwave systems experience delays between the transmission of a signal and its reception back to the earth (540 ms).

Characteristics

1. It uses frequency range between 11 GHz and 14 GHz.
2. Attenuation depends on frequency, power, antenna size and atmospheric condition.
3. The signals are affected by EMI effect, jamming and eavesdropping.
4. The installation of satellites is extremely difficult and the alignment of earth station antennas must be perfectly aligned.
5. The cost of building and launching is very high.
6. The satellites can provide point to point or broadcast services.
7. The message signal transmitted by the earth station to the satellite is called as an uplink signal.
8. It is amplified and down converted in frequency by the transponder and then re-transmitted back to various earth stations.
9. The signal from satellite to earth station is called as the downlink signal.
10. The uplink signal frequency is 6GHz and the downlink signal frequency is 4 GHz.
11. In the 6/4 GHz band, a typical satellite is assigned  a 500 MHz bandwidth which is divided among 12 transponders on the satellite.
12. Each transponder uses approximately 36 MHz bandwidth. A transponder can carry at least one TV channel, 1200 voice channelss or a digital data at the rate of 50 Mb/s .