A communication channel, provides the connection between transmitter and receiver

Different types of channel are:

1] Wire-line channel

2] Fiber optic channel

3] Wireless channel

4] Underwater channel

5] Storage channel.

The most important problem faced by all channel is addictive noise, which is sometimes called as thermal noise.

Wire-line channel :

• These channel use the pair of cables that carry the signal in electrical form telephone n/w is the best example.

• The wire line channels include the twisted pair line and co axial cables. Together they are called guided channel.

Co-axial cables:

• The connection of Co-axial cable is shown below. It consist of two concentric conductors separated by a dielectric material.

• The Co-axial cable was initially developed as backbone of analog telephone n/w’s where a single telephone cable would be used to carry more than 10,000 voice channels of a time. The most popular application of Co-axial cable is in cable TV system. The existing Co-axial cable has a range from 54 MHz to 500 MHz.

• One more application is Ethernet LAN.

Twisted pair cables :

• The construction of twisted pair cable is shown below:

• This is a commonly used medium and is cheaper than co-axial cable.

wireless channel:

• In radio communication, Tx radiates its o/p in the form of electromagnetic waves using communication system.

• These waves travel towards the Rx through free air.

• The propagation of EM waves takes place by means of one of the following types:

1] Ground wave

2] Sky wave

3]Space wave.

• Ground wave propagation is dominant mode of propagation in the frequency band as 30 KHz to 3 MH z.

• Sky wave propagation uses the ionosphere which is the layer of ions above earth atmosphere to reflect back the EM waves.

• Sky wave propagation or LOS is used for frequencies above 30 MHz.

Under water channels:

• The EM waves cannot propagate over long distances at extremely low frequency.

• But it is very expensive because large Tx are required to be used.

• For sea water skin depth is given by $\delta = \frac{50}{\sqrt{f}}$

Storage channels:

• The information storage and retrieval systems are very important in data handling activity.

• Magnetic tapes, digital audio and video tapes are used for storing large amount of data.

• Optical desks are used for storing computer data, music and video information, all these data characterized as storage channels.