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.
There are two types of channels, namely point-to-point channels and broadcast channels.
Example of point-to-point channels are wire lines, microwave links and optical fibres. Wire-lines operate by guided electromagnetic waves and they are used for local telephone transmission.
In case of microwave links, the transmitted signal is radiated as an electromagnetic wave in free space. Microwave links are used in long distance telephone transmission.
An optical fibre is a low-loss, well-controlled, guided optical medium. Optical fibres are used in optical communications.
Although these three channels operate differently, they all provide a physical medium for the transmission of signals from one point to another point. Therefore, for these channels, the term point-to-point is used.
On the other hand, the broadcast channel provides a capability where several receiving stations can be reached simultaneously from a single transmitter.
An example of a broadcast channel is a satellite in geostationary orbit, which covers about one third of the earth’s surface.
During the process of transmission and reception the signal gets distorted due to noise introduced in the system.
Noise is an unwanted signal which tend to interfere with the required signal. Noise signal is always random in character. Noise may interfere with signal at any point in a communication system. However, the noise has its greatest effect on the signal in the channel.