Q. What do you mean by communication channel?

The communication channel (transmissionmedium) is the physicalmediumthat is utilized to send the signal from the transmitter to the receiver. In wireless transmission, such as microwave radio, the transmission medium is usually the atmosphere or free space. Telephone channels, on the other hand, employ a variety of physical media such as wire lines and optical fiber cables. Irrespective of the type of physical medium for signal transmission, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms. For simplicity, the effects of these phenomena (attenuation, distortion, interference, noise, etc.) are shown at the center of Figure, since the transmission medium is often the most vulnerable part of a communication system, particularly over long distances.

Attenuation, caused by losses within the system, reduces the size or strength of the signal, whereas distortion is any alteration of the waveshape itself due to energy storage and/or non-linearities. Contamination by extraneous signals causes interference, whereas noise emanates from sources both internal and external to the system. To eliminate any one of these may pose a challenge to the design engineer.