Communication system basically consists of three elements :
(i) Transmitter : The transmitter converts the message (or information) signal into a suitable signal which may be passed on to a suitable medium called transmission channel. The signal processing for transmission usually involves modulation and coding.
(ii) Transmission Channel : It is the medium that carries the message in the form of a suitable signal from one place to another. It may be a pair of wires, a co-axial cable, a radio wave or a laser beam. Every channel involves loss of energy during transmission. That is why signal power goes on decreasing with increasing distance.
(iii) Receiver : Receiver converts the suitable signal prepared by transmitter into actual message (or information). Receiver operations include amplification of signal to compensate for the transmission loss, demodulation and decoding to reverse the signal processing performed by the transmitter.
The three components of a transmission system are sketched in the fig.