      It is a technique by which the total bandwidth is divided into a series of non- overlapping frequency bands and each of them is used to carry a separate signal.

      This allows a single transmission medium such as a cable or optical fiber to be shared by multiple independent signals. Another use is to carry separate serial bits or higher rate signal segments in parallel.

      The most natural example of frequency-division multiplexing is radio and television broadcasting. FDM is also used by telephone systems to transmit multiple telephone calls through high capacity trunklines, 

      The multiple information (modulation) signals that are sent over an FDM channel are called baseband signals.

      At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, that serves to "carry" information.

      The carrier has higher frequency than the baseband signal.

      The carrier and the baseband signal are combined in a modulator circuit. The modulator alters the carrier signal, in its amplitude, frequency, or phase, with respect to the baseband signal,

      The result of modulating the carrier with the baseband signal is to generate sub-frequencies near the carrier frequency, as the sum (fC + fB) and difference (fC − fB) of the frequencies.

      The information from the modulated signal is carried in sidebands present on each side of the carrier frequency.

      Hence, all the information carried by the channel is in a narrow band of frequencies clustered around the carrier frequency, this is called the pass band of the channel.