FREQUENCY DIVISION AND COUNTING

In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency sub-bands, each of which is used to carry a separate signal. These sub-bands can be used independently with completely different information streams, or used dependently in the case of information sent in a parallel stream. This allows a single transmission medium such as the radio spectrum, a cable or optical fiber to be shared by multiple separate signals.

The most natural example of frequency-division multiplexing is radio and television broadcasting, in which multiple radio signals at different frequencies pass through the air at the same time. Another example is cable television, in which many television channels are carried simultaneously on a single cable. FDM is also used by telephone systems to transmit multiple telephone calls through high capacity trunklines, communications satellites to transmit multiple channels of data on uplink and downlink radio beams, and broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses.

An analogous technique called wavelength division multiplexing is used in fiber optic communication, in which multiple channels of data are transmitted over a single optical fiber using different wavelengths (frequencies) of light.