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