Multiplexing enables several information (speech or data) channels to be carried simultaneously over one bearer, a wide band, single frequency microwave radio link for example. Both frequency and time division multiplex are common methods. On trunked radio systems where channels are allocated to users on demand the multiplexing is referred to as frequency- or time-division multiple access (FDMA, TDMA).
Frequency division multiplexing (FDM) used to be a common technique for sharing analogue trunk circuits between 12 (or multiples of 12) separate channels. This is an analogue technique that been superseded by the use of digital trunk circuits, where digital multiplexing is employed. FDM is still used on some satellite and microwave links, although most links are now using digital techniques. Some systems such as cellular telephones use separate transmit and receive frequencies, which is known as frequency division duplexing (FDD).