A multiplexing technique used in radio networks, FDMA derives multiple narrowband frequency channels from a wider band of assigned radio spectrum, much as frequency division multiplexing (FDM) operates in the electrical wireline domain. Using a technique known as frequency division duplex (FDD), a given call takes place on one pair of frequencies, with one for transmission in the forward direction and another for transmission in the reverse direction. At the same time, another call takes place on another pair of frequencies. The forward and reverse channels in each frequency pair are separated in frequency in order to avoid crosstalk and other forms of co-channel interference. Analog cellular systems, such as Advanced Mobile Phone System (AMPS), employ FDMA. Alternative multiplexing techniques employed in various cellular radio networks are code division multiple access (CDMA) and time division multiple access (TDMA). See also AMPS, channel, crosstalk, FDD, FDM, FDMA, frequency, multiplexer, narrowband, radio, spectrum, and TDMA.