A multiplexer (sometimes called a mux) is a communications device that multiplexes (combines) several signals for transmission over a single physical transmission channel. A demultiplexer completes the process by separating multiplexed signals from a channel line at the receiver. A multiplexer and demultiplexer are frequently combined into a single device that is capable of processing both outgoing and incoming signals. The communications channel may be shared between the multiplexed signals in a variety of ways, including Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM).
Time division multiplexing is a scheme in which multiple incoming digital signals are combined for transmission onto a single transmission line using interleaved time slots. Each incoming channel is allocated a specific time slot, and has full access to the transmission line during its allocated time slot. Some TDM systems allow for a variation in the number of signals being sent along the line, and will adjust the time interval of each slot to optimise the use of the available bandwidth.
Time division multiplexing
Analogue signals are often multiplexed using frequency-division multiplexing, in which the bandwidth of the carrier is divided into sub-channels, each having its own range of frequencies, enabling each sub-channel to carry a separate signal. Each incoming low-bandwidth signal is assigned a different sub-channel on the main channel. In order to prevent interference between adjacent sub-channels, small-bandwidth gaps, known as guard bands, are left between each sub-channel. If a large number of signals are required to be sent along a single long-distance communication link, a high-bandwidth carrier is required. The transmission system must be carefully designed to ensure that it can provide the necessary transmission characteristics.
Frequency division multiplexing
For fibre-optic channels, a variation of frequency division multiplexing, called wavelength division multiplexing (WDM), is used. As long as each incoming channel has a different frequency range, and none of the frequency ranges overlap, they can be multiplexed onto a long-haul fibre-optic transmission link. At the transmitting end, incoming optical signals are passed through a diffraction grating and combined for transmission over a high-capacity fibre-optic link. At the other end of the link, this combined signal is split into its constituent channels using another diffraction grating. An optical system of this type is completely passive, and therefore highly reliable. In WDM transmission systems, each channel will typically carry a number of time division multiplexed (TDM) signals.
Wavelength division multiplexing