A multiplexer (also known as a mux) is a device that performs multiplexing. Generically, it takes two or more input facilities and combines them onto a single, higher-capacity output facility. At the remote end of the higher speed facility, a demultiplexer reverses the process. Because most communication facilities are full duplex, most multiplexers contain a combination of a multiplexer and a demultiplexer.
Multiplexers can perform frequency division multiplexing, time division multiplexing, or code division multiplexing. Multiplexers are typically designed to work with a particular technology. For example, a T-1 mux multiplexes DS-0 and lower speed circuits onto a T-1 facility. An M13 multiplexer combined multiple T-1s onto a T-3 facility. A SONET add/drop multiplexer can combine DS-0, T-1, T-3, and lower-speed SONET circuits onto higher-speed SONET facilities, typically a SONET ring.
The primary purpose of a multiplexer is to reduce the cost of transmission facilities. For example, to deploy 24 DS-0 circuits from a central office to a customer location would required 24 (or possibly 48) twisted-pair cables. If they are multiplexed into a T-1, however, only two pair are required.
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