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A multiplexer (MUX) is a device allowing one or more low-speed analog or digital input signals to be selected, combined and transmitted at a higher speed on a single shared medium or within a single shared device. Thus, several signals may share a single device or transmission conductor such as a copper wire or fiber optic cable. A MUX functions as a multiple-input, single-output switch.
In telecommunications the combined signals, analog or digital, are considered a single-output higher-speed signal transmitted on several communication channels by a particular multiplex method or technique. With two input signals and one output signal, the device is referred to as a 2-to-1 multiplexer; with four input signals it is a 4-to-1 multiplexer; etc.
For analog signals in telecommunications (and signal processing), a time division multiplexer (TDM) may select multiple samples of separate analog signals and combine them into one pulse amplitude modulated (PAM) wide-band analog signal.
For digital signals in telecommunications on a computer network or with digital video, several variable bit-rate data streams of input signals (using packet mode communication) may be combined, or multiplexed, into one constant bandwidth signal. With an alternate method utilizing a TDM, a limited number of constant bit-rate data streams of input signals may be multiplexed into one higher bit-rate data stream.
A multiplexer requires a demultiplexer to complete the process, i.e. to separate multiplex signals carried by the single shared medium or device.
Often an multiplexer and a demultiplexer are combined into a single device (also often just called a multiplexer) allowing the device to process both incoming and outgoing signals. Alternately, a multiplexer’s single output may be connected to a demultiplexer’s single input over a single channel. Either method is often used as a cost-saving measure. Since most communication systems transmit in both directions, the single combined device, or two separate devices (in latter example), will be needed at both ends of the transmission line.
Other types of multiplexing technologies and processes include, but are not limited to: