Multiplexing should be understood not as a specific operation but rather as an end product that may be achieved with different methods, sometimes in combination. The rationale is to economize on limited resources, particularly cabling or wireless bandwidth. The archetypal format would be to use a single pair of telegraph or telephone wires to simultaneously convey two or more communication channels to the detriment of neither.
There are many examples of such usage in everyday electronics. CATV and ordinary telephone lines simultaneously carry audio, video and synch information and at the same time network-supplied power, sometimes originating at the opposite end of the line.
Another scenario is for numerous audio and/or RF signals to be conveyed along a pair of wires or coaxial cable. Simple resonant circuitry can be used to tune in discrete channels conveyed along a network cable or transmission line between antenna and receiver.
Properly speaking, multiplex technology is an order of magnitude more complex. It consists of various methods by which analog signals and/or digital data streams are actually combined and later separated, so they can share limited media resources. The first multiplexing was developed in the 1870s for use in telegraph cabling, and in 1910 it emerged as a telephone carrier mode.
Where there is multiplexing, at the receiver end there must be a device that separates the desired signal(s), and this is known as a demultiplexer.
The principle types of multiplexing include:
Space-division multiplexing, wherein separate point-to-point wiring pairs are used for different channels. They may share a common return conductor. In early telegraphy, the earth was used as a return path, but this was not entirely satisfactory.
Frequency-division multiplexing, wherein tuning is employed to select the desired analog signal.
Time-division multiplexing, primarily a digital technique, wherein data streams are divided and make use of the media resources during prescribed time intervals. Carrier-sense multiple-access is a subdivision of this technology, but it differs in that there are separate origins.
Polarization-division multiplexing wherein separation is via mechanical rotation of an antenna or analogous electronic means. This is used to extend the capacity of satellite-dish TV transmission.
Orbital angular momentum multiplexing is a new technique, which may in the future enhance the capacity of existing multiplex technologies.
Code-division multiplexing, wherein frequency hopping and direct-sequence spread-spectrum are among the possible forms. The global positioning system (GPS) makes use of this type of multiplexing.
Other methods of multiplexing are emerging. As long as there is human imagination, new more efficient methods will probably come on line, perhaps eventually alleviating the problem of limited bandwidth.
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