Multiplexing is a technique that combines multiple channels of information over a single circuit or transmission path, according to the book "Encyclopedia of Networking & Telecommunications." Many protocols fall under the category of multiplexers, among them TCP/IP, the primary protocol of the Internet. TCP/IP is a multiplexer because it allows a computer to transmit two or more simultaneous connections over a single network link. Several other types of multiplexing protocols exist.
Space Division Multiplexing
The simplest multiplexing method is space division multiplexing (SDM), in which many separate transmission paths are grouped into a common cable, according to the book "Performance Analysis of Telecommunications and Local Area Networks." For instance, a telephone cable that consists of hundreds of twisted pair wires constitutes an SDM system, since many conversations can be carried out through the same cable.
Frequency Division Multiplexing
Frequency Division Multiplexing, or FDM, was used in early telephone systems. It is an analog broadband transmission technique in which each data or voice signal is modulated at a different frequency, like signals from different radio stations. FDM divides the transmission circuit's frequency range into bands, with each band carrying a different signal.
Wavelength Division Multiplexing
A variant of FDM, wavelength division multiplexing (WDM) is used on fiber-optic cables. It utilizes infrared signals, specifically two or more wavelengths in the infrared range that are transmitted through the same fiber-optic cable. Infrared beams occupy different frequency ranges than radio signals and are a fixture of so-called optical networks.
Time Division Multiplexing
Time division multiplexing (TDM) works on the principle of a channel of data or voice being transmitted for a set amount of time, after which a second channel takes its turn to be transmitted, and so on. However, the time slots involved are very fast and short, and so it appears as if several channels are transmitted through the same channel simultaneously. Transmissions travel in bits, and each bit occupies a specific timeslot in the channel.
In time division multiplexing, there are several instances where the time slots may be empty if the channel has nothing to send. This could result in some channels being overtaxed while others are underused. Statistical time division multiplexing is a variant of TDM that dynamically allocates time slots and uses all of the lines more efficiently.