In telecommunications and computer networks, multiplexing (known as muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire. It originated in telegraphy, and is now widely applied in communications. The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side. A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX). Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

Techniques of Multiplexing

A multiplexing technique may be further extended into a multiple access method or channel access method, for example TDM into Time-division multiple access (TDMA) and statistical multiplexing into carrier sense multiple access (CSMA). A multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity.Multiplexing is provided by the Physical Layer of the OSI model, while multiple access also involves a media access control protocol, which is part of the Data Link Layer.

Multiplexing and multiple access

Modern telephone networks allow bandwidths in their channels that are much larger than those is needed for a digitalized telephone channel. Basically, a number of channels share a common transmission medium with the aim of reducing costs and complexity in the network. Multiplexing is defined as the process by which several signals from different channels share a channel with greater capacity. When the sharing is carried out with respect to a remote resource, such as a satellite, this is referred to as multiple access rather than multiplexing. There are various ways of performing this sharing:

# FDM/FDMA (Frequency Division Multiplexing/Frequency Division Multiple Access): Assigns a portion of the total bandwidth to each of the channels. # TDM/TDMA (Time Division Multiplexing/Time Division Multiple Access): Assigns all of the transport capacity sequentially to each of the channels. # CDMA (Code Division Multiplexing Access): In certain circumstances it is possible to transmit multiple signals in the same frequency and at the same time, with the receiver being responsible for separating them. This technique has been used for years in military technology, and is based on extending the spectrum of the signal and reducing the transmission power. # PDMA (Polarization Division Multiple Access): Given that polarization can be maintained, the polarization direction can be used as a multiple access technique, although when there are many obstacles noise can make it unsuitable, which is why it is not usually used in indoor installations. Outside, however, it is widely used to increase transmission rates in installations that use microwaves. # SDMA (Space Division Multiple Access): With directional aerials, the same frequency can be re-used provided the alignment of the aerials is correctly adjusted. There is a great deal of interference but this system lets frequencies obtain a high degree of reusability.