Multiplexing
In electronics, telecommunications and computer networks, multiplexing
referrers to the process where multiple analog messages signals or digital
data streams are combined into one signal. Analogue circuits are much harder
to design and analyze the digital ones because the designer must take into
account effects such as the gain, linearity and power handling of components,
the resistance, capacitance and inductance of PCB tracks, wires and connectors,
interference between signals, power supply stability and more. A digital
design is usually less critical than an analog because most digital components
will function correctly within a range of parameters whereas variations
will corrupt the outputs of an analogue circuit. In communications, a multiplexed
signal is transmitted over a communication channels,
which may be a physical transmission medium. Multiplexing divides the capacity
of the low-level communication channel into several higher-level logical
channels. There is a reverse process called demultiplexing, that extracts
the original channels on the receiver side.
This is attempting to share an expensive resource. Multiplexing allows several
analog signals to be processed by one analog-to-digital converter (ADC),
and in telecommunications, several phone calls may be transferred using
one wire. A multiplexer is a device that performs the multiplexing is called
a, and a device that performs the reverse process is called a demultiplexer
(DEMUX). The two most basic forms of multiplexing are time-division multiplexing
(TDM) and frequency-division multiplexing (FDM). In optical communications,
FDM is referred to as wavelength-division multiplexing (WDM). Variable bit
rate digital bit streams may be transferred efficiently over a fixed bandwidth
channel by means of statistical multiplexing, for example packet mode communication.
In addition, digital bit streams can be transferred over an analog channel
by means of code-division multiplexing. In wireless communications, multiplexing
can also be accomplished through alternating polarization (horizontal/vertical
or clockwise/counterclockwise) on each adjacent channel and satellite, or
through phased multi-antenna array combined with a Multiple-input multiple-output
communications (MIMO) scheme. 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 seven layer model or TCP/IP five layer
model, while multiple access also involves a media access control protocol,
which is part of the data link layer.
In addition, inverse multiplexing (IMUX) has the opposite aim as multiplexing,
namely to break one data stream into several, and transfer
them simultaneously over several communication channels, and recreate the
original data stream.
Fiber in the loop (FITL) is a common method of multiplexing, which uses
optical fiber as the backbone. It not only connects POTS phone lines with
the rest of the PSTN, but also replaces DSL by connecting directly to Ethernet
wired into the home.
