Multiplexing is the process of sending data from multiple source over shared physical medium, multiplexer is used to represent multiplexing mechanism. Similarly, demultiplexing is inverse process of multiplexing; demultiplexer separates data received from multiple sources and sends to intended receivers; demultiplex is technique used to denote demultiplexing. There are four types of multiplexing techniques.
Frequency Division Multiplexing (FDM)
This is the simplest form of multiplexing, set of senders transmit electromagnetic single without disruption, each one is allotted separate frequency or carrier channel. This forms basis how radio stations work.
“Because carrier waves on separate frequencies do not interfere, frequency division multiplexing provides each sender and receiver pair with a private communication channel over which any modulation scheme can be used” Comer, D. (2009).
One other variation of FDM is Hierarchical FDM which allows shifting frequencies using hardware, the output of one stage becomes input of others and so on.
Wavelength Division Multiplexing (WDM)
Wavelength Division Multiplexing is another form frequency division multiplexing which applies to Fiber Optic transmission. This depends on wavelength of light, in physics when light passes through prism the output rays speared out, this can work in reverse or der as well; if set of lights directed towards prism then prism combines the beams to make signal beam of light, that how WDM works.
“When frequency division multiplexing is applied to optical fiber, prisms are used to combine or separate individual wavelengths of light, and the result is known as wavelength division multiplexing” Comer, D. (2009).
Time Division Multiplexing (TDM)
Time Division Multiplexing which using time for multiplexing instead of relying on electromagnetic wave properties. The simple flow how TDM works; between N senders and receivers, it sends N bits in sequence. Synchronous TDM is another variation of TDM which is most widely used today over internet; it sends data in round-robin fashion.
“The synchronous TDM mechanism used for digital telephone calls includes a framing bit at the beginning of each round. The framing sequence of alternating 1s and 0s insures that a demultiplexer either remains synchronized or detects the error” Comer, D. (2009).
Time Division Multiplexing has another variation called Hierarchical TDM which works like Hierarchical FDM. One of the challenges in Synchronous TDM is unfilled slots, that is when a source doesn’t have data in timely manner at time of sending.
Code Division Multiplexing (CDM)
Code Division Multiplexing is another form of multiplexing which doesn’t really on physical properties of medium, instead based on mathematical formula; values from orthogonal vector spaces can be combined and separated without interference. CDM is used in cellphone networks and some satellite transmission; the variation used in cell phone is specifically called Code Division Multi-Access (CDMA). In CDM delay is lower when network is utilized highly.