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Multiplexing Methods
- Multiplexing is a technology that is used to combine and send multiple digital signals
simultaneously across a single physical transmission channel (wire, fiber, or link).
- Multiplexing occurs on the physical layer, as it combines multiple signals across a single
physical channel.
- Time-Division Multiplexing (TDM) is a technique for assigning bandwidth on a single wire to
data from several channels based on pre-assigned time slots. Bandwidth is allocated to each
channel based on time slots, regardless of whether there is data to be transferred. Bandwidth will
be wasted when there is no data to transfer. The SDH/SONET is an example application of TDM.
Note: Statistical Time-Division Multiplexing (STDM) is an advanced version of TDM.
- Frequency-Division Multiplexing (FDM) is a form of signal multiplexing where multiplex
baseband signals are modulated on different frequency carrier waves and combined together to
create a composite signal. Data channels are allocated bandwidth based on the signal frequency
of the traffic. Ex: FM radio.
- The concept corresponding to FDM in the optical domain is known as Wavelength-Division
Multiplexing (WDM). Wavelength-Division Multiplexing (WDM) and Dense WDM
(DWDM) are technologies that are being used in fiber-optic communications which multiplex
multiple optical carrier signals of a single fiber optic by using different wavelengths (colours) of
laser light to carry different signals. Data channels are allocated bandwidth based on wavelength
(inverse of frequency).
- With Statistical-Division Multiplexing (SDM), the link sharing is adapted to the instantaneous
traffic demands of the data streams that are translated over each channel (bandwidth is
dynamically allocated to data channels). SDM is an alternative to multiplexing methods that
create a fixed sharing of a link, eg: TDM and FDM.
- Asynchronous Time-Division Multiplexing (ATDM) is differs from normal TDM in which the
time slots are assigned when necessary rather than pre-assigned to certain transmitters.
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