Time division multiplexing

From Hill2dot0

Time division multiplexing (TDM) is commonly used to transport multiple digital signals over a single facility. Because the frequency spectrum of a digital signal cannot easily be confined, such signals are amenable to multiplexing only in the time domain. In TDM, a single facility is split into multiple time slots of equal size. Each pair of users is assigned a time slot when the circuit is established; that time slot remains dedicated to them for the life of the circuit.

In TDM, the passband is not divided; rather, each user gets the use of the entire passband, but only for a short time. Thus, rather than dividing up the use of the medium in frequency, it is divided in time. The users simply take turns. The accompanying visual shows an example of TDM providing six channels.

TDM schemes vary according to how much data each input places in each time slot (typically either a character or a single bit). Character interleaving permits the multiplexers to support asynchronous devices without having to transmit start and stop bits across the channel, and can therefore improve efficiency. Multiplexers can also be configured to support sources with different data rates by assigning more time slots to some sources. To maintain synchronization between the multiplexers, extra bits in some predefined pattern are typically added to the stream.

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