Statistical time division multiplexing
Statistical Time Division Multiplexing (STDM) is a variant of time division multiplexing (TDM). In TDM, digital signals from a number of users are transported over a single wideband facility by time slicing the available bandwidth of that facility. Simple TDM assumes a synchronous nature to the multiplexing; the time slices are all of equal size (typically a few bits) and the users cycle through the time intervals in a fixed cycle. This gives each user a constant bandwidth channel with fixed delay characteristics. Synchronous Optical Network (SONET) and T-carrier systems are excellent examples of this kind of synchronous TDM system.
It is important to note the importance of the time slots being the same size and the users cycling in a fixed pattern. This allows the demultiplexer at the egress end of the circuit to distinguish one user from another and correctly demultiplex their traffic. Each user is distinguished in time. One might call this a form of implicit addressing. As long as the demultiplexer can correctly locate the time slots in the incoming bit stream, it can distinguish two separate channels by the time at which they arrive.
In statistical TDM (STDM), time slots are assigned to users on a first come, first served basis. User messages are queued in a pool of buffers and the buffers empty into time slots as slots become available on the shared medium. Thus, any user can be assigned to any time slot at any time. Because time slots are not dedicated to a particular user, it is not possible for the demultiplexer to locate a particular user simply by when the bits arrive. Instead, each time slot must carry an explicit address that identifies the user for that time slot. This addressing is overhead and is necessary to identify the receivers.
If each time slot contains an implicit address, there is also no requirement for the time slots to be exactly the same size. They can be, but they can also be allowed to vary in size. If they vary in size, however, it is critical that the transmitter provide adequate information about how long any particular time slot happens to be. This is called “framing.”
The objective of STDM technology is efficient use of bandwidth on the shared facility. If there is enough traffic being generated by users, there will be no empty time slots on the medium. In addition, STDM tends to provide bandwidth to those users who need it most (i.e., those users with a lot of traffic to send), what is sometimes referred to as “bandwidth on demand.”
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