Statistical Time-Division Multiplexing (STDM)

Statistical time-division multiplexing (STDM) shares a communication like TDM, but like the TDM, the time slots are not allocated to any channel. In the case of TDM, time slots are allocated to channels and reserved even if there is no data to send.  This allocation is just a waste of bandwidth, and Statistical time-division multiplexing overcomes this inefficiency of standard TDM.  Statistical time-division multiplexing achieved this with the use of intelligent devices that are capable of identifying when a terminal is idle.

STDM was specially developed to overcome TDM’s inefficiency. It uses a variable time slot length, allowing channels to use any free space to send their information. It uses a buffer memory to store the data during peak traffic temporarily. STDM requires each transmission to carry identification information or a channel identifier.

Statistical time-division multiplexing is the same as TDM, except that every signal is assigned a slot based on priority and demand. Standard TDM and other circuit-switching techniques are executed at the physical layer in the OSI and TCP/IP model. Still, Statistical time-division multiplexing is executed at the data link layer and above.

The STDM is used to transport multiple data, audio, and video streams with different data rates to be broadcast across a bandwidth-limited channel. The Frame relay packet-switching and X.25 protocols, in which the packets have different lengths, are examples of this. The STDM is an example of an Asynchronous Transfer Mode packet-switched protocol.

In the asynchronous transfer mode, single streams are classified into variable time segments and transmitted using the asynchronous time-division multiplexing procedure. In this mode, the order of the bit transmission is not defined. Each time slot carries channel information to separate the packet in the demultiplexer procedure. The figure below illustrates the STDM process.