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STDM, or statistical time division multiplexing, is one method for transmitting several types of data simultaneously across a single transmission cable or line (such as a T1 or T3 line). STDM is often used for managing data being transmitted via a local area network (LAN) or a wide area network (WAN). In these situations, the data is often simultaneously transmitted from any number of input devices attached to the network, including computers, printers, or fax machines.
STDM can also be used in telephone switchboard settings to manage the simultaneous calls going to or coming from multiple, internal telephone lines.
The concept behind STDM is similar to TDM, or time division multiplexing. TDM allows multiple users or input devices to transmit or receive data simultaneously by assigning each device the same, fixed amount of time on one of many "channels" available on the cable or line. The TDM method works well in many cases, but does not always account for the varying data transmission needs of different devices or users.
For example, a busy laser printer shared by many users might need to receive or transmit data 80-90% of the time at a much higher transmission rate than a seldom-used, data-entry computer attached to the same T-1 line. With TDM, even though the printer's transmission needs are greater, both devices would still be allocated the same duration of time to transmit or receive data.
In comparison to TDM, the STDM method analyzes statistics related to the typical workload of each input device (printer, fax, computer) and determines on-the-fly how much time each device should be allocated for data transmission on the cable or line. In the above example, STDM would allocate more time to the group printer, based on its past and current transmission needs and less time to the data-entry computer. Many believe the STDM method is a more efficient use of total bandwidth available than the TDM method.
The main statistics used in STDM are: each input device's peak data rates (in kbps, or kilobytes per second), and each device's duty factors (which is the percentage of time the device typically spends either transmitting or receiving data).
Besides TDM and STDM, other methods for simultaneously transmitting data on the same channel include Wavelength Division Multiplexing (WDM) and Frequency Division Multiplexing (FDM).
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