Asynchronous time-division multiplexing, or statistical time-division multiplexing, is designed to avoid wastage. Asynchronous means something different in multiplexing than it means in other areas of data communications.
Asynchronous TDM allows a number of lower speed input lines to be multiplexed to a single higher speed line. The total speed of the input lines can be greater than the capacity of the path. In a synchronous system, if we have n input lines, the frame contains a fixed number of at least n time slots. In an asynchronous system if we have n input lines, the frame contains no more than m slots (m less than n). Asynchronous TDM supports the same number of input lines as synchronous TDM with a lower capacity link. Asynchronous TDM can support more devices than synchronous TDM.
The number of time slots in an synchronous TDM frame is based on a statistical analysis of the number of input lines that are likely to be transmitting at any given time. Rather than being preassigned, each slot is available to any of the attached input lines that has data to send. The multiplexer scans the input lines, accepts portions of data until a frame is filled, and then sends the frame across the link. If there are not enough data to fill all the slots in a frame, the frame is transmitted only partially filled; thus full link capacity may not be used 100 percent of the time. The ability to allocate time slots dynamically, coupled with the lower ratio of time slots to input lines, greatly reduces the likelihood and degree of waste.
Consider three cases when five computers are sharing a data link using asynchronous TDM. In the first case, only three of the five computers have data to send (the average scenario for this system, as indicated by the fact that a frame size of three slots was chosen). In the second case, four lines are sending data, one more than the number of slots per frame. In the third case, all lines are sending data. In each case, the multiplexer scans the devices in order, from 1 to 5, filling time slots as it encounters data to be sent.
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Asynchronous TDM can accommodate traffic of varying data rates by varying the length of the time slots. Stations transmitting at a faster data rate can be given a longer a slot and they increase over head of the system.
