Asynchronous or Statistical Time Division Multiplexing
As we have seen synchronous TDM does not guarantee that the full capacity of a link in sued. In fact it is more likely that only a portion of the time slots are in use at a given instant. Because the time slots are reassigned and fixed whenever a connected device is not transmitting, the corresponding slot is empty and that much capacity of the path is wasted. For example imagine that we have multiplexed the output of 20 identical computer onto a single line. Suing synchronous TDM the speed of that line must be at least 20 times the speed of each input line. But what if only 10 computers are in use at a time ? half of the capacity of the line is wasted.
Statistical time division multiplexing is designed to avoid this type of waste. As with the term synchronous. The term asynchronous means some things different in multiplexing that it means in other areas of data communications. Here it flexible or not fixed.
Like synchronous TDM asynchronous TDM allows a number of lower speed input lines to be multiplexed to a single higher speed line. Unlike synchronous TDM however in asynchronous TDM the total speed of the input can be greater than the capacity of the path. In a synchronous systems if we have in input lines. The frame contains a fixed number of a least m time slots. In an asynchronous systems. If we have n input line the frame contains no more than m slots with m less than n. In this way asynchronous TDM supports the same number of input lines as synchronous TDM with a lower capacity link or given the same link. Asynchronous TDM can support more devices than synchronous TDM.
figure Asynchronous TDM
The number of time slots in an asynchronous TDM fame 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 reassigned each slot is available to any of the attached input lines that has data to seen. The multiplexer scans the input lines accept portions of data until a frame is felled and then sends the frame across the link. If there are not enough data to fill all the slot s in a frame the frame a transmitted only partially filled thus full link capacity may not be used 100 percent of the time. But the ability to allocate time slots dynamically coupled with the lower ration of the slots to input greatly reduces the likelihood and degree of waste.