with additional bits, to make it as integer multple of other device.
By the technique called bit stuffing, the multiplexer adds extra bits to a device's source stream to speed relationships among the various devices into integer multiples of each other.
At receiver, exactly reverse operation in performed. The demultiplexer removes the message from frame and pass it to the approximate receiving device.
Asynchronous time division multiplexing. This type of TDM is also called statistical TDM as it avoids the redundant bits requirement. In synchronus TDM, the total speed of the input lines can be greater than the capacity of the link. In synchrons TDM, for n channel input, there are atleast n time slots. In asynchronous TDM, for n input channel, the number of time slots is less than n (say m). Hence with the same link, asynchronous TDM can support more devices than synchronous TDM. The number of asynchrons TDM frame (m) is based on a statistical analysis of the number of input lines that are likely to the transmitting at any given time. The operation of asynchronous TDM is explained with two cases.
Case (i). Fig. 10.3, shows the multiplexing of asynchorous TDM where there are 5 input lines and of five only three input channels are sending data.
In the above figure, 3 time slots are used in each frame. For the first three frames, the input is symmetrically distributed among all the channels. In fourth frame 3rd channel not having data and hence channel 1 data (A) is filled in 3rd time slot. This causes the absence of fixed positional relationships.
Hence, each time slot must carry a bit or bits to send data to a particular output line. This additional bit is attached by the multiplexer and discarded by the demultiplexer once it has been read. This is called addressing. This causes the system to become inefficient and additional overhead. For this case, in a synchronous TDM system, six frames (max data of A in channel 1) and five time slots (for 5 channels) requires 30 time slots. But only 13 time slots would have been filled in asynchronous system.
Case (ii). Fig. 10.4 shows the multiplexing of asynchronous TDM where there are 5 input lines and of five, four channels are sending data.
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