Time-division multiplexing (TDM)

Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent time slots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.

In the diagram to the right you can see the 24 channels (CH 1 through CH 24) of a standard T1. At a given point in time (t), every channel provides an input value (not really, but that's a more complicated subject for later). These input values are represented in this diagram by the letters of the alphabet next to each channel in the diagram.

The data passes through the multiplexing device and emerges from the other side as a time-division multiplexed signal that contains the inputs from all the other channels (t+1). Note that all 24 pieces of information are still being transmitted, but are being transmitted together in the same amount of time as when they were part of their original channels.