Time-Division Multiplexing

Time-Division Multiplexing

Time-division multiplexing (TDM) divides a channel into time slots that are allocated to the data streams to be transmitted, as illustrated in Figure 3.4. If the sender and receiver agree on the time-slot assignments, the receiver can easily recover and reconstruct the original data streams.

TDM transmits the multiplexed signal in baseband mode. Interestingly, this process makes it possible to multiplex a TDM multiplexed signal as one of the data channels on an FDM system.

Conventional TDM equipment utilizes fixed-time divisions and allocates time to a channel, regardless of that channel’s level of activity. If a channel isn’t busy, its time slot isn’t being fully utilized. Because the time divisions are programmed into the configurations of the multiplexors, this technique often is referred to as synchronous TDM.

If using the capacity of the data medium more efficiently is im-portant, a more sophisticated technique, statistical time-division multiplexing (StatTDM), can be used. A stat-mux uses the time-slot technique but allocates time slots based on the traffic demand on the individual channels, as illustrated in Figure 3.5. Notice that Channel B is allocated more time slots than Channel A, and that Channel C is allocated the fewest time slots. Channel D is idle, so no slots are allocated to it. To make this procedure work, the data transmitted for each time slot includes a control field that identifies the channel to which the data in the time slot should be assigned.