time-division multiplexing (TDM)

Definition of time-division multiplexing (TDM) in The Network Encyclopedia.

What is TDM (Time-division Multiplexing)?

Time-division multiplexing, or TDM, is a multiplexing method for transmitting multiple data streams in a single communication path.

How It Works

In time-division multiplexing (TDM), the data from different input channels is divided into fixed-length segments and then combined in round-robin fashion into a single output data stream, which can then be transmitted over a single channel transmission system and demultiplexed at the destination location. The segments can be created by the multiplexer itself or can be inherent in the input channel signals, such as fixed-length frames. For example, if input streams A, B, and C are divided into segments as shown here

A -> A1, A2, A3,...
B -> B1, B2, B3,...
C -> C1, C2, C3,...

the output stream will look like this:

MUX(ABC) -> A1, B1, C1, A2, B2, C2, A3, B3, C3,...

One weakness in this mechanism is that if an input channel does not have anything important to carry for a time, empty segments are inserted into the output stream anyway. For example, if channel A is not transmitting data, one-third of the output channel is not being used. You can overcome this weakness by using a more sophisticated multiplexing technique called statistical multiplexing.


TDM is used in T1 lines to enable them to simultaneously carry 24 data channels by interleaving data into portions of a single 193-bit frame. For example, bits 1 through 8 represent channel 1, bits 9 through 16 represent channel 2, and so on to bits 185 through 192 for channel 24, plus bit 193 for synchronization. This framing process occurs 8000 times per second, producing a total throughput of 1.544 Mbps.

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