Time division multiplexing explained

Time division multiplexing (TDM) is a digital multiplexing technique in which several low-rate digital signals share one high-rate link by occupying it in different, non-overlapping time slots. The whole bandwidth is used by one source at a time, in turn.

Operation

• Each input stream is divided into small units (a bit, byte or character).
• The multiplexer takes one unit from each input in turn and places it in a slot. One slot from each input makes one frame.
• Frames are sent one after another. The demultiplexer at the receiver uses a synchronised clock to deliver each slot to the correct output line.
• The link rate must be at least the sum of the input rates. With n inputs of rate R each, the link rate is n × R, and each slot is 1/n as long as the original unit duration.
• Extra framing bit(s) at the start of each frame keep sender and receiver synchronised.

 A: A1 A2 A3 ..   --->|       |
 B: B1 B2 B3 ..   --->| MUX   |-- A1 B1 C1 | A2 B2 C2 | A3 ..
 C: C1 C2 C3 ..   --->|       |   [frame 1]  [frame 2]
                                  (slots)
                                      |
                              link --> DEMUX --> A, B, C

Types

• Synchronous TDM: fixed slot for every input in every frame, even if idle.
• Asynchronous (statistical) TDM: slots given only to active inputs, with addresses.

Other points

• Interleaving can be at bit level or byte level.
• Problems: unequal rates (solved by multilevel multiplexing, multiple-slot allocation or pulse stuffing/bit padding) and idle slots wasted.
• Example: T1 carries 24 voice channels, 8 bits each per frame, 8000 frames/s: (24×8+1) × 8000 = 1.544 Mbps.