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TDM vs statistical TDM and STDM frame

Short answer

TDM is simple and constant-delay but wastes slots when inputs are idle. Statistical TDM fills frames only from active inputs, tagging each piece with address and length, so it uses less capacity for bursty traffic but adds overhead, buffering and variable delay. Its frame is Flag, Address, Control, sub-frames (Address, Length, Data), FCS, Flag.

What are the strength and weakness of TDM and statistical TDM? How statistical TDM recover weakness of TDM? Explain STDM with its frame format.

In synchronous TDM every input channel gets a fixed time slot in every frame whether or not it has data. In statistical (asynchronous) TDM slots are given only to inputs that currently have data, so the link capacity is shared on demand.

Strengths and weaknesses of synchronous TDM

Strengths

  • Simple multiplexer and demultiplexer; position of a slot identifies the channel, so no address overhead.
  • Fixed, constant delay; ideal for voice and other constant-bit-rate traffic.
  • Easy synchronisation and well-standardised (T1, E1).

Weaknesses

  • Wasted capacity: idle sources still get slots; for bursty data traffic most slots go empty.
  • Link rate must equal the sum of all input rates, even if most are idle.
  • Inflexible: number of channels and rates fixed.

Strengths and weaknesses of statistical TDM

Strengths

  • Much better link utilisation; the link rate can be less than the sum of input rates.
  • Supports more terminals on the same line for bursty data.
  • Flexible, can handle variable rates.

Weaknesses

  • Each slot must carry an address (and length), adding overhead.
  • Needs buffers; when many inputs are active at once, data is queued, so delay is variable and buffers may overflow.
  • More complex and costly multiplexer; not ideal for real-time voice.

How statistical TDM overcomes the weakness of TDM

The main weakness of TDM is that slots for idle inputs are wasted. A statistical TDM scans the input buffers and fills the frame only with data from active inputs, tagging each piece with its source address. Since on average only a fraction of terminals are active, a link of lower capacity serves the same terminals, and no capacity is wasted on silence.

 Inputs   A: data   B: idle   C: data   D: idle
 Sync TDM frame:   | A | B(empty) | C | D(empty) |
 Stat TDM frame:   | A,data | C,data |

STDM frame format

Statistical TDM usually uses an HDLC-type frame:

+----+------+-----+----------------------+-----+----+
|Flag| Addr | Ctl | sub-frames (data)    | FCS |Flag|
+----+------+-----+----------------------+-----+----+
                  |                      |
         +--------+                      +-----+
         | Addr | Len | Data | Addr | Len | Data |
         +------+-----+------+------+-----+------+
  • Flag: marks the start and end of frame (01111110).
  • Address / Control: link-level fields of the frame.
  • Sub-frames: each holds the source address, optional length field and data of one active input. With one source per frame, only address + data is needed; with many, address, length and data are repeated.
  • FCS: frame check sequence (CRC) for error detection.

Address and length fields are the price paid for the gain in efficiency.

✓ Answer checked by Claude Opus 5.5 (high effort)

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