--- title: "TDM vs statistical TDM and STDM frame" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:tdm-statistical-tdm" source: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-statistical-tdm site: BCT Engineering --- # TDM vs statistical TDM and STDM frame Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2080 Chaitra, 4+2+2 marks; 2075 Asoj, 4+2+2 marks (asked 2 times) - Video: [Time Division Multiplexing (TDM) | Synchronous & Statistical TDM](https://www.youtube.com/watch?v=r6HqspymMms) — A Z Computing - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-statistical-tdm - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question What are the strength and weakness of TDM and statistical TDM? How statistical TDM recover weakness of TDM? Explain STDM with its frame format. ## 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. ## Answer 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. ```text 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: ```text +----+------+-----+----------------------+-----+----+ |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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main weakness of synchronous TDM? A. It needs an address in every slot B. Slots of idle inputs are wasted C. It has variable delay D. It cannot carry voice Answer: B. Each input has a fixed slot whether or not it has data, so capacity is wasted for bursty traffic. (Key point: Weakness of sync TDM) 2. How does statistical TDM overcome that weakness? A. It fills frames only with data from active inputs, tagging each with an address B. It gives each input a longer fixed slot C. It removes buffers D. It increases the link rate to the sum of inputs Answer: A. Only active inputs get slots, so the link rate can be less than the sum of input rates. (Key point: How STDM fixes it) 3. What is the price paid for STDM efficiency? A. Fixed delay B. Address (and length) overhead, buffers and variable delay C. No synchronisation D. Link rate must equal the sum of inputs Answer: B. Each slot carries an address, and buffering makes delay variable and less suited to real-time voice. (Key point: Cost of STDM) 4. In an HDLC-type STDM frame, what does each sub-frame contain? A. Flag and FCS B. Only data, identified by position C. Control field and CRC D. Source address, optional length and data of one active input Answer: D. Sub-frames hold address, length and data; flags mark frame boundaries and FCS gives error detection. (Key point: STDM frame format) ## Related in Telecommunications - [TDM frame, time slot and 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-frame-24-channel) — Telecommunications, chapter 3 - [TDM principle and T1 carrier](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-principle-t1-carrier) — Telecommunications, chapter 3 ## Related in other subjects - [T1 and E1 TDM-PCM hierarchy with frame diagrams](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-e1-frame-diagrams) — Communication Systems, chapter 5 - [E1 frame, TDM hierarchy and signaling rate](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-frame-hierarchy) — Communication Systems, chapter 5 - [Multiplexing; synchronous vs asynchronous TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-compare-sync-async-tdm) — Data Communication, chapter 6, asked 6× - [Short note: FDM vs TDM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm-vs-tdm) — Communication Systems, chapter 5