--- title: "TDM types and pulse stuffing" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:tdm-types-pulse-stuffing" source: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-types-pulse-stuffing site: BCT Engineering --- # TDM types and pulse stuffing Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2081 Baisakh, 3+2+3 marks - Video: [Multilevel multiplexing, pulse stuffing, synchronization bit in Time division multiplexing](https://www.youtube.com/watch?v=MzV5hk8AgmU) — Mathur Lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-types-pulse-stuffing - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question What is Time Division Multiplexing (TDM)? What are the types of TDM? Explain it. What is pulse stuffing and why it needed in TDM? ## Short answer Time Division Multiplexing divides time into frames and slots, each input using full bandwidth during its slot. Types include synchronous (fixed slot per channel) and statistical (slots only when data is present). Pulse stuffing adds dummy bits to align independent tributary clocks to the common multiplexer clock, avoiding slip‑induced data loss. ## Answer **Time Division Multiplexing (TDM)** is a technique in which several signals share one channel by occupying it in turn: time is divided into frames, and each frame into time slots, one slot per input. Each input uses the full channel bandwidth during its slot. Digital telephony uses PCM TDM: each voice channel is sampled 8000 times/s, so a frame is 125 µs long. ```text |<-------------- one frame ------------->| | S1 | S2 | S3 | ... | SN | S1 | S2 | ... slot slot slot S_i = time slot of channel i ``` #### Types of TDM 1. **Synchronous TDM:** every input has a fixed slot in every frame, even if it has no data. The slot position identifies the channel. Simple and good for constant-rate voice (T1, E1), but slots of idle inputs are wasted. 2. **Statistical (asynchronous) TDM:** slots are given only to inputs that have data, and each slot carries the source address. The link rate can be less than the sum of input rates; suits bursty data but needs buffers and address overhead. 3. By the way the frame is built, TDM may also be: - **Bit interleaved** – one bit from each tributary in turn (used in PDH higher-order multiplexers). - **Word (byte) interleaved** – one 8-bit word from each channel in turn (used in T1/E1, SDH). Example: in E1, 30 voice channels plus 2 control slots are byte interleaved: $32 \times 8 \times 8000 = 2.048$ Mbps. #### Pulse stuffing and why it is needed **Pulse stuffing (justification)** is the insertion of extra dummy bits into a tributary stream so that its rate exactly matches the slot rate of a higher-order multiplexer. Need: - Tributaries (e.g. four E1 signals into an E2) come from different sources with independent clocks, which differ slightly (plesiochronous). - The multiplexer reads them at one common rate. Without correction, buffers would underflow or overflow, causing **slips** (lost/repeated bits) and frame loss. - The multiplexer is therefore run slightly faster than any tributary; whenever a tributary's buffer is nearly empty, a **stuff bit** is inserted. **Stuffing control bits**, repeated three times and decoded by majority vote, tell the demultiplexer where stuff bits are, so it can remove them and recover the original clock with a PLL. - This is why higher PDH rates exceed the sum of inputs: $8.448 > 4 \times 2.048 = 8.192$ Mbps. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What distinguishes synchronous TDM from statistical TDM? A. Sync TDM gives every input a fixed slot in every frame; statistical gives slots only to active inputs with addresses B. Sync TDM uses addresses, statistical does not C. Statistical TDM has fixed delay D. They are identical Answer: A. Sync TDM wastes idle slots; statistical TDM shares capacity on demand but needs addressing and buffers. (Key point: Sync vs statistical TDM) 2. Which is word (byte) interleaved TDM? A. PDH higher-order multiplexers taking one bit per tributary B. T1/E1, taking one 8-bit word per channel in turn C. FDM groups D. Statistical TDM only Answer: B. T1/E1 and SDH are byte interleaved, whereas PDH higher-order multiplexers are bit interleaved. (Key point: Interleaving) 3. Why is pulse stuffing needed in TDM? A. To save bandwidth B. To add encryption C. Tributaries from different sources have slightly different clocks (plesiochronous), so rates must be matched to avoid slips D. To reduce attenuation Answer: C. Independent clocks differ slightly; stuffing prevents buffer underflow or overflow. (Key point: Why stuffing) 4. Why is E2 at 8.448 Mbps more than 4 x 2.048 Mbps? A. Rounding error B. The extra bits carry frame alignment, stuffing control and alarms because the multiplexer runs slightly faster than the tributaries C. It carries four extra voice channels D. The tributaries are 2.112 Mbps Answer: B. 4 x 2.048 = 8.192 Mbps; the 256 kbps extra is overhead plus stuffing capacity. (Key point: Higher-rate evidence) ## Related in Telecommunications - [Pulse stuffing in TDM synchronization](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-stuffing-tdm-sync) — Telecommunications, chapter 3 - [Pulse stuffing and complete TDM PCM system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-pulse-stuffing-tdm-pcm) — Telecommunications, chapter 3 - [Pulse stuffing; LED vs LASER sources](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-stuffing-led-laser) — Telecommunications, chapter 2 - [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 - [Multiplexing techniques and TDM standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-techniques-tdm-standards) — Telecommunications, chapter 3 - [Time division switching and space switch](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-division-space-switch) — Telecommunications, chapter 4 ## Related in other subjects - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3× - [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× - [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3× - [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2×