--- title: "TDM principle and T1 carrier" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:tdm-principle-t1-carrier" source: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-principle-t1-carrier site: BCT Engineering --- # TDM principle and T1 carrier Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2069 Chaitra, 2+5 marks - Video: [T1 Carrier System (Tdm Scheme )(हिन्दी)](https://www.youtube.com/watch?v=NxFN-Rgt63Q) — LEARN AND GROW - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-principle-t1-carrier - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question Explain the working principle of TDM. Describe T1 carrier system showing the frame structure and different multiplexing levels. ## Short answer TDM assigns each signal a time slot within a recurring frame; a commutator samples channels sequentially at ≥2fmax. The T1 system frames 24 voice channels (8 bits each) plus one framing bit every 125 µs, yielding 1.544 Mbps. Higher levels: T2 6.312 Mbps (4 T1s), T3 44.736 Mbps (7 T2s), T4 274.176 Mbps (6 T3s). ## Answer #### Working principle of TDM In **Time Division Multiplexing**, several signals share one line by taking turns. Time is divided into **frames**, and each frame into **time slots**; each channel gets one slot per frame and uses the full line bandwidth during it. ```text ch1 -[LPF]-o o-[LPF]- ch1 ch2 -[LPF]-o \ / o-[LPF]- ch2 (rotating) link (rotating) chN -[LPF]-o / switch ------> \ o-[LPF]- chN commutator decommutator ^ ^ +-- clock / sync ----+ ``` 1. Each signal is band-limited by an LPF. 2. A commutator samples the channels in turn at $f_s \geq 2 f_m$ (8 kHz for voice), so one frame lasts $1/f_s$ = 125 µs. 3. The samples are coded (PCM) and sent with framing bits. 4. A decommutator at the receiver, kept in step by the framing pattern, routes each slot to its own output, where an LPF restores the signal. #### T1 carrier system **Frame structure:** 24 voice channels × 8 bits + 1 framing bit. ```text 1 bit 8 bits 8 bits 8 bits +-----+-------+-------+-----+--------+ | F | Ch1 | Ch2 | ... | Ch24 | +-----+-------+-------+-----+--------+ <--------- 193 bits in 125 us --------> F = framing bit ``` $$ \begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frame rate} &= 8000\ \text{frames/s}\ (T = 125\ \mu\text{s}) \\ R_{T1} &= 193 \times 8000 = 1.544\ \text{Mbps} \end{aligned} $$ - Framing bits over 12 frames (superframe) give the alignment pattern; robbed-bit signalling uses the LSB of each channel in frames 6 and 12. - µ-law coding, AMI/B8ZS line code, repeaters about every 1.8 km. **Multiplexing levels:** | Level | Made from | Voice channels | Bit rate | Overhead | |---|---|---|---|---| | DS0 | 1 voice channel | 1 | 64 kbps | – | | DS1 (T1) | 24 DS0 | 24 | 1.544 Mbps | 8 kbps (framing) | | DS2 (T2) | 4 DS1 | 96 | 6.312 Mbps | 136 kbps | | DS3 (T3) | 7 DS2 | 672 | 44.736 Mbps | 552 kbps | | DS4 (T4) | 6 DS3 | 4032 | 274.176 Mbps | 5.76 Mbps | $T2 = 4 \times T1 + 136$ kbps overhead; $T3 = 7 \times T2 + 552$ kbps; $T4 = 6 \times T3 + 5.76$ Mbps (framing and stuffing bits). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In TDM, how does each channel use the line? A. It uses a part of the bandwidth all the time B. It uses the full bandwidth during its own time slot in each frame C. It uses a unique wavelength D. It uses a unique code Answer: B. Time is split into frames and slots; each channel gets one slot per frame and the full line bandwidth in it. (Key point: TDM principle) 2. Why does one TDM frame for voice last 125 µs? A. Each channel is sampled at fs = 8 kHz, so frame = 1/fs B. There are 24 channels C. The line rate is 1.544 Mbps D. The framing bit lasts 125 µs Answer: A. One sample per channel per frame means frame time is 1/8000 s = 125 µs, set by the sampling rate. (Key point: Frame duration) 3. How does the receiving decommutator stay in step? A. By the framing pattern B. By the LPF C. By the companding law D. By the line repeaters' power Answer: A. The framing bits let the receiver find slot boundaries so each slot goes to its own output. (Key point: Receiver synchronisation) ## Related in Telecommunications - [TDM and T1/E1 carrier standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-t1-e1-carriers) — Telecommunications, chapter 3 - [WDM and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-t-carrier-t1) — Telecommunications, chapter 3, asked 2× - [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2× - [Digital carrier system and multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-carrier-multiplexing) — Telecommunications, chapter 3 - [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 ## Related in other subjects - [T1 digital carrier system; adaptive delta modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-hierarchy-adm) — Communication Systems, chapter 5 - [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 - [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× - [TDM vs FDM for voice; E1 and T1 hierarchies](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Atdm-fdm-voice-e1-t1) — Communication Systems, chapter 5