--- title: "TDM and T1/E1 carrier standards" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:tdm-t1-e1-carriers" source: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-t1-e1-carriers site: BCT Engineering --- # TDM and T1/E1 carrier standards Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2079 Chaitra, 2+6 marks - Video: [T1 and E1 Circuits Explained | Basics of Digital Transmission](https://www.youtube.com/watch?v=z5f_vS-UVHk) — Experienced Networking - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/tdm-t1-e1-carriers - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question What is Time Division Multiplexing? Explain digital carrier standards T1 and E1 with proper elaboration. ## Short answer Time Division Multiplexing assigns each channel a short time slot in a repeating frame. The T1 carrier combines 24 voice channels at 1.544 Mbps using µ‑law and robbed‑bit signaling, while the E1 carrier combines 30 channels at 2.048 Mbps using TS0 for synchronization and TS16 for signaling with A‑law. ## Answer **Time Division Multiplexing (TDM)** is a technique in which several signals share one transmission line by taking turns in time: each channel is given a short time slot in a repeating frame and uses the whole bandwidth of the line during its slot. For voice, each channel is sampled at 8 kHz and coded into 8-bit PCM words, so a frame lasts 125 µs. #### T1 carrier (North American DS1) - 24 voice channels, each 8 bits per frame, plus 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:** the F bits of successive frames form a fixed pattern (in the 12-frame D4 superframe, 100011011100) used to find frame boundaries. - **Signalling:** robbed-bit; in frames 6 and 12 the LSB of each channel carries signalling (A and B bits), so voice uses 7⅚ bits on average. - **Companding:** µ-law, $\mu = 255$. - **Line code:** bipolar AMI, with B8ZS to avoid long zero runs; 1.544 Mbps over twisted pair with repeaters about every 1.8 km. - Efficiency: $24 \times 64 = 1.536$ Mbps payload, only 8 kbps overhead. #### E1 carrier (European CEPT-1) - 32 time slots of 8 bits each. ```text +-----+-----+-----+------+------+-----+------+ | TS0 | TS1 | ... | TS15 | TS16 | ... | TS31 | +-----+-----+-----+------+------+-----+------+ sync <-voice 1-15-> sig <-voice 16-30-> <------ 32 x 8 = 256 bits in 125 us -------> ``` $$ \begin{aligned} \text{Bits per frame} &= 32 \times 8 = 256\ \text{bits} \\ R_{E1} &= 256 \times 8000 = 2.048\ \text{Mbps} \end{aligned} $$ - **TS0:** frame-alignment word (0011011) in alternate frames, alarms and spare bits in the others. - **TS16:** channel-associated signalling, shared among 30 channels over a **16-frame multiframe** (4 bits per channel per multiframe), or used as a 64 kbps common-channel signalling link. - **Voice:** TS1–TS15 and TS17–TS31 = 30 channels. - **Companding:** A-law ($A = 87.6$). **Line code:** HDB3. #### Comparison summary | Point | T1 | E1 | |---|---|---| | Channels | 24 | 30 | | Bits/frame | 193 | 256 | | Rate | 1.544 Mbps | 2.048 Mbps | | Framing | 1 bit/frame | TS0 | | Signalling | Robbed bit | TS16 | | Law / line code | µ-law / AMI, B8ZS | A-law / HDB3 | Higher levels are formed by multiplexing: 4 T1 → T2 (6.312 Mbps), 7 T2 → T3 (44.736 Mbps); 4 E1 → E2 (8.448 Mbps), 4 E2 → E3 (34.368 Mbps). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a PCM voice TDM frame 125 microseconds long? A. Each channel is coded in 125 bits B. Each voice channel is sampled 8000 times per second C. The line rate is 125 kbps D. There are 125 channels Answer: B. 8 kHz sampling gives one sample every 1/8000 s = 125 us. (Key point: TDM and frame time) 2. How is signalling carried in T1? A. TS16 slot B. Separate signalling wire C. Robbing the LSB of each channel in frames 6 and 12 of the superframe D. Extra framing bits only Answer: C. Robbed-bit signalling makes voice use 7 5/6 bits on average. (Key point: T1 signalling) 3. How are the 32 E1 slots used? A. TS0 alignment, TS16 signalling, other 30 slots voice B. 32 voice channels C. 24 voice and 8 signalling D. TS0 voice, TS31 sync Answer: A. TS0 is frame alignment/alarms and TS16 signalling, leaving 30 voice channels. (Key point: E1 slot use) 4. Which pairing of law and line code is correct? A. T1: A-law, HDB3; E1: mu-law, AMI B. T1: mu-law (255) with AMI/B8ZS; E1: A-law (87.6) with HDB3 C. Both use A-law D. Both use HDB3 Answer: B. North American T1 uses mu-law and AMI/B8ZS; European E1 uses A-law and HDB3. (Key point: Companding and line code) ## Related in Telecommunications - [TDM principle and T1 carrier](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-principle-t1-carrier) — 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 - [Digital carrier system and multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-carrier-multiplexing) — Telecommunications, chapter 3 - [North American T1 vs European E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3At1-e1-hierarchy-compare) — Telecommunications, chapter 3, asked 2× - [T1 and E1 frame structures and bit rates](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3At1-e1-frame-structures) — 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 ## Related in other subjects - [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 - [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 - [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 - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3×