--- title: "North American T1 vs European E1 hierarchy" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:t1-e1-hierarchy-compare" source: https://www.bctengineer.com/q/semester_8/telecommunications/t1-e1-hierarchy-compare site: BCT Engineering --- # North American T1 vs European E1 hierarchy Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2081 Chaitra, 8 marks; 2073 Chaitra, 8 marks (asked 2 times) - Video: [North American (T1), Europe (E1) System & Hierarchy Principles of Communication Board Soln In Nepali](https://www.youtube.com/watch?v=XiEB5dKF_Ec) — OPengineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/t1-e1-hierarchy-compare - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question Compare North American TDM and the European E1 TDM hierarchy by clearly indicating data rates and numbers of users with the help of proper diagrams. ## Short answer T1 frames 24 channels plus one framing bit (193 bits at 8 kHz), giving 1.544 Mbps, with robbed-bit signalling; higher levels are T2 6.312, T3 44.736, T4 274.176 Mbps. E1 uses 32 time slots (30 voice plus TS0 and TS16), giving 2.048 Mbps, with separate signalling; higher levels E2 8.448, E3 34.368, E4 139.264, E5 564.992 Mbps. ## Answer The **North American (T/DS) hierarchy** (AT&T/ANSI) and the **European E hierarchy** (CEPT/ITU-T G.732/G.751) are the two plesiochronous digital hierarchies (PDH). Both sample each voice channel at 8 kHz with 8-bit PCM (64 kbps) and use a 125 µs frame, but they differ in frame size, signalling and the multiplexing steps. #### North American T1 frame 24 channels × 8 bits + 1 framing bit per frame. ```text 1 bit 8 bits 8 bits 8 bits +-----+-------+-------+-----+--------+ | F | Ch1 | Ch2 | ... | Ch24 | +-----+-------+-------+-----+--------+ <--------- 193 bits in 125 us --------> F = framing bit ``` $$ \begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frame rate} &= 8000\ \text{frames/s}\ (T = 125\ \mu\text{s}) \\ R_{T1} &= 193 \times 8000 = 1.544\ \text{Mbps} \end{aligned} $$ Signalling is "robbed bit": the least significant bit of each channel in every 6th frame (12-frame superframe D4, or 24-frame ESF). #### European E1 frame 32 time slots × 8 bits; TS0 for frame alignment, TS16 for signalling (CAS) and TS1–15, TS17–31 for 30 voice channels. ```text +-----+-----+-----+------+------+-----+------+ | TS0 | TS1 | ... | TS15 | TS16 | ... | TS31 | +-----+-----+-----+------+------+-----+------+ sync <-voice 1-15-> sig <-voice 16-30-> <------ 32 x 8 = 256 bits in 125 us -------> ``` $$ \begin{aligned} \text{Bits per frame} &= 32 \times 8 = 256\ \text{bits} \\ R_{E1} &= 256 \times 8000 = 2.048\ \text{Mbps} \end{aligned} $$ #### North American hierarchy ```text 24 x DS0 --> [MUX] --> T1 1.544 Mbps (24 ch) 4 x T1 --> [M12] --> T2 6.312 Mbps (96 ch) 7 x T2 --> [M23] --> T3 44.736 Mbps (672 ch) 6 x T3 --> [M34] --> T4 274.176 Mbps (4032 ch) ``` | 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 | #### European hierarchy ```text 30 x 64k --> [MUX] --> E1 2.048 Mbps (30 ch) 4 x E1 --> [MUX] --> E2 8.448 Mbps (120 ch) 4 x E2 --> [MUX] --> E3 34.368 Mbps (480 ch) 4 x E3 --> [MUX] --> E4 139.264 Mbps (1920 ch) 4 x E4 --> [MUX] --> E5 564.992 Mbps (7680 ch) ``` | Level | Made from | Voice channels | Bit rate | Overhead | |---|---|---|---|---| | E0 | 1 voice channel | 1 | 64 kbps | – | | E1 | 30 voice + 2 TS | 30 | 2.048 Mbps | 128 kbps (TS0, TS16) | | E2 | 4 E1 | 120 | 8.448 Mbps | 256 kbps | | E3 | 4 E2 | 480 | 34.368 Mbps | 576 kbps | | E4 | 4 E3 | 1920 | 139.264 Mbps | 1.792 Mbps | | E5 | 4 E4 | 7680 | 564.992 Mbps | 7.936 Mbps | Extra bits at each higher level carry frame alignment, stuffing (justification) and control, as the tributaries are not exactly synchronous. #### Comparison | Point | North American (T) | European (E) | |---|---|---| | Primary rate | 1.544 Mbps | 2.048 Mbps | | Channels at primary level | 24 | 30 (+2 overhead slots) | | Bits per frame | 193 | 256 | | Framing | 1 bit per frame (F bit) | Whole time slot TS0 | | Signalling | Robbed bit (in voice LSB) | Separate slot TS16 | | Companding law | µ-law ($\mu = 255$) | A-law ($A = 87.6$) | | Multiplexing factor | 4, 7, 6 (irregular) | 4 at every level | | Higher levels | 6.312, 44.736, 274.176 Mbps | 8.448, 34.368, 139.264, 564.992 Mbps | | Channels per level | 96, 672, 4032 | 120, 480, 1920, 7680 | | Line code (primary) | AMI / B8ZS | HDB3 | The E system wastes no voice bits on signalling and has a regular ×4 structure; the T system gives slightly higher efficiency at the primary level (only 8 kbps overhead) but degrades voice LSBs. Interworking needs converters (e.g. µ-law ↔ A-law). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the T1 bit rate obtained? A. 32 x 8 x 8000 = 2.048 Mbps B. 24 x 8 x 8000 = 1.536 Mbps C. (24 x 8 + 1) x 8000 = 1.544 Mbps D. (24 x 8 + 1) x 4000 = 0.772 Mbps Answer: C. 193 bits per frame at 8000 frames/s gives 1.544 Mbps. (Key point: T1 rate calculation) 2. In an E1 frame, which time slots carry synchronisation and signalling, and how many voice channels result? A. TS0 sync, TS16 signalling, 30 voice channels B. TS1 sync, TS31 signalling, 32 voice channels C. TS0 signalling, TS16 sync, 30 voice channels D. Framing bit and robbed bit, 24 voice channels Answer: A. E1 has 32 slots; TS0 is alignment, TS16 signalling, leaving 30 voice slots. (Key point: E1 structure) 3. How does T1 signalling differ from E1? A. T1 uses a dedicated slot, E1 robs bits B. T1 robs the LSB of voice channels, while E1 uses a separate slot TS16 C. Both rob bits D. Neither carries signalling Answer: B. Robbed-bit signalling degrades voice LSBs in T1; E1 has a separate signalling slot. (Key point: Signalling difference) 4. Which statement about the multiplexing hierarchies is correct? A. T hierarchy uses x4 at every level B. E hierarchy factors are 4, 7, 6 C. T hierarchy goes 1.544, 6.312, 44.736, 274.176 Mbps with irregular factors; E goes 2.048, 8.448, 34.368, 139.264 with x4 each D. Both give the same rates at each level Answer: C. T uses 4, 7, 6; E has a regular x4 at every level. (Key point: Hierarchy structure) ## Related in Telecommunications - [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 and T1/E1 carrier standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-t1-e1-carriers) — Telecommunications, chapter 3 - [European TDM system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aeuropean-tdm-system) — 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 - [FDM and TDM with block diagrams](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-block) — 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 - [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 - [FDMA vs TDMA; T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-tdma-t1-e1) — Communication Systems, chapter 5 - [Constellation diagram significance; E1 vs T1](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aconstellation-significance-e1-t1) — Communication Systems, chapter 5