--- title: "Short note: E1 digital hierarchy" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:short-note-e1-hierarchy" source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-e1-hierarchy site: BCT Engineering --- # Short note: E1 digital hierarchy Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2076 Chaitra (CS II), 5 marks - 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_6/communication_systems/short-note-e1-hierarchy - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question Write a short note on the E1 digital hierarchy. ## Short answer E1 carries 30 voice channels at 2.048 Mb/s, using 32 time slots per frame. The hierarchy multiplexes four lower-level streams each step, reaching 564.992 Mb/s at E5. ## Answer **E1** is the ITU-T (European) standard TDM-PCM digital carrier. It multiplexes **30 voice channels** of 64 kb/s plus 2 overhead channels into a **2.048 Mb/s** stream. It is used in Europe and most other countries, including Nepal. #### E1 frame - Speech is sampled at 8 kHz, so a frame lasts 125 µs. - Each frame has **32 time slots × 8 bits = 256 bits**. - **TS0**: frame alignment word and alarm bits. - **TS16**: signalling (channel-associated). 16 frames form a 2 ms multiframe. - **TS1–TS15 and TS17–TS31**: 30 voice channels. ```text |<-------- one frame = 125 us = 256 bits -------->| +-----+-----+-----+-----+------+------+-----+------+ | TS0 | TS1 | ... |TS15 | TS16 | TS17 | ... | TS31 | +-----+-----+-----+-----+------+------+-----+------+ sync voice voice signal voice voice each time slot = 8 bits; 32 slots per frame voice channels: TS1-TS15 and TS17-TS31 (30) ``` $$ R_b = 32 \times 8 \times 8000 = 2.048\ \text{Mb/s} $$ #### E1 digital hierarchy Four lower-level streams are combined at each step. Extra bits for framing and justification (bit stuffing) are added. | Level | Made of | Voice channels | Bit rate | |---|---|---|---| | E1 | 30 voice + 2 | 30 | 2.048 Mb/s | | E2 | 4 × E1 | 120 | 8.448 Mb/s | | E3 | 4 × E2 | 480 | 34.368 Mb/s | | E4 | 4 × E3 | 1920 | 139.264 Mb/s | | E5 | 4 × E4 | 7680 | 564.992 Mb/s | E1 uses A-law companding ($A = 87.6$) and the HDB3 line code. Compared with T1 (24 channels, 1.544 Mb/s), E1 carries more channels and has a separate slot for signalling, so all 8 bits of each voice slot stay available for speech. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many bits are in one E1 frame, and how long does a frame last? A. 193 bits in 125 µs B. 256 bits in 125 µs C. 256 bits in 2 ms D. 248 bits in 125 µs Answer: B. An E1 frame has 32 time slots of 8 bits = 256 bits, sent every 125 µs (8 kHz sampling). (Key point: E1 frame structure) 2. Which time slots of an E1 frame carry frame alignment and signalling respectively? A. TS1 for alignment, TS31 for signalling B. TS16 for alignment, TS0 for signalling C. TS0 for alignment, TS16 for signalling D. TS0 for both Answer: C. TS0 carries the frame alignment word and alarm bits; TS16 carries channel-associated signalling. The other 30 slots carry voice. (Key point: Role of time slots) 3. What is the E1 bit rate, and how is it obtained? A. 1.544 Mb/s = 193 x 8000 B. 2.048 Mb/s = 256 x 8000 C. 2.048 Mb/s = 30 x 64 kb/s D. 8.448 Mb/s = 4 x 2.048 Mb/s Answer: B. Rb = 32 slots x 8 bits x 8000 frames/s = 2.048 Mb/s. The 30 voice channels alone would give only 1.92 Mb/s. (Key point: E1 bit rate) 4. Why is E2 = 8.448 Mb/s and not exactly 4 x 2.048 = 8.192 Mb/s? A. Extra framing and justification (stuffing) bits are added when 4 streams are combined B. E2 uses a higher sampling rate C. E2 uses 9 bits per sample D. Each E1 is sped up by 3% Answer: A. At each level, four lower streams are multiplexed and extra framing and bit-stuffing bits are added, raising the rate above 4 times. (Key point: Higher-order hierarchy) ## Related in Communication Systems - [E1 digital hierarchy; SQNR in uniformly quantized PCM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-hierarchy-sqnr-uniform-pcm) — Communication Systems, chapter 5, asked 2× - [Differences between T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-vs-e1) — Communication Systems, chapter 5 - [E1 hierarchy; PCM SQNR at 32 kbps](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-pcm-4khz-1-1v-32kbps) — Communication Systems, chapter 5 - [DPCM vs delta modulation; E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adpcm-vs-dm-e1) — Communication Systems, chapter 4 - [E1 hierarchy; PCM of 4.2 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-pcm-tv-4-2mhz-512) — Communication Systems, chapter 4 - [Short note: T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-t1-e1-hierarchy) — Communication Systems, chapter 5 ## Related in other subjects - [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× - [E1 telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Ae1-hierarchy) — Computer Networks and Security, chapter 2 - [Short note: Analog vs digital mux hierarchy](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aanalog-vs-digital-mux-hierarchy) — Data Communication, chapter 6 - [Digital hierarchy of T-carriers](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3At-carriers-hierarchy) — Data Communication, chapter 6