---
title: "Short note: T1 and E1 hierarchy"
subject: "Communication Systems"
chapter: 5
key: "semester_6/communication_systems:short-note-t1-e1-hierarchy"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-t1-e1-hierarchy
site: BCT Engineering
---

# Short note: T1 and E1 hierarchy

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2081 Chaitra, 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-t1-e1-hierarchy
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Write a short note on T1 and E1 hierarchy used in telephony.

## Short answer

T1 carries 24 voice channels, one framing bit, 193 bits per 125 µs frame, rate 1.544 Mbps; E1 uses 32 slots (30 voice, TS0 sync, TS16 signalling), 2.048 Mbps. Multiplexing: T1 DS1→DS2→DS3→DS4, E1→E2→E3→E4. T1 uses µ-law, E1 A-law.

## Answer

T1 and E1 are the two standard **TDM-PCM digital carrier systems** used in telephony. Each one interleaves several 64 kb/s PCM voice channels into one serial stream. T1 is the North American (Bell) standard. E1 is the European (ITU-T) standard, and Nepal Telecom uses it too. Both sample speech at 8 kHz with 8 bits per sample, so one frame lasts $1/8000 = 125$ µs.

#### T1 system
- 24 voice channels × 8 bits + 1 framing bit = **193 bits per frame**.
- $R_b = 193 \times 8000 = 1.544$ Mb/s.
- Signalling is "robbed bit": the LSB of each channel in frames 6 and 12 of a superframe.

#### E1 system
- 32 time slots × 8 bits = **256 bits per frame**. TS0 carries frame sync, TS16 carries signalling, and the other 30 slots carry voice.
- $R_b = 256 \times 8000 = 2.048$ Mb/s.

#### Hierarchies
Higher levels are made by multiplexing lower-level streams. Extra bits are added for framing and bit stuffing, so each higher rate is a little more than the sum of its inputs.

| T1 level | Rate (Mb/s) | Channels | E1 level | Rate (Mb/s) | Channels |
|---|---|---|---|---|---|
| DS1 (T1) | 1.544 | 24 | E1 | 2.048 | 30 |
| DS2 = 4 DS1 | 6.312 | 96 | E2 = 4 E1 | 8.448 | 120 |
| DS3 = 7 DS2 | 44.736 | 672 | E3 = 4 E2 | 34.368 | 480 |
| DS4 = 6 DS3 | 274.176 | 4032 | E4 = 4 E3 | 139.264 | 1920 |

T1 uses µ-law companding ($\mu = 255$) and the AMI/B8ZS line code. E1 uses A-law ($A = 87.6$) and the HDB3 line code.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How many bits are in a T1 frame and what is the rate?
   A. 256 bits, 2.048 Mb/s
   B. 193 bits, 1.544 Mb/s
   C. 192 bits, 1.536 Mb/s
   D. 193 bits, 2.048 Mb/s
   Answer: B. 24 x 8 + 1 framing bit = 193 bits, times 8000 frames/s = 1.544 Mb/s. (Key point: T1 frame)

2. In E1, which time slots carry frame sync and signalling?
   A. TS1 sync, TS31 signalling
   B. TS0 sync, TS16 signalling
   C. TS16 sync, TS0 signalling
   D. Both in TS0
   Answer: B. TS0 carries frame alignment and TS16 carries signalling, leaving 30 voice slots. (Key point: E1 slot use)

3. Why is the DS2 rate 6.312 Mb/s rather than 4 x 1.544 = 6.176 Mb/s?
   A. Extra bits for framing and bit stuffing are added
   B. The sampling rate is raised
   C. Voice is coded with more bits
   D. Companding adds bits
   Answer: A. Higher levels add framing and justification bits, so the rate is a little more than the sum of inputs. (Key point: Higher-order rates)

4. Which pairing of standard, companding and line code is correct?
   A. T1: A-law, HDB3; E1: mu-law, AMI
   B. T1: mu-law (255), AMI/B8ZS; E1: A-law (87.6), HDB3
   C. T1: mu-law, HDB3; E1: A-law, B8ZS
   D. Both use mu-law and AMI
   Answer: B. T1 uses mu-law with AMI/B8ZS; E1 uses A-law with HDB3. (Key point: Standards and coding)

## Related in Communication Systems

- [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
- [Companding; T1 hierarchy of TDM-PCM telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acompanding-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
- [Differences between T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-vs-e1) — Communication Systems, chapter 5
- [FDM and FDM hierarchy in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-hierarchy-telephony) — Communication Systems, chapter 5, asked 4×
- [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×

## 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×
- [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
- [E1 telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Ae1-hierarchy) — Computer Networks and Security, chapter 2