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

# Differences between T1 and E1 hierarchy

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2075 Asoj (CS II), 4 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/t1-vs-e1
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Explain the differences between T1 and E1 digital hierarchy.

## Short answer

T1 uses 24 channels at 1.544 Mb/s with µ-law companding and robbed-bit signaling. E1 uses 30 channels at 2.048 Mb/s with A-law and a dedicated signalling slot TS16.

## Answer

T1 and E1 are TDM-PCM digital carrier systems. Both use 8 kHz sampling, 8-bit samples and 125 µs frames, but they differ in channel count, framing and rates.

| Point | T1 (North America, Japan) | E1 (Europe, ITU-T, Nepal) |
|---|---|---|
| Voice channels | 24 | 30 (+2 overhead slots) |
| Time slots per frame | 24 + 1 framing bit | 32 |
| Bits per frame | $24 \times 8 + 1 = 193$ | $32 \times 8 = 256$ |
| Frame duration | 125 µs | 125 µs |
| Bit rate | 1.544 Mb/s | 2.048 Mb/s |
| Framing | 1 F-bit per frame, pattern over 12/24 frames | TS0 carries frame alignment word |
| Signalling | Robbed-bit (LSB of frames 6 and 12) | Separate slot TS16 (CAS) |
| Multiframe | Superframe 12 frames / ESF 24 frames | 16 frames (2 ms) |
| Companding law | µ-law, $\mu = 255$ | A-law, $A = 87.6$ |
| Line code | AMI / B8ZS | HDB3 |
| Higher levels | 6.312, 44.736, 274.176 Mb/s | 8.448, 34.368, 139.264, 564.992 Mb/s |

## Check yourself

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

1. Which pair correctly gives bits per frame for T1 and E1?
   A. T1 = 256, E1 = 193
   B. T1 = 193, E1 = 256
   C. T1 = 192, E1 = 256
   D. T1 = 193, E1 = 248
   Answer: B. T1: 24 x 8 + 1 framing bit = 193 bits. E1: 32 x 8 = 256 bits. Both frames last 125 µs. (Key point: Bits per frame)

2. How does T1 carry signalling compared with E1?
   A. T1 uses a separate slot TS16; E1 robs bits
   B. Both use a separate slot
   C. T1 robs the LSB of certain frames (robbed-bit); E1 uses separate slot TS16
   D. Neither carries signalling
   Answer: C. T1 uses robbed-bit signalling (LSB of frames 6 and 12), while E1 has a dedicated signalling slot TS16 (CAS). (Key point: Signalling method)

3. Which combination is correct?
   A. T1: mu-law (255), AMI/B8ZS; E1: A-law (87.6), HDB3
   B. T1: A-law, HDB3; E1: mu-law, B8ZS
   C. T1: mu-law, HDB3; E1: A-law, B8ZS
   D. Both use A-law with AMI
   Answer: A. T1 uses mu-law with mu = 255 and AMI/B8ZS; E1 uses A-law with A = 87.6 and HDB3. (Key point: Companding and line code)

4. Which statement about T1 and E1 is true?
   A. They use different sampling rates (T1 8 kHz, E1 10 kHz)
   B. E1 has fewer voice channels than T1
   C. Both use 8 kHz sampling and 125 µs frames, but T1 = 1.544 Mb/s and E1 = 2.048 Mb/s
   D. Both have the same bit rate of 2.048 Mb/s
   Answer: C. Both sample at 8 kHz with 8-bit samples and 125 µs frames; they differ in slots per frame (24 vs 32), giving 1.544 vs 2.048 Mb/s. (Key point: Bit rates)

## Related in Communication Systems

- [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
- [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
- [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
- [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×
- [Short note: E1 digital hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-e1-hierarchy) — 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

## 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