---
title: "E1 telephone hierarchy"
subject: "Computer Networks and Security"
chapter: 2
key: "semester_7/computer_networks_and_security:e1-hierarchy"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/e1-hierarchy
site: BCT Engineering
---

# E1 telephone hierarchy

Computer Networks and Security · Chapter 2: Physical Layer

- Asked in: 2073 Shrawan, 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_7/computer_networks_and_security/e1-hierarchy
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-2

## Question

Explain the E1 telephone hierarchy system.

## Short answer

E1 is a TDM hierarchy with 32 slots per frame, 8000 frames/s, giving 2.048 Mbps; 30 voice channels, one sync, one signalling. Higher levels E2 (8.448 Mbps) to E5 (564.992 Mbps) each multiplex four lower levels.

## Answer

**E1** is the European (ITU-T) standard digital telephone hierarchy based on time division multiplexing (TDM) of 64 kbps voice channels. It is used in Nepal, India and most countries outside North America and Japan (those two use the T1 hierarchy).

#### E1 frame
- Voice is sampled at 8000 samples/s with 8 bits per sample, giving a 64 kbps channel (DS0).
- An E1 frame has **32 time slots** of 8 bits each = 256 bits. Frames are sent 8000 times per second.
- Slot 0 is for frame synchronisation and alarms; slot 16 is for signalling. The remaining **30 slots** carry user voice or data.
- Rate = 32 × 64 kbps = **2.048 Mbps**.

```text
 | TS0 | TS1 ... TS15 | TS16 | TS17 ... TS31 |
 | sync|    voice     | sign.|    voice      |
```

#### Higher levels (each multiplexes four of the lower level)
| Level | Channels | Rate |
|---|---|---|
| E1 | 30 voice | 2.048 Mbps |
| E2 | 4 × E1 = 120 | 8.448 Mbps |
| E3 | 4 × E2 = 480 | 34.368 Mbps |
| E4 | 4 × E3 = 1920 | 139.264 Mbps |
| E5 | 4 × E4 = 7680 | 564.992 Mbps |

The rates are slightly more than 4 times the lower level because of extra framing and justification bits.

## Check yourself

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

1. How many time slots and bits are in an E1 frame?
   A. 24 slots, 193 bits
   B. 32 slots of 8 bits = 256 bits
   C. 30 slots, 240 bits
   D. 16 slots, 128 bits
   Answer: B. E1 has 32 time slots of 8 bits, 256 bits per frame. (Key point: E1 frame)

2. What is the E1 rate and how is it obtained?
   A. 1.544 Mbps
   B. 2.048 Mbps = 32 x 64 kbps
   C. 2.048 Mbps = 30 x 64 kbps
   D. 8.448 Mbps
   Answer: B. Frames are sent 8000 times per second, 256 x 8000 = 2.048 Mbps = 32 x 64 kbps. (Key point: E1 rate)

3. Which slots carry synchronisation and signalling?
   A. Slot 0 sync, slot 16 signalling
   B. Slot 1 sync, slot 15 signalling
   C. Slot 31 sync, slot 0 signalling
   D. All slots carry voice
   Answer: A. Slot 0 is for sync/alarms and slot 16 for signalling, leaving 30 voice slots. (Key point: Special slots)

4. How is E2 formed?
   A. 2 x E1
   B. 4 x E1 = 120 channels, 8.448 Mbps
   C. 8 x E1
   D. 30 x E1
   Answer: B. Each level multiplexes four of the lower level (E2 = 4 x E1). (Key point: Higher levels)

## Related in other subjects

- [FDM hierarchy in telephone system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-hierarchy-telephone) — Data Communication, chapter 6
- [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
- [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
- [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3×