---
title: "ISDN and its channel types"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:isdn-channel-types"
source: https://www.bctengineer.com/q/semester_8/telecommunications/isdn-channel-types
site: BCT Engineering
---

# ISDN and its channel types

Telecommunications · Chapter 8: Data Communication

- Asked in: 2081 Chaitra, 8 marks; 2073 Chaitra, 6+2 marks (asked 2 times)
- Video: [CN19: Integrated Services Digital Network (ISDN) | Types of ISDN Services| Basic rate interface, PRI](https://www.youtube.com/watch?v=VQXlODgeCP8) — University Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/isdn-channel-types
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Explain ISDN (integrated services digital network) and the types of ISDN channels in detail.

## Short answer

ISDN is a fully digital network with separate D-channel signalling for fast call setup. Its channel types include B channels (64 kbps) for user data, D channels (16 kbps for BRI, 64 for PRI) for signalling and low-speed data, and H channels (384-1920 kbps) for high-speed applications like video conferencing.

## Answer

**ISDN (Integrated Services Digital Network)** is a fully digital network defined by ITU-T (I-series Recommendations) in which voice, data, fax and video share the same digital subscriber line. Signalling is carried separately on a D channel, giving fast call setup and many supplementary services.

#### Main features

- End-to-end digital connection over the existing copper loop.
- Standard user–network interfaces (BRI, PRI) and reference points (R, S, T, U).
- Several devices (up to 8) on one line and simultaneous calls.
- Common-channel signalling: DSS1 (Q.931) on the D channel to the user, SS7 in the network.

#### Types of ISDN channels

**1. B channel (bearer channel), 64 kbps**

- Carries user information: PCM voice, circuit-switched data, packet data (X.25), fax.
- Four kinds of connection on B: circuit-switched, packet-switched, semi-permanent (leased), and multiplexed lower-rate data.
- Two B channels can be combined (bonding) to give 128 kbps.

**2. D channel (data/signalling channel), 16 or 64 kbps**

- 16 kbps in BRI, 64 kbps in PRI.
- Carries **common-channel signalling** for the B channels using LAPD (layer 2) and Q.931 (layer 3).
- When idle, it can carry low-speed packet data and telemetry.

**3. H channels (high-speed)**

| Channel | Rate | Equivalent |
|---|---|---|
| H0 | 384 kbps | 6 B channels |
| H11 | 1536 kbps | 24 B (T1 payload) |
| H12 | 1920 kbps | 30 B (E1 payload) |

Used for video-conferencing, high-speed data, LAN interconnection and fast fax.

#### Channel structures (access interfaces)

```text
 BRI:  | B1 64k | B2 64k | D 16k |  = 144 kbps
 PRI:  | 30 x B 64k | D 64k | sync | = 2.048 Mbps
```

| Interface | Structure | Total rate |
|---|---|---|
| Basic Rate (BRI) | 2B + D(16) | 144 kbps (192 kbps line rate at S/T) |
| Primary Rate, Europe | 30B + D(64) | 2.048 Mbps |
| Primary Rate, N. America | 23B + D(64) | 1.544 Mbps |

BRI suits homes and small offices; PRI connects PBXs and large users. B-ISDN (ATM based, 155/622 Mbps) was the broadband extension.

## Check yourself

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

1. What is the B channel in ISDN used for?
   A. High-speed video at 384 kbps
   B. Carrying user information at 64 kbps
   C. Signalling only at 16 kbps
   D. Synchronisation only
   Answer: B. B channels carry PCM voice, circuit data, packet data and fax at 64 kbps each. (Key point: B channel)

2. Which statement about the D channel is correct?
   A. It carries only voice
   B. It carries common-channel signalling (LAPD, Q.931) and is 16 kbps in BRI, 64 kbps in PRI
   C. It exists only in B-ISDN
   D. It is 64 kbps in BRI and 16 kbps in PRI
   Answer: B. D carries signalling for the B channels and, when idle, low-speed packet data. (Key point: D channel)

3. H0 is 384 kbps. How many B channels is that equivalent to?
   A. 24
   B. 2
   C. 6
   D. 30
   Answer: C. 384 / 64 = 6 B channels; H11 = 24 B and H12 = 30 B. (Key point: H channels)

4. What is the user rate of BRI?
   A. 1.536 Mbps
   B. 64 kbps
   C. 144 kbps (2B + D16)
   D. 2.048 Mbps
   Answer: C. 2 x 64 + 16 = 144 kbps (192 kbps line rate at S/T with framing). (Key point: BRI rate)

## Related in Telecommunications

- [ISDN architecture, services and channels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-architecture-channels) — Telecommunications, chapter 8, asked 2×
- [Short note: ISDN structure, services, reference points](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-isdn-structure-services) — Telecommunications, chapter 8
- [ISDN interfaces and channels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-interfaces-channels) — Telecommunications, chapter 8, asked 2×
- [Basic ISDN services and architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-services-architecture) — Telecommunications, chapter 8
- [Short note: Architecture of ISDN network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-isdn-architecture) — Telecommunications, chapter 8
- [Analog vs digital multiplexing; 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aanalog-digital-mux-24-channel) — Telecommunications, chapter 3

## Related in other subjects

- [Advantages of digital communication; source and channel encoders](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-advantages-source-channel-encoders) — Communication Systems, chapter 1
- [ISDN and its architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aisdn-architecture) — Computer Networks and Security, chapter 2