---
title: "Short note: ISDN"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:short-note-isdn"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-isdn
site: BCT Engineering
---

# Short note: ISDN

Telecommunications · Chapter 8: Data Communication

- Asked in: 2076 Chaitra, 4 marks; 2072 Chaitra, 4 marks; 2072 Kartik, 5 marks; 2070 Chaitra, 9 marks; 2068 Bhadra (old course), 10 marks (asked 5 times)
- Video: [ISDN - Integrated Services Digital Network](https://www.youtube.com/watch?v=nhupbJ8t4N4) — Sunny Classroom
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-isdn
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Write a short note on ISDN.

## Short answer

ISDN (Integrated Services Digital Network) provides end-to-end digital connectivity for voice, data, and video over standard interfaces with out-of-band D-channel signalling. Basic Rate Interface offers 2B+D (144 kbps); Primary Rate offers 30B+D (2.048 Mbps) in Europe or 23B+D (1.544 Mbps) in North America. It supports bearer, teleservices, and supplementary services but has largely been replaced by broadband.

## Answer

**ISDN (Integrated Services Digital Network)** is a set of ITU-T standards (I-series) for a fully digital network that carries voice, data, fax and video over the same digital subscriber line, using standard interfaces and common-channel signalling. It gives end-to-end digital connectivity.

#### Features

- Digital transmission from the user to the exchange.
- Several services on one line, with several simultaneous calls.
- Out-of-band signalling on a separate D channel (DSS1 to the user, SS7 inside the network).
- Standard user–network interfaces and fast call setup.

#### Channels

| Channel | Rate | Use |
|---|---|---|
| B (bearer) | 64 kbps | voice, data, fax |
| D (signalling) | 16 or 64 kbps | signalling, packet data |
| H0 | 384 kbps | video |
| H11 | 1536 kbps | high-speed data |
| H12 | 1920 kbps | high-speed data |

#### Access types

- **Basic Rate Interface (BRI):** 2B + D = 2×64 + 16 = 144 kbps (192 kbps with framing at the S/T point); for homes and small offices.
- **Primary Rate Interface (PRI):** 30B + D = 2.048 Mbps (Europe/Nepal, E1) or 23B + D = 1.544 Mbps (North America, T1); D = 64 kbps; for PBXs and large users.

#### Architecture (reference model)

```text
        R        S        T        U
 [TE2]---[TA]--+
               |--[NT2]---[NT1]=======[LT/ET]
 [TE1]---------+  (PBX)   (NTU)  2-wire  exchange
               S bus    4-wire   loop
```

- **TE1:** ISDN terminal; **TE2:** non-ISDN terminal with **TA** (terminal adapter).
- **NT1:** physical/line termination at the customer site; **NT2:** intelligent device such as a PBX.
- **R, S, T, U:** reference points between these units; U is the two-wire loop to the exchange.

#### Services

- **Bearer services:** circuit-mode and packet-mode transport.
- **Teleservices:** telephony, fax, videotex, teletex.
- **Supplementary services:** caller ID, call waiting, call forwarding, conference.

#### Advantages and limitations

- Faster and better-quality connections than analog modems; several services on one pair; quick call setup.
- Data rates are low by modern standards; costly equipment; largely replaced by DSL and fibre. **Broadband ISDN (B-ISDN)**, based on ATM, was the high-speed extension.

## Check yourself

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

1. What is the Basic Rate Interface (BRI) of ISDN?
   A. 30B + D = 2.048 Mbps
   B. 23B + D = 1.544 Mbps
   C. 2B + D = 144 kbps
   D. H0 = 384 kbps
   Answer: C. BRI is 2B + D (2x64 + 16 = 144 kbps). (Key point: BRI structure)

2. Which Primary Rate Interface is used in Europe/Nepal?
   A. 23B + D at 1.544 Mbps
   B. 30B + D at 2.048 Mbps
   C. 2B + D
   D. H12 only
   Answer: B. E1-based PRI uses 30B + D = 2.048 Mbps; T1-based 23B + D is North American. (Key point: PRI in Nepal)

3. What is the D channel used for?
   A. Carrying voice only
   B. Video at 384 kbps
   C. Signalling and packet data
   D. Fax only
   Answer: C. D carries out-of-band signalling (DSS1) and packet data. (Key point: D channel)

4. In the ISDN reference model, what is NT2?
   A. The non-ISDN terminal
   B. An intelligent device such as a PBX, whereas NT1 is physical line termination
   C. The two-wire loop
   D. A terminal adapter
   Answer: B. NT2 is an intelligent device like a PBX; NT1 is line termination; TA adapts TE2. (Key point: NT1 vs NT2)