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

# Short note: ISDN structure, services, reference points

Telecommunications · Chapter 8: Data Communication

- Asked in: 2076 Asoj, 4 marks
- 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/short-note-isdn-structure-services
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Write a short note on ISDN structure, services, reference points and channels.

## Short answer

ISDN provides end-to-end digital services over standard interfaces. Reference points are R, S, T, U; channels are B (64 kbps), D (signalling), and H (high-speed). BRI is 2B+D, PRI is 30B+D. Services include bearer, teleservices, and supplementary services.

## Answer

**ISDN (Integrated Services Digital Network)** is an ITU-T standard digital network that gives end-to-end digital connections for voice, data, fax and video over the telephone subscriber line, through a small set of standard user–network interfaces.

**Structure (reference model)**
```text
TE1 ------------ S --[NT2]-- T --[NT1]-- U --[LT/ET]
TE2 -R-[TA]----- S
```
- **TE1:** ISDN terminal (digital phone); **TE2:** non-ISDN terminal; **TA:** terminal adapter.
- **NT2:** customer switching (PBX, LAN); **NT1:** line termination at customer premises.
- **LT/ET:** line and exchange termination at the exchange.

**Reference points:** **R** (TE2–TA, non-ISDN e.g. RS-232), **S** (TE–NT2), **T** (NT2–NT1), **U** (NT1–exchange, 2-wire loop).

**Channels**

| Channel | Rate | Use |
|---|---|---|
| B | 64 kbps | User voice/data |
| D | 16 or 64 kbps | Signalling (Q.931), low-speed packets |
| H0 / H11 / H12 | 384 / 1536 / 1920 kbps | High-speed data, video |

- **BRI:** 2B + D(16) = 144 kbps (192 kbps with framing).
- **PRI:** 30B + D(64) = 2.048 Mbps (Europe/Nepal) or 23B + D = 1.544 Mbps (USA).

**Services**
- **Bearer services:** pure transport – circuit-mode 64 kbps speech/data, packet-mode, frame-mode.
- **Teleservices:** complete services incl. terminals – telephony, Group 4 fax, teletex, videotex.
- **Supplementary services:** add-ons – calling line ID, call waiting, call forwarding, conference, closed user group.

## Check yourself

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

1. Which reference point lies between NT1 and the exchange (2-wire loop)?
   A. S
   B. T
   C. R
   D. U
   Answer: D. U is the 2-wire interface between NT1 and the exchange line termination; R is TE2-TA, S is TE-NT2, T is NT2-NT1. (Key point: Reference points)

2. Which pair is correct for the ISDN access structures?
   A. BRI = 2B + D(16) = 144 kbps; PRI (Europe) = 30B + D(64) = 2.048 Mbps
   B. BRI = 2B + D(64) = 192 kbps; PRI = 23B + D = 2.048 Mbps
   C. BRI = 30B + D = 144 kbps; PRI = 2B + D = 2.048 Mbps
   D. BRI = B + D(16) = 80 kbps; PRI = 30B + D = 1.544 Mbps
   Answer: A. BRI is 2x64+16 = 144 kbps; European PRI is 30B + D(64) on an E1 at 2.048 Mbps. (Key point: BRI and PRI rates)

3. What distinguishes a teleservice from a bearer service?
   A. A teleservice only transports information, without terminal functions
   B. A teleservice is a complete service including terminal functions, e.g. telephony or Group 4 fax
   C. A teleservice is an optional add-on like call waiting
   D. A teleservice uses only the D channel
   Answer: B. Bearer services only transport; teleservices include terminals (telephony, fax); supplementary services are add-ons. (Key point: Bearer vs teleservice)

4. What is the main use of the ISDN D channel?
   A. Carrying user voice
   B. Carrying video only
   C. Signalling (Q.931) and low-speed packet data
   D. Line synchronisation only
   Answer: C. The D channel carries signalling and low-speed packets, while B channels carry user information. (Key point: Role of D channel)

## 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×
- [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
- [ISDN and its channel types](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-channel-types) — Telecommunications, chapter 8, asked 2×

## Related in other subjects

- [GSM frame structure, traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-traffic-control-channels) — Wireless Communication, chapter 8, asked 4×
- [GSM frame structure and channels during a call](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-call-channels) — Wireless Communication, chapter 8