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

# ISDN service connections

Telecommunications · Chapter 8: Data Communication

- Asked in: 2071 Chaitra, 10 marks
- 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/isdn-service-connections
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

What are the ISDN service connections? Explain.

## Short answer

ISDN provides circuit-switched (on-demand B channel), packet-switched (X.25 on B or D channel), semi-permanent (leased 64 kbps), and frame-mode (frame relay) connections. Bearer services (layers 1-3) offer circuit/packet transport, teleservices add terminal functions (telephony, fax), and supplementary services (CLIP, call forwarding) enhance basic services.

## Answer

**ISDN (Integrated Services Digital Network)** gives digital end-to-end connections over the subscriber line for many services (voice, data, fax, video) through one standard interface. The connections and services ISDN offers can be described by **(1) the types of connection** it can set up, **(2) the access channels and interfaces** used, and **(3) the service classes** (bearer, tele- and supplementary services) defined in ITU-T I.200 series.

#### Architecture behind the connections
```text
 Customer premises                   Network
TE1 --S--+
         [NT2]--T--[NT1]--U--+--[ISDN exchange]
TE2-R-[TA]-S--+              |   |     |     |
                             | circuit packet CCS/SS7
                             | switched switched signalling
```
- **TE1** ISDN terminal, **TE2** non-ISDN terminal with **TA**, **NT2** PBX/LAN, **NT1** line termination, **LT/ET** at the exchange.
- Reference points R, S, T, U separate these groups.
- The exchange sends user B-channel traffic to the circuit-switched or packet-switched network and the D-channel signalling to the common channel signalling (SS7) network.

#### 1. Types of ISDN connection
1. **Circuit-switched connection:** a B channel (64 kbps) is set up on demand using D-channel signalling (Q.931 SETUP, ALERTING, CONNECT, RELEASE). Used for telephony, fax, video calls, file transfer.
2. **Packet-switched connection:** user data is carried as X.25 packets either on a B channel to a packet handler or directly on the D channel (low-speed data, e.g. POS terminals, alarm systems).
3. **Semi-permanent (leased) connection:** a B channel is set up by the operator for a long time without call signalling; acts like a 64 kbps leased line.
4. **Frame-mode connection:** frame relay service with LAPF framing over B or H channels.

#### 2. Channels and access interfaces

| Channel | Rate | Purpose |
|---|---|---|
| B | 64 kbps | User data/voice, circuit or packet |
| D | 16 kbps (BRI), 64 kbps (PRI) | Signalling, packet data |
| H0 | 384 kbps (6B) | Video conferencing |
| H11 / H12 | 1536 / 1920 kbps | High-speed data, LAN links |

- **BRI = 2B + D** = 144 kbps user rate (192 kbps line rate at S/T): homes, small offices.
- **PRI = 30B + D** = 2.048 Mbps (E1) or **23B + D** = 1.544 Mbps (T1): PBXs, businesses.

#### 3. ISDN services
**(a) Bearer services** – only transport the information across the network between interfaces (OSI layers 1–3); the user chooses the terminal.
- Circuit-mode: 64 kbps unrestricted digital, 64 kbps speech, 3.1 kHz audio (for modems and fax), 2×64 kbps, 384/1536/1920 kbps.
- Packet-mode: virtual call and permanent virtual circuit (X.25, connection-oriented); a separate connectionless packet service; user signalling.
- Frame-mode: frame relay and frame switching.

**(b) Teleservices** – complete services including terminal functions (layers 1–7):
- Telephony (3.1 kHz and 7 kHz wideband speech)
- Group 4 facsimile
- Teletex and telex
- Videotex, videotelephony, video conferencing

**(c) Supplementary services** – added to a bearer or teleservice, not sold alone:
- Calling line identification presentation/restriction (CLIP/CLIR)
- Call waiting, call hold, call forwarding (busy / no reply / unconditional)
- Three-party and conference calling
- Closed user group, direct dialling-in, advice of charge, user-to-user signalling

#### Example
A small office has a BRI line. One B channel carries a phone call (circuit-mode telephony teleservice) while the other carries a 64 kbps Internet/file-transfer connection; the D channel handles signalling for both and also shows the caller's number (CLIP supplementary service). A PC with only an RS-232 port connects through a TA at the R point.

#### Advantages of ISDN connections
- Fully digital, so clear speech and error-free data.
- Fast call set-up (1–2 s) because of out-of-band D-channel signalling.
- Several services and calls on one line at the same time.
- Standard interfaces let terminals from different makers work together.

## Check yourself

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

1. How is an ISDN circuit-switched connection set up?
   A. By the operator permanently, without signalling
   B. On demand using a B channel with D-channel Q.931 signalling
   C. By X.25 packets on the D channel
   D. By frame relay on H channels only
   Answer: B. A B channel is set up on demand using D-channel messages (SETUP, ALERTING, CONNECT, RELEASE). (Key point: Circuit-switched connection)

2. What is a semi-permanent ISDN connection?
   A. A call that lasts a few seconds
   B. A B channel set up by the operator for a long time without call signalling, like a leased line
   C. A packet call on the D channel
   D. A supplementary service
   Answer: B. It acts like a 64 kbps leased line with no per-call signalling. (Key point: Semi-permanent connection)

3. Which describes bearer services?
   A. Complete services with terminal functions, layers 1-7
   B. Add-ons such as call waiting
   C. Transport of information between interfaces only, OSI layers 1-3
   D. Services that exist only on PRI
   Answer: C. Bearer services only carry information (layers 1-3); teleservices cover layers 1-7. (Key point: Bearer service scope)

4. Which statement about supplementary services is correct?
   A. They are sold alone as independent services
   B. They are added to a bearer or teleservice, e.g. CLIP, call forwarding, conference
   C. They replace the D channel
   D. They are only used for fax
   Answer: B. Supplementary services such as CLIP or call waiting add features to bearer or teleservices and are not sold alone. (Key point: Supplementary service)