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

# Basic ISDN services and architecture

Telecommunications · Chapter 8: Data Communication

- Asked in: 2069 Chaitra, 4+4 marks
- Video: [ISDN | Integrated Services Digital Network | Introduction, Applications, Services | Computer Network](https://www.youtube.com/watch?v=ltjn49tKrfo) — Dr. Ankit Verma
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/isdn-services-architecture
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Describe the basic services in ISDN with its architecture.

## Short answer

ISDN bearer services provide circuit-mode (64 kbps) and packet-mode transport; teleservices include telephony, fax, and video; supplementary services add features like caller ID and call forwarding. BRI (2B+D) and PRI (30B+D) interfaces connect TE1/TA via S/T reference point to NT2/NT1, with 2B1Q U interface to the exchange.

## Answer

**ISDN (Integrated Services Digital Network)** is an ITU-T standard all-digital network that provides voice and non-voice services over the subscriber line through a small set of standard user–network interfaces.

#### Basic services in ISDN
**1. Bearer services** – carry information between user–network interfaces without changing it (OSI layers 1–3).
- Circuit-mode: 64 kbps unrestricted data, 64 kbps speech, 3.1 kHz audio, multiples of 64 kbps.
- Packet-mode: X.25 virtual calls and permanent virtual circuits on B or D channel.
- Frame-mode: frame relay.

**2. Teleservices** – complete services including the terminal (layers 1–7).
- Telephony, Group 4 fax, teletex, videotex, videotelephony.

**3. Supplementary services** – extra features added to the above.
- Caller ID (CLIP), call waiting, call forwarding, call hold, conference calls, closed user group, advice of charge.

**Channel services (access)**

| Access | Structure | Rate |
|---|---|---|
| BRI | 2B + D (16 kbps) | 144 kbps (192 kbps line) |
| PRI (E1) | 30B + D (64 kbps) | 2.048 Mbps |
| PRI (T1) | 23B + D (64 kbps) | 1.544 Mbps |

B = 64 kbps user channel; D = signalling and low-speed packets; H channels (384, 1536, 1920 kbps) for high-speed data.

#### ISDN architecture
```text
 Customer premises                   Network
TE1 --S--+
         [NT2]--T--[NT1]--U--+--[ISDN exchange]
TE2-R-[TA]-S--+              |   |     |     |
                             | circuit packet CCS/SS7
                             | switched switched signalling
```

**Functional groups**
- **TE1:** ISDN terminal (digital phone, ISDN fax).
- **TE2:** non-ISDN terminal (analogue phone, PC with RS-232).
- **TA:** terminal adapter; converts TE2 signals to ISDN.
- **NT2:** intelligent premises equipment (PBX, LAN controller) – switching, concentration, layer 2–3 functions.
- **NT1:** physical termination of the line – 2-wire to 4-wire conversion, line coding, timing, power feeding.
- **LT/ET:** line and exchange termination at the central office.

**Reference points**
- **R:** TE2–TA (non-ISDN).
- **S:** TE1/TA–NT2, 4-wire 192 kbps bus (up to 8 terminals).
- **T:** NT2–NT1.
- **U:** NT1–exchange over the 2-wire loop (2B1Q, 160 kbps).

**Network side:** the ISDN exchange connects to a circuit-switched network, a packet-switched network and a common-channel signalling (SS7) network. User signalling on the D channel (Q.931) is converted to SS7 ISUP between exchanges, giving fast call set-up and the supplementary services.

## Check yourself

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

1. Which lists the three ISDN service categories?
   A. Bearer, tele- and supplementary services
   B. BRI, PRI and H channel services
   C. Circuit, packet and frame services only
   D. Voice, video and fax only
   Answer: A. ISDN services are bearer services, teleservices and supplementary services. (Key point: Three basic services)

2. Which belongs to packet-mode bearer services?
   A. 64 kbps speech
   B. X.25 virtual calls and permanent virtual circuits
   C. Group 4 fax
   D. Call forwarding
   Answer: B. Packet-mode bearer services include X.25 virtual calls and PVCs; speech is circuit-mode and fax is a teleservice. (Key point: Packet-mode bearer)

3. What does NT2 do?
   A. Intelligent premises equipment (PBX/LAN) doing switching, concentration and layer 2-3 functions
   B. Terminates the line at the exchange
   C. Converts a non-ISDN terminal
   D. Feeds power over the loop only
   Answer: A. NT2 is a PBX or LAN controller with switching and higher-layer functions. (Key point: NT2 function)

4. How is user D-channel signalling handled between exchanges?
   A. Q.931 is converted to SS7 ISUP
   B. It is sent on the B channel unchanged
   C. It is dropped
   D. It is converted to X.25
   Answer: A. Q.931 on the D channel is converted to SS7 ISUP between exchanges, giving fast call set-up. (Key point: Signalling inside the network)

## 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
- [Short note: Architecture of ISDN network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-isdn-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

- [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
- [Web services architecture; SOAP and WSDL roles](https://www.bctengineer.com/question.md?key=semester_7%2Fweb_technology%3Aweb-services-soap-wsdl) — Web Technology, chapter 8