---
title: "ISDN and its architecture"
subject: "Computer Networks and Security"
chapter: 2
key: "semester_7/computer_networks_and_security:isdn-architecture"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/isdn-architecture
site: BCT Engineering
---

# ISDN and its architecture

Computer Networks and Security · Chapter 2: Physical Layer

- Asked in: 2071 Chaitra, 2+6 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_7/computer_networks_and_security/isdn-architecture
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-2

## Question

What is ISDN? Explain the ISDN architecture in detail with example.

## Short answer

ISDN is a digital network offering BRI (2B+D, 144 kbps) and PRI (30B+D, 2.048 Mbps). Architecture includes TE1/TA, NT2, NT1 and exchange line termination with reference points S, T, U; B channels carry user data, D channel signalling.

## Answer

#### ISDN
**ISDN (Integrated Services Digital Network)** is a set of ITU-T standards for end-to-end digital transmission of voice, data, video and fax over the ordinary telephone network, using a single digital access line.

**Services/channels**
- **B channel:** 64 kbps, carries user data or voice.
- **D channel:** 16 kbps (BRI) or 64 kbps (PRI), carries signalling and control.
- **BRI (Basic Rate Interface):** 2B + D = 144 kbps (for homes and small offices).
- **PRI (Primary Rate Interface):** 30B + D (E1, 2.048 Mbps) in Europe/Nepal, or 23B + D (T1) in North America.

#### Architecture
```text
 TE1 ---------- S --+
                     |   +-----+  T  +-----+  U  +----------+
                     +---| NT2 |-----| NT1 |-----| Local    |--> ISDN
 TE2 --R-- [TA] - S -+   |(PBX)|     |     |     | exchange |    network
                         +-----+     +-----+     +----------+
```

Components:
- **TE1 (Terminal Equipment 1):** ISDN-compatible device such as a digital phone.
- **TE2:** non-ISDN device (an analog phone, PC with serial port).
- **TA (Terminal Adapter):** converts TE2 signals to ISDN format.
- **NT1 (Network Termination 1):** physical and electrical termination at the customer premises; connects the 4-wire user side to the 2-wire local loop.
- **NT2:** intelligent device (PBX, router) that performs switching and concentration at layers 2 and 3. Often combined with NT1 as NT12.
- **LT/ET (Line Termination, Exchange Termination):** the exchange end of the local loop.

**Reference points:** R (between TE2 and TA), S (TE1/TA to NT2), T (NT2 to NT1), U (NT1 to exchange line).

#### Example
A small office has an ISDN BRI line. A digital phone (TE1) and a PC with an ISDN adapter connect to the NT1 using the S/T bus. One B channel carries a phone call while the other carries Internet data at 64 kbps, and the D channel sets up the calls.

## Check yourself

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

1. What is the ISDN BRI structure and rate?
   A. 30B + D = 2.048 Mbps
   B. 2B + D = 144 kbps
   C. 23B + D
   D. 1B + 2D
   Answer: B. BRI = 2 x 64 + 16 = 144 kbps. (Key point: BRI)

2. What does the D channel carry?
   A. User voice only
   B. Signalling and control
   C. Video only
   D. Error codes only
   Answer: B. The D channel carries signalling and call setup. (Key point: D channel)

3. What does a terminal adapter (TA) do?
   A. Converts TE2 (non-ISDN) signals to ISDN format
   B. Connects to local exchange
   C. Does switching in a PBX
   D. Amplifies signal
   Answer: A. The TA lets non-ISDN devices such as analog phones use ISDN. (Key point: TA and TE2)

4. What is the role of NT1?
   A. Physical and electrical termination at customer premises, connecting 4-wire user side to 2-wire local loop
   B. Converting analog phones
   C. Layer 3 switching of PBX
   D. Signalling server
   Answer: A. NT1 terminates the line at the customer premises. (Key point: NT1)

## Related in other subjects

- [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: Architecture of ISDN network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-isdn-architecture) — Telecommunications, chapter 8
- [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×