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

# ISDN interfaces and channels

Telecommunications · Chapter 8: Data Communication

- Asked in: 2078 Bhadra, 4+4 marks; 2074 Chaitra, 8 marks (asked 2 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/isdn-interfaces-channels
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Explain ISDN with different interfaces. Also mention its channels.

## Short answer

ISDN provides digital connections through defined functional groups and interfaces: R, S, T, U. Access interfaces are Basic Rate Interface (BRI: 2B+D at 144 kbps) and Primary Rate Interface (PRI: 30B+D at 2.048 Mbps or 23B+D at 1.544 Mbps). Its channels include B (64 kbps), D (16/64 kbps signalling), and H channels for high bandwidth.

## Answer

**ISDN (Integrated Services Digital Network)** is a digital network standardised by ITU-T that provides end-to-end digital connections for voice and non-voice services over the subscriber line, through a small set of standard user–network interfaces.

#### ISDN reference configuration and interfaces

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

**Functional groups:** TE1 (ISDN terminal), TE2 (non-ISDN terminal), TA (terminal adapter), NT1 (line termination at customer site), NT2 (PBX or controller), LT/ET (exchange side).

**Interfaces (reference points)**

| Point | Between | Description |
|---|---|---|
| R | TE2 – TA | non-ISDN interface (e.g. RS-232, analog) |
| S | TE1/TA – NT2 | 4-wire bus, up to 8 terminals, 192 kbps (BRI) |
| T | NT2 – NT1 | same electrical form as S; customer–network boundary |
| U | NT1 – exchange | 2-wire local loop, 160 kbps (2B1Q) for BRI |

When there is no NT2, S and T merge into the **S/T interface**.

**User–network access interfaces**

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

#### ISDN channels

| Channel | Rate | Use |
|---|---|---|
| B | 64 kbps | voice, data, fax (user information) |
| D | 16 kbps (BRI) / 64 kbps (PRI) | signalling (LAPD, Q.931), packet data |
| H0 | 384 kbps | video, high-speed data |
| H11 | 1536 kbps | high-speed data (T1) |
| H12 | 1920 kbps | high-speed data (E1) |

## Check yourself

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

1. Which interface lies between a non-ISDN terminal TE2 and the terminal adapter TA?
   A. R
   B. S
   C. T
   D. U
   Answer: A. R is the non-ISDN interface, e.g. RS-232 or analog. (Key point: R reference point)

2. When do the S and T points merge into the S/T interface?
   A. When a TA is used
   B. When PRI is used
   C. When there is no NT2
   D. When there is no NT1
   Answer: C. Without an NT2 (PBX) the S and T points are the same, giving the S/T interface. (Key point: S/T merge)

3. Which pairing of interface and typical user is correct?
   A. BRI for PBXs and large users, PRI for homes
   B. PRI only in North America
   C. BRI for homes and small offices, PRI for PBXs and large customers
   D. Both only for homes
   Answer: C. BRI (2B+D) suits small users; PRI (30B+D or 23B+D) suits PBXs and large customers. (Key point: BRI vs PRI use)

4. What is the D channel rate in BRI and PRI respectively?
   A. 16 kbps and 16 kbps
   B. 64 kbps and 16 kbps
   C. 16 kbps and 64 kbps
   D. 64 kbps and 64 kbps
   Answer: C. BRI uses D(16 kbps); PRI uses D(64 kbps). (Key point: D channel rate)

## Related in Telecommunications

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