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

# Interfaces available in ISDN

Telecommunications · Chapter 8: Data Communication

- Asked in: 2071 Shrawan, 5 marks
- Video: [User Interface | BRI PRI | Basic Primary Rate Interfaces | ISDN Subscriber Access | Computer Network](https://www.youtube.com/watch?v=tfYfnhj_jCs) — 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-interfaces
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Describe the interfaces available in ISDN.

## Short answer

ISDN offers Basic Rate Interface (2B+D, 144 kbps user rate, 192 kbps on S/T bus) over a 2-wire loop, and Primary Rate Interface (30B+D E1 2.048 Mbps or 23B+D T1 1.544 Mbps) on 4-wire. Reference points R (non-ISDN), S/T (4-wire bus), and U (2-wire loop, 160 kbps 2B1Q) define the physical interfaces.

## Answer

ISDN offers standard **user–network interfaces**, defined by the channel structure (BRI and PRI) and by the **reference points** between functional groups at the customer premises.

#### 1. Basic Rate Interface (BRI) – 2B + D
- Two B channels of 64 kbps for voice/data and one D channel of 16 kbps for signalling and low-speed packet data.
- User rate $2 \times 64 + 16 = 144$ kbps; with framing and synchronisation the S/T line rate is **192 kbps**. On the U interface 160 kbps with 2B1Q coding.
- Works over the existing 2-wire telephone loop. Used by homes and small offices.

#### 2. Primary Rate Interface (PRI)
- **30B + D** (Europe, Nepal): 30 × 64 + 64 (D) + 64 (sync) = **2.048 Mbps** on an E1.
- **23B + D** (USA, Japan): 23 × 64 + 64 + 8 framing = **1.544 Mbps** on a T1.
- D channel here is 64 kbps. Used to connect PBXs and LANs. B channels may be grouped into **H channels**: H0 = 384 kbps, H11 = 1536 kbps, H12 = 1920 kbps.

#### 3. Reference points (physical interfaces)
```text
 TE1 ------------- S --+
                       [NT2]-- T --[NT1]-- U --[LT/ET]
 TE2 -- R --[TA]-- S --+   (PBX)    (NTE)    exchange
```

| Point | Between | Description |
|---|---|---|
| R | TE2 and TA | Non-ISDN interface, e.g. RS-232, V.24, analogue |
| S | TE1/TA and NT2 | 4-wire, 192 kbps (I.430), up to 8 terminals on a bus |
| T | NT2 and NT1 | Same electrical form as S; S and T combine (S/T) if no NT2 |
| U | NT1 and exchange (LT) | 2-wire loop, 160 kbps, 2B1Q (in the USA the NT1 is customer-owned, so U is the network boundary) |
| V | LT and ET | Inside the exchange |

The NT1 converts the 2-wire U interface to the 4-wire S/T bus. The D channel at these interfaces uses LAPD (Q.921) and Q.931 call control messages.

## Check yourself

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

1. What is the BRI user rate and its line rate at S/T?
   A. 144 kbps user; 192 kbps at S/T
   B. 192 kbps user; 144 kbps at S/T
   C. 128 kbps user; 144 kbps at S/T
   D. 2.048 Mbps user; 192 kbps at S/T
   Answer: A. 2x64 + 16 = 144 kbps user rate; framing and synchronisation raise the S/T line rate to 192 kbps. (Key point: BRI structure)

2. How is the European PRI rate of 2.048 Mbps made up?
   A. 23 x 64 + 64 + 8 framing
   B. 30 x 64 + 64 (D) + 64 (sync)
   C. 2 x 64 + 16
   D. 32 x 64 + 16
   Answer: B. 30 B channels, one 64 kbps D channel and one 64 kbps sync slot give 32 x 64 = 2.048 Mbps. (Key point: PRI 2.048 Mbps)

3. What does reference point R lie between?
   A. NT2 and NT1
   B. TE2 (non-ISDN) and the TA
   C. NT1 and the exchange
   D. LT and ET
   Answer: B. R is the non-ISDN interface (e.g. RS-232) between TE2 and the terminal adapter. (Key point: Reference point R)

4. Which describes the U interface?
   A. 4-wire bus at 192 kbps for 8 terminals
   B. 2-wire loop between NT1 and exchange at 160 kbps with 2B1Q coding
   C. Interface inside the exchange
   D. Non-ISDN V.24 link
   Answer: B. U is the 2-wire subscriber loop between NT1 and the exchange, 160 kbps using 2B1Q. (Key point: U interface)

5. Which are the H channel rates?
   A. H0 = 64, H11 = 144, H12 = 192 kbps
   B. H0 = 384, H11 = 1536, H12 = 1920 kbps
   C. H0 = 384, H11 = 2048, H12 = 1544 kbps
   D. H0 = 16, H11 = 64, H12 = 128 kbps
   Answer: B. H0 = 6 B channels = 384 kbps; H11 = 1536 kbps; H12 = 1920 kbps. (Key point: H channels)

## Related in Telecommunications

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