--- title: "Short note: Architecture of ISDN network" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:short-note-isdn-architecture" source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-isdn-architecture site: BCT Engineering --- # Short note: Architecture of ISDN network Telecommunications · Chapter 8: Data Communication - Asked in: 2075 Asoj, 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/short-note-isdn-architecture - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question Write a short note on architecture of ISDN network. ## Short answer ISDN architecture includes functional groups TE1, TE2 with TA, NT2, NT1, and LT/ET, connected at reference points R, S, T, U. Access is BRI (2B+D) or PRI (30B+D). The exchange routes B channels to circuit or packet networks and uses SS7 for out-of-band D-channel signalling. ## Answer **ISDN (Integrated Services Digital Network)** is an all-digital network defined by ITU-T that carries voice, data, fax and video over one subscriber line using standard interfaces. Its architecture has the user–network access, the ISDN exchange, and separate networks for circuit, packet and signalling traffic. ```text Customer premises | Network TE1 -S-+ | [NT2]-T-[NT1]-U--------+--[LT/ET ISDN exchange] TE2-R-[TA]-S-+ | | | | | circuit packet SS7 | network network (CCS) ``` **Functional groups** - **TE1:** ISDN-ready terminal (digital phone, ISDN PC card). - **TE2:** non-ISDN terminal (analogue phone, RS-232 PC); needs a **TA (terminal adapter)**. - **NT2:** intelligent customer equipment – PBX, LAN controller; does switching and concentration. - **NT1:** physical termination of the 2-wire loop; converts 2-wire U to 4-wire S/T, line coding, timing, power. - **LT/ET:** line and exchange termination at the ISDN exchange. **Reference points:** R (TE2–TA), S (TE–NT2), T (NT2–NT1), U (NT1–LT). **Access interfaces** - **BRI:** 2B + D = 2×64 + 16 = 144 kbps; for homes and small offices. - **PRI:** 30B + D = 2.048 Mbps (E1) or 23B + D = 1.544 Mbps (T1); for PBXs and businesses. **Network side:** the exchange routes B channels into the circuit-switched network or a packet-switched network, and carries D-channel signalling (Q.931 / LAPD) into **common channel signalling (SS7)** between exchanges. Signalling is thus out-of-band, giving fast call set-up and supplementary services. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a terminal adapter (TA) needed? A. To connect a non-ISDN terminal (TE2) to the ISDN B. To terminate the 2-wire loop at the exchange C. To switch calls inside a PBX D. To carry SS7 signalling Answer: A. A TA converts signals of a non-ISDN terminal (TE2) so it can use the ISDN interface. (Key point: Role of TA) 2. Which is a function of NT1? A. Converting 2-wire U to 4-wire S/T with line coding, timing and power B. Routing packets between exchanges C. Running the Q.931 protocol at the exchange D. Acting as a PBX switch Answer: A. NT1 is the physical termination: 2-wire to 4-wire conversion, line coding, timing and power feeding. (Key point: Function of NT1) 3. What does the network side do with D-channel signalling? A. Mixes it into the B channel B. Carries it into the common channel signalling network (SS7) between exchanges C. Discards it after call set-up D. Sends it over the packet network only Answer: B. D-channel Q.931 signalling is carried over SS7 between exchanges, giving fast call set-up and supplementary services. (Key point: Out-of-band signalling) 4. What distinguishes NT2 from NT1? A. NT2 is intelligent premises equipment (PBX/LAN) doing switching and concentration B. NT2 is only a cable C. NT2 sits in the exchange D. NT2 converts 2-wire to 4-wire Answer: A. NT2 does switching and concentration (PBX, LAN controller); NT1 is only the physical line termination. (Key point: NT2 versus NT1) ## 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× - [Basic ISDN services and architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-services-architecture) — Telecommunications, chapter 8 - [CCS7 network architecture and terminology](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs7-architecture-terminologies) — Telecommunications, chapter 5 - [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 - [Meaning of network architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Anetwork-architecture-meaning) — Computer Networks and Security, chapter 1 - [SUN NFS architecture and operation](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Asun-nfs-architecture) — Distributed System, chapter 2, asked 3× - [Redundant hierarchical network design for bank](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aredundant-network-bank-four-departments) — Internet and Intranet, chapter 5