--- title: "ISDN architecture, services and channels" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:isdn-architecture-channels" source: https://www.bctengineer.com/q/semester_8/telecommunications/isdn-architecture-channels site: BCT Engineering --- # ISDN architecture, services and channels Telecommunications · Chapter 8: Data Communication - Asked in: 2079 Chaitra, 1+3+2+2 marks; 2075 Chaitra, 2+4+2 marks (asked 2 times) - 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-architecture-channels - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question What is ISDN? Explain the architecture, services offered and different channels used in ISDN. ## Short answer ISDN is an ITU-T standard digital network offering end-to-end digital connections for voice, data, and video over standard interfaces. Its architecture defines functional groups (TE, NT) and reference points (R, S, T, U). It provides bearer, teleservices, and supplementary services, using B (64 kbps), D (16/64 kbps signalling), and H channels (384-1920 kbps). ## Answer #### ISDN **ISDN (Integrated Services Digital Network)** is an ITU-T standard digital network that provides end-to-end digital connections for voice, data, fax and video over the same subscriber line, using standard user–network interfaces and out-of-band (D-channel) signalling. #### Architecture ```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 (Terminal Equipment 1):** ISDN-compatible terminal (digital phone, ISDN PC card). - **TE2:** non-ISDN terminal (analog phone, RS-232 device). - **TA (Terminal Adapter):** converts TE2 signals to ISDN format. - **NT1 (Network Termination 1):** layer-1 device at the customer premises; terminates the two-wire loop, line coding, power, timing. - **NT2:** intelligent layer 2–3 device such as a digital PBX or LAN switch that concentrates terminals. - **LT/ET:** line termination and exchange termination at the central office. **Reference points** - **R:** between TE2 and TA (non-ISDN interface). - **S:** between TE1/TA and NT2 (4-wire, up to 8 terminals on a bus). - **T:** between NT2 and NT1. - **U:** between NT1 and the exchange (2-wire loop, 160 kbps for BRI using 2B1Q coding). #### Services offered 1. **Bearer services:** transport of information without change: circuit-mode (64 kbps speech, unrestricted digital) and packet-mode (X.25 on B or D channel). 2. **Teleservices:** complete services including terminal functions: telephony, telefax (Group 4), teletex, videotex, videotelephony. 3. **Supplementary services:** added to basic services: calling line identification, call waiting, call forwarding, call hold, conference call, closed user group, advice of charge. #### Channels | Channel | Rate | Function | |---|---|---| | B | 64 kbps | user voice/data, circuit or packet | | D | 16 kbps (BRI), 64 kbps (PRI) | signalling (DSS1), low-speed packet data | | H0 | 384 kbps | video, high-speed data | | H11 | 1536 kbps | T1-based high-speed | | H12 | 1920 kbps | E1-based high-speed | **Interfaces** - **BRI:** 2B + D = 144 kbps user rate (192 kbps at S/T with overhead). - **PRI:** 30B + D = 2.048 Mbps (Europe, Nepal) or 23B + D = 1.544 Mbps (North America). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does ISDN provide? A. Wireless broadband only B. End-to-end digital connections for voice, data, fax and video over the same subscriber line C. Only packet data using X.25 D. Analog voice only over a dedicated line Answer: B. ISDN is a digital network with standard interfaces and out-of-band D-channel signalling. (Key point: ISDN definition) 2. What is the role of NT1 in the ISDN architecture? A. Non-ISDN terminal B. A digital PBX at layer 3 C. Converter for analog phones D. Layer-1 device at the customer premises that terminates the two-wire loop Answer: D. NT1 terminates the loop (line coding, power, timing); NT2 is the PBX and TA converts TE2 signals. (Key point: NT1 role) 3. Which reference point lies between NT1 and the exchange, using a 2-wire loop at 160 kbps (2B1Q) for BRI? A. R B. T C. S D. U Answer: D. U is the NT1-to-exchange 2-wire loop; R is TE2-TA, S is TE/TA-NT2, T is NT2-NT1. (Key point: Reference points) 4. What is the PRI structure used in Europe and Nepal, and its total rate? A. 30B + D(64) = 2.048 Mbps B. 2B + D(16) = 144 kbps C. 6B + D = 384 kbps D. 23B + D(64) = 1.544 Mbps Answer: A. European PRI is 30B + D(64) = 2.048 Mbps; 23B + D gives 1.544 Mbps in North America. (Key point: PRI rates) ## Related in Telecommunications - [Basic ISDN services and architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-services-architecture) — Telecommunications, chapter 8 - [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 - [ISDN interfaces and channels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aisdn-interfaces-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 - [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 - [Substandard sand offered with cost-saving share](https://www.bctengineer.com/question.md?key=semester_8%2Fengineering_professional_practice%3Asubstandard-sand-aggregate-share) — Engineering Professional Practice, chapter 7