--- title: "Conventional exchange functions; PCM switching advantages" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:conventional-exchange-pcm-advantages" source: https://www.bctengineer.com/q/semester_8/telecommunications/conventional-exchange-pcm-advantages site: BCT Engineering --- # Conventional exchange functions; PCM switching advantages Telecommunications · Chapter 4: Digital Switching - Asked in: 2074 Chaitra, 4+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/conventional-exchange-pcm-advantages - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question What are the basic functions of a conventional exchange? Write economic and technical advantages of PCM switching compared to its analog switching. ## Short answer A conventional exchange performs line identification, digit reception, digit analysis, busy testing, path interconnection, ringing, supervision, signaling, and charging. PCM switching is economically better due to shared time-slot crosspoints, no per-exchange codecs, cheap VLSI, low maintenance; technically it gives no noise accumulation, constant loss, less crosstalk, integrated transmission and switching, ISDN/data, SS7, and SPC features. ## Answer #### Basic functions of a conventional exchange 1. **Attending (identification):** continuously monitor all lines and detect a call request (off-hook) and the calling line. 2. **Information receiving:** send dial tone and receive the dialled digits (pulse or DTMF). 3. **Information processing:** analyse the digits to find the destination and route. 4. **Busy testing:** check whether the called line or a trunk is free. 5. **Interconnection (switching):** set up a speech path through the switching network between the calling and called line or trunk. 6. **Alerting:** send ringing current to the called party and ring-back tone to the caller. 7. **Supervision:** watch the call for answer and clear-down, and release the path when either party hangs up. 8. **Information sending:** send address and line signals to other exchanges for outgoing calls. 9. **Charging and metering:** record call duration and destination for billing. #### Advantages of PCM (digital) switching over analog switching **Economic advantages** - **Shared crosspoints:** one time-shared crosspoint or memory location serves many calls, so far fewer components are needed. - **No codecs between exchanges** in an integrated digital network: digital trunks are switched directly, removing costly A/D and D/A conversion at each exchange. - **VLSI hardware:** memories and logic chips are cheap, small and low power, so exchanges need less floor space, air conditioning and power. - **Lower maintenance cost:** no moving parts, self-diagnosis and remote maintenance by software. - **Quick installation and easy expansion** in modular units. **Technical advantages** - **No noise or loss accumulation:** signals are regenerated, so quality does not depend on the number of switching stages or distance. - **Constant transmission loss** through the exchange, independent of path. - **Less crosstalk** and better speech quality. - **Integration of transmission and switching** on the same PCM format (2.048 Mbps E1). - **Data and ISDN services** carried directly, as everything is already digital. - **Fast call set-up** and easy use of common channel signaling (SS7). - **SPC features:** call waiting, forwarding, conference, itemised billing. - **High reliability** through duplicated processors and self-checking. (Issues to note: need for network synchronisation, a codec and hybrid on every analogue line card, and echo control.) ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which function sets up the speech path between calling and called lines? A. Attending B. Interconnection C. Supervision D. Alerting Answer: B. Interconnection sets up the path through the switching network. (Key point: Interconnection function) 2. What is the role of supervision in an exchange? A. Detecting the call request B. Sending ringing current C. Watching for answer and clear-down and releasing the path D. Recording call duration Answer: C. Supervision monitors answer and hang-up and releases the connection. (Key point: Supervision) 3. Why does PCM switching have no noise or loss accumulation over many stages? A. Signals are regenerated B. Analog amplifiers are used C. Fewer stages are used D. Lines are shorter Answer: A. Digital signals are regenerated, so quality is independent of stages or distance. (Key point: Noise accumulation) 4. Which is an economic advantage of PCM switching in an integrated digital network? A. Codecs are needed at every exchange B. Digital trunks are switched directly, removing A/D and D/A at each exchange C. Moving contacts reduce wear D. More floor space is available Answer: B. No codecs are needed between exchanges, which saves cost. (Key point: Economic advantage of integrated network) ## Related in Telecommunications - [PCM switching and three-stage crosspoints](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apcm-switching-crosspoints-4x5) — Telecommunications, chapter 4 - [Telephone exchange and switching hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-switching-hierarchy) — Telecommunications, chapter 1, asked 3× - [Exchange roles and demerits of manual switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-roles-manual-demerits) — Telecommunications, chapter 1 - [PCM switching and stored program control](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apcm-switching-spc) — Telecommunications, chapter 4 - [Switching functions and three-stage crosspoints](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-functions-crosspoints-4x4) — Telecommunications, chapter 4, asked 2× - [Digital switching system and its functions](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-switching-functions) — Telecommunications, chapter 4 ## Related in other subjects - [PCM and analog-to-digital conversion](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apcm-working) — Data Communication, chapter 5, asked 4× - [Basic functions of real-time kernel](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Areal-time-kernel-functions) — Embedded System, chapter 6, asked 2× - [Switching compared with multiplexing](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aswitching-vs-multiplexing) — Computer Networks and Security, chapter 2 - [PCM vs DM SQNR for 6cos(5000πt) with 128 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apcm-dm-6cos5000pit-128) — Communication Systems, chapter 4