--- title: "Dual processor modes in centralized SPC" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:dual-processor-modes-spc" source: https://www.bctengineer.com/q/semester_8/telecommunications/dual-processor-modes-spc site: BCT Engineering --- # Dual processor modes in centralized SPC Telecommunications · Chapter 4: Digital Switching - Asked in: 2079 Bhadra, 6 marks; 2076 Asoj, 4 marks; 2073 Chaitra, 8 marks; 2069 Chaitra, 6 marks (asked 4 times) - Video: [What is SPC? What is standby mode? What is synchronous duplex mode?what is load sharing mode?](https://www.youtube.com/watch?v=HTMpYWaJJdw) — Neha tutorials - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/dual-processor-modes-spc - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question Explain the various modes of operation of dual processor configuration used in a centralized digital exchange. ## Short answer Dual processors operate in standby (active/standby, slow takeover, possible call loss), synchronous duplex (both run same program, comparator detects faults, immediate takeover), or load-sharing mode (each handles part of load, exclusion device avoids conflict, higher capacity but complex software). ## Answer In **centralised SPC**, a single processor controls the whole exchange, so its failure would stop all calls. To give high availability (an exchange may be down only about 2 hours in 40 years), the central processor is **duplicated**. The two processors work in one of three modes. #### 1. Standby mode ```text +-------+ +-------+ | P1 | | P2 | |active | |standby| +---+---+ +---+---+ +----[shared memory / disk]----+ | exchange equipment ``` - One processor (P1) is **active** and handles all calls; the other (P2) is idle in **standby**. - Both have access to a common secondary memory; the active one periodically copies the exchange state (call data, tables) to it. - When P1 fails, P2 takes over, loads the latest state and continues. - **Cold standby:** P2 starts from the last saved data, so calls being set up are lost. **Hot standby:** P2 is kept updated and takes over almost at once. - Simple, but takeover takes time and transient calls may be lost. #### 2. Synchronous duplex (match) mode ```text +-------+ comparator +-------+ | P1 |<------C------->| P2 | | M1 | | M2 | +---+---+ +---+---+ +-------- exchange --------+ ``` - Both processors run the **same program on the same data, in step**, each with its own memory (M1, M2). - A **comparator** checks their results continuously. One processor's outputs actually control the exchange. - If the results disagree, a fault is suspected; both run diagnostics. The faulty processor is isolated and the good one continues alone, with almost no loss of calls. - Gives fast fault detection and no takeover delay, but both processors do the same work, so no extra capacity, and a transient fault may be hard to locate. #### 3. Load-sharing mode ```text +-------+ exclusion +-------+ | P1 |<-----ED------->| P2 | | calls | (shared | calls | | 1,3,5 | data) | 2,4,6 | +---+---+ +---+---+ +-------- exchange --------+ ``` - Each processor handles part of the traffic; an incoming call is given randomly or in turn to one of them. - Both share the exchange resources and data; an **exclusion device (ED)** stops them from seizing the same resource at the same time. - Each can handle the full load. If one fails, the other carries all the traffic (with possible congestion at peak). - Gives **higher capacity** in normal operation and good reliability, but software is more complex. #### Comparison | Mode | Normal capacity | Takeover | Complexity | |---|---|---|---| | Standby | One processor | Slowest, calls may be lost | Simple | | Synchronous duplex | One processor | Immediate | Comparator needed | | Load sharing | Up to two processors | Quick, other takes all load | Exclusion logic, complex software | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the central processor duplicated in a centralised SPC exchange? A. To double the number of subscribers B. To give higher voice quality C. A single processor failure would stop all calls D. To avoid using memory Answer: C. One processor controls everything, so duplication gives the high availability required. (Key point: Why duplicate processors) 2. In synchronous duplex (match) mode, what does the comparator do? A. Shares calls between the processors B. Checks the results of both processors running the same program in step to detect faults C. Selects the cheaper processor D. Copies data to disk Answer: B. Both run the same program in step; a mismatch signals a fault, giving fast detection and no takeover delay. (Key point: Synchronous duplex mode) 3. In load-sharing mode, what is the exclusion device (ED) for? A. Stops both processors from seizing the same resource at the same time B. Compares outputs of the processors C. Stores the standby data D. Splits the bandwidth Answer: A. Processors share resources and data, so the ED prevents simultaneous seizure of one resource. (Key point: Load-sharing exclusion device) 4. Which mode has the slowest takeover, where transient calls may be lost? A. Load sharing B. Synchronous duplex C. All are equal D. Standby (cold) Answer: D. In standby mode the idle processor must load saved state, so takeover is slowest. (Key point: Standby versus others) ## Related in Telecommunications - [SPC and dual processor architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspc-dual-processor) — Telecommunications, chapter 4 - [Role of exchange and rotary dial operation](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-role-rotary-dial) — Telecommunications, chapter 1 - [Digital exchange and three-stage crosspoints](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-exchange-crosspoints) — Telecommunications, chapter 4 - [Time switch in digital exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-switch-digital-exchange) — Telecommunications, chapter 4 - [Working principle of a digital telephone exchange](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-telephone-exchange-principle) — Telecommunications, chapter 4, asked 4× - [Digital exchange principles; non-blocking 3-stage](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-exchange-nonblocking-3-stage) — Telecommunications, chapter 4 ## Related in other subjects - [Dual-purpose processor: median and variance](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Adual-purpose-median-variance) — Embedded System, chapter 2 - [Dual-purpose processor: factorial and prime](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Adual-purpose-factorial-prime) — Embedded System, chapter 2 - [Modes of operation of 8255](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3A8255-modes) — Microprocessor, chapter 4 - [Modem and its modes of operation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodem-modes) — Communication Systems, chapter 7