--- title: "Short note: Stored program control" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:short-note-spc-digital-exchange" source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-spc-digital-exchange site: BCT Engineering --- # Short note: Stored program control Telecommunications · Chapter 4: Digital Switching - Asked in: 2080 Chaitra, 4 marks - Video: [Stored Program Control](https://www.youtube.com/watch?v=0tMisQZbz6c) — Dipanjan De - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-spc-digital-exchange - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question Write a short note on Stored Program control (SPC) in digital exchange. ## Short answer Stored Program Control (SPC) uses a computer program to control an exchange instead of hard-wired logic, handling call setup, signaling, billing, and maintenance. Centralized SPC employs one duplicated processor, while distributed SPC shares control among many processors for better reliability and easier expansion. ## Answer **Stored Program Control (SPC)** means that the exchange is controlled by a digital computer (processor) that runs a program stored in memory, instead of by hard-wired relay logic as in Strowger and crossbar exchanges. All digital exchanges use SPC. **What the program does** - Scans subscriber lines and trunks to detect off-hook, on-hook and digits. - Analyses digits, finds a free path through the switching network and sets it up. - Sends ringing, tones and signaling messages, and supervises the call. - Records call data for charging (billing) and traffic statistics. - Runs maintenance, fault diagnosis and administration tasks. **Types** - **Centralised SPC:** one main processor (usually duplicated for reliability) controls the whole exchange. Dual-processor modes are standby, synchronous duplex and load sharing. - **Distributed SPC:** control is shared among many smaller processors (line-group processors, call processors, etc.). It is more reliable and easier to expand; most modern exchanges use it. **Advantages** - New services (call forwarding, call waiting, abbreviated dialling, conference calls) are added by changing software. - Easy change of subscriber data and routing through a terminal. - Automatic fault detection and remote maintenance. - Fast call set-up and support for common channel signaling (SS7). - Smaller size, lower power use and better reliability. **Disadvantages:** high initial software cost, and a software fault can affect the whole exchange, so careful testing and processor redundancy are needed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does SPC differ from Strowger and crossbar control? A. It uses a processor running a stored program instead of hard-wired relay logic B. It uses more relays C. It has no switching network D. It works only for analogue lines Answer: A. SPC replaces hard-wired logic with a program in memory. (Key point: SPC vs earlier exchanges) 2. Why is distributed SPC preferred in modern exchanges? A. It uses a single processor B. Control is shared among many processors, which is more reliable and easier to expand C. It needs no software D. It avoids all faults Answer: B. Many smaller processors share the work so one failure affects less, and expansion is easy. (Key point: Distributed SPC) 3. What is a main disadvantage of SPC? A. Cannot add services B. High initial software cost, and a software fault can affect the whole exchange C. No billing D. No fault detection Answer: B. Software is costly to develop and a fault can spread, so testing and processor redundancy are needed. (Key point: Disadvantage) 4. Which task is NOT done by the SPC program? A. Digit analysis and path finding B. Charging records and traffic statistics C. Converting speech samples into analogue by hand-wiring D. Fault diagnosis and maintenance Answer: C. The program scans lines, sets up paths, signals, charges and maintains; it does not wire analogue conversion. 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