---
title: "SPC and dual processor architecture"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:spc-dual-processor"
source: https://www.bctengineer.com/q/semester_8/telecommunications/spc-dual-processor
site: BCT Engineering
---

# SPC and dual processor architecture

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2076 Chaitra, 2+5 marks
- Video: [Centralized SPC](https://www.youtube.com/watch?v=G9ATZ_68LVc) — Anjali Ponamala
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/spc-dual-processor
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

What do you mean by stored program control (SPC)? Explain different modes of dual processor architecture used in an electronic switching system using centralized SPC.

## Short answer

SPC means an exchange is controlled by a processor running stored programs. Centralized SPC duplicates the processor in three modes: standby, synchronous duplex with comparator, and load-sharing with common memory. Dual systems greatly improve availability, with MTBF_D = MTBF^2/(2 MTTR).

## Answer

#### Stored program control
**Stored Program Control (SPC)** is the control of an exchange by a processor running programs stored in memory. Call processing, routing, charging and maintenance are done in software, so new features can be added by changing the program.

#### Dual-processor architecture in centralised SPC
In centralised SPC one processor controls the whole exchange, so a failure would stop all calls. The processor is therefore duplicated. The two processors (P1 and P2) share or copy the memory and work in one of three modes.

```text
  +------+   +------+
  |  P1  |   |  P2  |
  +--+---+   +---+--+
     |  +----+   |
     +--|Comp|---+     (comparator in
     |  +----+   |      sync duplex)
  +--+---+   +---+--+
  |  M1  |   |  M2  |   (memories)
  +------+   +------+
        \     /
     exchange hardware
```

**1. Standby mode**
- One processor is active; the other is idle on standby.
- The standby takes over when the active one fails.
- The memory is shared or the active processor regularly copies its data to a secondary store, so the standby can reload the current call data.
- Simple, but calls being set up at the time of failure may be lost.

**2. Synchronous duplex mode**
- Both processors run the same program at the same time, each with its own memory, and get the same inputs.
- A **comparator** checks their outputs after each step. If they agree, normal working continues.
- On a mismatch, both run diagnostic programs; the faulty one is taken out of service and the healthy one continues alone.
- Gives fast fault detection and no loss of calls, but transient faults may cause needless check routines.

**3. Load-sharing mode**
- Each processor handles part of the traffic (e.g. calls are given to them alternately or at random).
- They share a common memory and use an **exclusion device (ED)** so both do not seize the same resource at once.
- If one fails, the other carries the full load (with possible reduced grade of service at the busy hour).
- Gives higher call-handling capacity than standby or synchronous modes.

#### Availability
For one processor, $A = \dfrac{MTBF}{MTBF + MTTR}$. For a dual system the mean time between system failures becomes
$$
MTBF_D = \frac{MTBF^2}{2\,MTTR}
$$
so unavailability falls from about $MTTR/MTBF$ to about $2\,MTTR^2/MTBF^2$. This large gain is why all centralised SPC exchanges use duplicated processors.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is the processor duplicated in centralised SPC?
   A. A single processor failure would stop all calls
   B. To double the speech memory
   C. To halve the crosspoints
   D. To avoid software
   Answer: A. One processor controls the whole exchange, so redundancy is needed for reliability. (Key point: Why duplicate)

2. In synchronous duplex mode, what detects a fault?
   A. A comparator checks the outputs of the two processors after each step
   B. The standby timer
   C. An exclusion device
   D. A scanner
   Answer: A. Both run the same program on the same inputs; a mismatch triggers diagnostics and the faulty one is removed. (Key point: Synchronous duplex)

3. What is the purpose of the exclusion device in load-sharing mode?
   A. Stops both processors seizing the same resource at once
   B. Compares outputs
   C. Copies data to a standby
   D. Provides clock
   Answer: A. Both processors share a common memory and handle part of the traffic, so the ED prevents resource clashes. (Key point: Load sharing)

4. What is the drawback of standby mode?
   A. Calls being set up at the time of failure may be lost
   B. It needs a comparator
   C. It has no spare processor
   D. Both processors do work at once
   Answer: A. The standby has to reload call data after a failure, so calls in set-up can be lost. (Key point: Standby weakness)

## Related in Telecommunications

- [Dual processor modes in centralized SPC](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adual-processor-modes-spc) — Telecommunications, chapter 4, asked 4×
- [PCM switching and stored program control](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apcm-switching-spc) — Telecommunications, chapter 4
- [Short note: Stored program control](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-spc-digital-exchange) — Telecommunications, chapter 4