---
title: "PCM switching and stored program control"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:pcm-switching-spc"
source: https://www.bctengineer.com/q/semester_8/telecommunications/pcm-switching-spc
site: BCT Engineering
---

# PCM switching and stored program control

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2081 Baisakh, 4+4 marks
- Video: [Stored program control (SPC)](https://www.youtube.com/watch?v=QVASQsD-5-k) — EC Video
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/pcm-switching-spc
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

What are the advantages and issues of PCM switching? Briefly elaborate the concept of Store Program Control (SPC).

## Short answer

PCM switching integrates digital transmission, uses cheap RAM time switches, avoids repeated A/D conversion, and supports ISDN. Issues include network synchronisation, per-line codec cost, delay, and control complexity. SPC uses a stored-program computer to control an exchange, enabling new services, easy subscriber data changes, automatic diagnostics, and fast call setup.

## Answer

#### PCM (digital) switching
In **PCM switching**, speech is switched in its digital form: 64 kbps PCM samples are moved between time slots and highways by time and space switches, without converting back to analogue.

**Advantages**
- Direct integration with digital PCM transmission (E1/T1): no codec or 2W/4W conversion at every exchange, so no noise or loss build-up.
- Time switches made of cheap RAM; very few crosspoints; small size and low power.
- Four-wire switching throughout, so no hybrid echo problems inside the network.
- Easy to switch data and $n \times 64$ kbps channels; supports ISDN.
- Fast connection, high reliability, easy software control and maintenance.

**Issues (problems)**
- **Synchronisation:** all exchanges must be clock-locked; clock differences cause **slips** (lost or repeated frames).
- **Delay:** each time switch adds up to 125 µs; many in tandem add delay and need echo control.
- **Codec per line:** analogue subscriber lines need BORSCHT functions and a codec on each line card, which is costly.
- **Jitter and wander** on incoming links need elastic buffers.
- **Control complexity:** path search over slots and highways needs powerful processors and software.
- Interworking with analogue exchanges and signalling conversion.

#### Stored Program Control (SPC)
**SPC** is the control of a telephone exchange by a digital computer (processor) that runs a stored program, instead of hard-wired relays or electromechanical logic.
```text
  [processor] <--> [program + data memory]
       |
  [scanners / distributors / signal units]
       |
  switching network  <-->  lines and trunks
```
- **Scanners** read the state of lines and trunks (off-hook, digits); the processor runs call-processing programs, consults the **data memory** (subscriber class, routing tables, charging) and orders the switching network through **distributors/markers**.
- **Centralised SPC:** one main processor (duplicated in standby, synchronous-duplex or load-sharing mode). **Distributed SPC:** many processors, each controlling part of the exchange or a set of functions.

Benefits: new services (call forwarding, waiting, conference, abbreviated dialling) by software change; easy changes of subscriber data and routing; automatic fault diagnosis and traffic statistics; centralised maintenance; accurate billing; faster call set-up; smaller size.

## Check yourself

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

1. Why must all exchanges in a PCM switching network be clock-locked?
   A. Clock differences cause slips (lost or repeated frames)
   B. Clock differences cause hybrid echo
   C. Clock differences increase the number of crosspoints
   D. Clock differences remove the need for a codec
   Answer: A. If exchange clocks differ, buffers over- or under-run, giving slips: frames lost or repeated. (Key point: Synchronisation issue)

2. Which is a genuine advantage of switching speech in digital PCM form?
   A. It needs a codec and 2W/4W conversion at every exchange
   B. Time switches use cheap RAM and few crosspoints
   C. It removes the need for any processor
   D. It gives no delay at all
   Answer: B. Digital time switches are made of RAM, so they need very few crosspoints and are small and low power. (Key point: Advantage of PCM switching)

3. What is Stored Program Control (SPC)?
   A. Control of the exchange by hard-wired relay logic
   B. Control of the exchange by a processor running a program held in memory
   C. Storing speech samples in memory for later playback
   D. A method of clock synchronisation
   Answer: B. SPC means a computer runs stored call-processing programs instead of hard-wired relays. (Key point: Definition of SPC)

4. In an SPC exchange, how do scanners and distributors/markers fit in?
   A. Scanners set up the path; distributors read line states
   B. Scanners read line and trunk states; the processor orders the network via distributors/markers
   C. Both only store billing data
   D. Both replace the data memory
   Answer: B. Scanners detect off-hook and digits; the processor decides and commands the switching network through distributors/markers. (Key point: Role of scanners and distributors)

5. How does SPC let an exchange add a new service such as call forwarding?
   A. By rewiring the relays
   B. By changing the software
   C. By adding more crosspoints
   D. By replacing the codecs
   Answer: B. Features are in the stored program, so they are added by a software change. (Key point: Benefit of SPC)

## Related in Telecommunications

- [Short note: Stored program control](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-spc-digital-exchange) — Telecommunications, chapter 4
- [SPC and dual processor architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspc-dual-processor) — Telecommunications, chapter 4
- [PCM switching and three-stage crosspoints](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apcm-switching-crosspoints-4x5) — Telecommunications, chapter 4
- [Conventional exchange functions; PCM switching advantages](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aconventional-exchange-pcm-advantages) — Telecommunications, chapter 4

## Related in other subjects

- [Stored program concept and processing cycle](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Astored-program-processing-cycle) — Microprocessor, chapter 1, asked 6×
- [Simple program illustrating OOP concept](https://www.bctengineer.com/question.md?key=semester_3%2Fobject_oriented_programming%3Aoop-simple-program) — Object Oriented Programming, chapter 1
- [PID tuning and computer control issues](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-tuning-practical-issues) — Embedded System, chapter 7
- [ADC804 with 8255 control word and program](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc804-8255-program) — Instrumentation II, chapter 4