---
title: "Time and space switches; STTS and TSST"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:time-space-switch-combinations"
source: https://www.bctengineer.com/q/semester_8/telecommunications/time-space-switch-combinations
site: BCT Engineering
---

# Time and space switches; STTS and TSST

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2081 Bhadra, 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/time-space-switch-combinations
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

What are the roles of time-switches and space-switches in digital exchanges? How do combinations of these switches (e.g., STTS, TSST) enhance the capabilities of telecommunications networks?

## Short answer

A time switch (TSI) moves PCM samples between time slots on the same highway; a space switch connects input to output highways in the same time slot. Combined in TST, STS, TSST, they scale to large exchanges by offering many alternative paths, low blocking, low cost via RAM, reliability, and flexibility for ISDN and data services.

## Answer

#### Role of time switches
A **time switch** (time slot interchanger, TSI) moves PCM samples from one time slot to another on the same highway.
```text
 incoming slots -> [speech memory] -> outgoing slots
   write in order     ^ read address
                [control memory]
```
- Incoming samples are written into a **speech memory** in slot order (sequential write) and read out in the order set by a **control memory** (random read), or the reverse.
- It connects channel $i$ to channel $j$ on the same highway with a delay of up to one frame (125 µs).
- Built from RAM, so it is cheap, and it is non-blocking for one highway; the size is limited by memory speed ($c$ slots × 2 accesses within 125 µs).

#### Role of space switches
A **space switch** is a crosspoint matrix (electronic gates) that connects an input highway to an output highway, **keeping the same time slot**.
- Each crosspoint is opened or closed every time slot according to its control memory, so it is shared by many calls (time-shared space switching).
- It enlarges the switch to many highways but cannot change time slots; used alone it blocks when two calls need the same output in the same slot.

#### How combinations enhance capability
Real exchanges have hundreds of highways, so time and space switches are combined in multi-stage networks:

- **TS / ST (two stage):** simple, but blocking is high.
- **TST (time–space–time):** incoming TSI moves the call to any free internal slot, the space stage crosses highways, the outgoing TSI moves it to the wanted slot. The number of alternative paths equals the number of internal slots, so blocking is very low and memory is cheap. Standard in large exchanges.
- **STS (space–time–space):** space stages spread calls over $k$ time modules; suits smaller systems.
- **TSST / TSSST:** two or more space stages in the middle allow very large numbers of highways (large transit exchanges) without huge single matrices.
- **STTS:** two time stages in the middle give more slot-changing freedom and paths between groups of highways, useful in modular/distributed designs.

Benefits of these combinations:
1. **Scalability:** tens of thousands of lines and trunks switched in one network by adding modules.
2. **Low blocking:** many alternative paths (internal slots × middle arrays); with time expansion near non-blocking.
3. **Lower cost and size:** fewer crosspoints; most switching done in cheap RAM.
4. **Reliability:** alternative paths and duplicated planes let traffic avoid faults.
5. **Flexibility:** supports any-to-any connection of 64 kbps channels, broadcast/conference connections and $n \times 64$ kbps data paths, which digital networks and ISDN need.

## Check yourself

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

1. How does a time switch (TSI) move a sample to another slot?
   A. By closing crosspoints between highways
   B. By changing the frequency of the sample
   C. By converting it to analogue
   D. By writing in order into a speech memory and reading out in the order set by a control memory (or the reverse)
   Answer: D. Sequential write and random read (or the reverse) using speech and control memories changes the slot. (Key point: Time switch operation)

2. What cannot a space switch do on its own?
   A. Change the time slot of a sample
   B. Connect two highways
   C. Be time-shared
   D. Use a control memory
   Answer: A. A space switch keeps the same time slot, so used alone it blocks when two calls need the same output in the same slot. (Key point: Space switch limitation)

3. What is a key benefit of combining time and space stages (e.g. TST)?
   A. Fewer alternative paths
   B. More alternative paths (internal slots), low blocking and low cost via RAM
   C. Larger crosspoint matrices
   D. No need for control
   Answer: B. Combinations give many alternative paths and do most of the switching in cheap RAM, allowing scalability. (Key point: Why combine switches)

## Related in Telecommunications

- [Space and time switches; STS vs TST](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-time-switch-sts-tst) — Telecommunications, chapter 4
- [Time and space switch drawbacks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-space-switch-drawbacks) — Telecommunications, chapter 4
- [Time division switching and space switch](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-division-space-switch) — Telecommunications, chapter 4
- [S and T switches; 3-stage vs 2-stage](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3As-t-switches-3-stage) — 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
- [Drawbacks of manual telephone exchanges](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amanual-exchange-drawbacks) — Telecommunications, chapter 1

## Related in other subjects

- [Time and space mean speed from 20 spot speeds](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aspot-speed-20-observations-travel-time) — Transportation Engineering II, chapter 1
- [Four-segment instruction pipeline and space-time diagram](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Afour-segment-pipeline) — Computer Organization and Architecture, chapter 4, asked 5×
- [Time and space mean speed from 14 travel times](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atime-space-mean-speed-14-vehicles) — Transportation Engineering II, chapter 1
- [Time/space mean speed and density from spot speeds](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aspot-speed-20-values-time-space) — Transportation Engineering II, chapter 1