---
title: "Short note: TST and STS"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:short-note-tst-sts"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-tst-sts
site: BCT Engineering
---

# Short note: TST and STS

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2074 Asoj, 4 marks
- Video: [Lecture - 10 Basic Time Division Switching Networks, Space-Time-Space (S-T-S) &Time-Space-Time (TST)](https://www.youtube.com/watch?v=peoFGpGCR34) — Engineering Jnana
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-tst-sts
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

Write a short note on TST and STS switches.

## Short answer

TST and STS are three-stage digital switches providing many alternative paths. TST has input TSI, space stage, output TSI; alternative paths are internal time slots l, blocking lower, non-blocking if l>=2c-1. STS has input S, centre TSIs, output S; alternative paths are k TSIs, blocking higher, non-blocking if k>=2N-1. TST is preferred for large exchanges.

## Answer

**TST** and **STS** are three-stage digital switches that combine time switches (TSIs) and time-multiplexed space switches (S) so that every call has many alternative paths. They solve the high blocking of two-stage ST and TS switches.

```text
 TST:  link-->[T]-->[ S ]-->[T]-->link
 STS:  link-->[ S ]-->[T..T]-->[ S ]-->link
```

**TST (Time–Space–Time)**
- Input TSI moves the sample from its external slot to a free internal slot $x$; the space stage switches it in slot $x$ to the right output highway; the output TSI moves it to the required output slot.
- Alternative paths = number of internal time slots $l$.
- Blocking: $P_B = [1-(1-p')^2]^l$, $p' = pc/l$; non-blocking if $l \ge 2c - 1$.

**STS (Space–Time–Space)**
- The input space stage sends the sample to one of $k$ centre TSIs; that TSI changes its slot; the output space stage sends it to the right outgoing link.
- Alternative paths = number of centre TSIs $k$.
- Blocking: $P_B = [1-(1-p')^2]^k$, $p' = pN/k$; non-blocking if $k \ge 2N - 1$.

| Point | TST | STS |
|---|---|---|
| Expansion | Time (cheap) | Space (costly) |
| Blocking | Lower | Higher |
| Best for | Large, busy exchanges | Small exchanges |

TST is more widely used because time expansion only needs faster memory, giving lower blocking at lower cost.

## Check yourself

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

1. Why are TST and STS used instead of two-stage ST or TS switches?
   A. They use fewer time slots
   B. They give many alternative paths and so lower blocking
   C. They need no control memory
   D. They avoid the need for PCM
   Answer: B. Two-stage switches have one path and block heavily; three-stage switches give several alternative paths. (Key point: Purpose of three-stage switches)

2. In a TST switch, what does the input TSI do?
   A. Moves the sample to a free internal slot
   B. Moves the sample to another highway
   C. Selects a centre TSI
   D. Decodes the output address
   Answer: A. The input TSI shifts the sample from its external slot to a free internal slot x, used by the space stage. (Key point: TST operation)

3. How many alternative paths do TST and STS have, respectively?
   A. k and l
   B. l internal slots and k centre TSIs
   C. c and N
   D. 2c-1 and 2N-1 always
   Answer: B. TST has l internal time slots as paths; STS has k centre TSIs. (Key point: Alternative paths)

4. Why is TST more widely used than STS?
   A. Time expansion only needs faster memory, which is cheaper than space expansion
   B. It has no blocking at all
   C. It uses fewer slots
   D. It needs no space stage
   Answer: A. TST gets low blocking by time expansion (cheap), whereas STS needs more TSIs and crosspoints. (Key point: Why TST is preferred)

## 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
- [Clos network and STS/TST blocking](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aclos-network-sts-tst-blocking) — Telecommunications, chapter 4
- [Derive STS and TST blocking probabilities](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asts-tst-blocking-derivation) — Telecommunications, chapter 4
- [Multistage switching; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultistage-tst-sts-compare) — Telecommunications, chapter 4
- [TST vs STS and three-stage design](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atst-sts-design-crosspoints) — Telecommunications, chapter 4
- [Telephone exchange structure; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-structure-tst-sts) — Telecommunications, chapter 4