---
title: "Short note: ST switch"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:short-note-st-switch"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-st-switch
site: BCT Engineering
---

# Short note: ST switch

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2081 Baisakh, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-st-switch
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

Write a short note on ST switch.

## Short answer

An ST switch is a two-stage digital switch with a space stage followed by a time stage. A sample enters via the space stage in its arrival time slot, and a time-slot interchanger shifts it to the desired output slot. It blocks when that slot is already occupied on the output highway; adding a third stage (STS or TST) provides alternative paths and reduces blocking.

## Answer

An **ST switch** is a two-stage digital switch in which a **space (S) stage** is followed by a **time (T) stage**. The S stage is a time-multiplexed space switch that connects incoming TDM highways to outgoing highways; each outgoing highway has a time slot interchanger (TSI) that moves the sample to the required output slot.

```text
 TDM in            S stage           T stage     TDM out
 I1 ------->+----------------+--->[TSI 1]---> O1
 I2 ------->|  time-shared   |--->[TSI 2]---> O2
 ..         |  crosspoint    |     ...         ..
 IN ------->|  matrix (N x N)|--->[TSI N]---> ON
            +----------------+
                  ^
            control memory (per output column)
```

**Working:** a sample arriving in slot $i$ on input highway $I_a$ must go to slot $j$ on output highway $O_b$. In slot $i$ the S-stage control memory closes crosspoint $(a,b)$, so the sample enters TSI $b$, which writes it and reads it out in slot $j$.

**Drawback (blocking):** the space stage must move the sample in the *same* time slot $i$ in which it arrives. If another call from a different input already uses slot $i$ to reach output highway $b$, the new call is blocked, even though slot $j$ on $O_b$ is free. There is only **one path** for each connection, so blocking is high under heavy load. TS switches have the same problem the other way round.

**Remedy:** add a third stage to get several alternative paths, giving **STS** or **TST** switches. The TST form is preferred in practice (e.g. in most large digital exchanges) because time expansion is cheaper than space expansion.

## Check yourself

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

1. What is an ST switch?
   A. A time stage followed by a space stage
   B. A space stage followed by a time stage with a TSI per output highway
   C. Two space stages in series
   D. Two TSIs in series with no space stage
   Answer: B. The S stage connects input highways to output highways, then each output highway has a TSI. (Key point: ST structure)

2. Why does an ST switch block?
   A. The space stage must pass the sample in the same slot i in which it arrives, so only one path exists
   B. The TSI memory is too slow
   C. The control memory is too small
   D. Too many highways share one clock
   Answer: A. If slot i towards output highway b is already used by another call, the new call is blocked even if slot j on that highway is free. (Key point: Cause of blocking)

3. How is the blocking of ST switches reduced?
   A. Remove the TSIs
   B. Add a third stage to give alternative paths (STS or TST)
   C. Reduce the frame size
   D. Use only one input highway
   Answer: B. A third stage provides several alternative paths; TST is preferred because time expansion is cheaper than space expansion. (Key point: Remedy)

## Related in Telecommunications

- [ST and TS drawbacks solved by STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ast-ts-drawbacks-sts) — Telecommunications, chapter 4
- [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — Telecommunications, chapter 4