--- title: "Space and time switches; STS vs TST" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:space-time-switch-sts-tst" source: https://www.bctengineer.com/q/semester_8/telecommunications/space-time-switch-sts-tst site: BCT Engineering --- # Space and time switches; STS vs TST Telecommunications · Chapter 4: Digital Switching - Asked in: 2073 Chaitra, 8 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/space-time-switch-sts-tst - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question What is a space switch and time switch? How STS switch is differ than TST switch? Explain with telephone switching diagram. ## Short answer A space switch changes highway but not time slot; a time switch changes time slot but not highway. STS uses input S, centre TSIs, output S, with k alternative paths and higher blocking; TST uses input TSI, centre S, output TSI, with l alternative slots and lower blocking. Example: A slot3 highway1 to B slot17 highway4 via TST internal slot9 or centre TSI2. ## Answer #### Space switch A **space switch (S)** connects physically separate input lines to output lines. In a digital exchange it is a **time-multiplexed space switch**: an $N \times M$ crosspoint (or multiplexer) matrix whose crosspoints are changed every time slot by a control memory, so one crosspoint carries different calls in different slots. It changes the *highway* of a sample but not its time slot. #### Time switch A **time switch (T)** or **TSI** changes the *time slot* of a sample on one TDM highway. It writes the incoming samples into a speech memory and reads them out in a different order under control of a control memory. It changes the slot but not the highway. ```text Space switch: I1 slot5 --> O3 slot5 Time switch: slot5 --> slot12 (same highway) ``` #### How STS differs from TST ```text STS: links -->[ S ]-->[TSI 1]-->[ S ]--> links [TSI 2] [TSI k] TST: links -->[TSI]-->[ S ]-->[TSI]--> links (N) (l internal (N) slots) ``` **STS working:** in the external slot $i$, the input S stage sends the sample to a free centre TSI $m$. TSI $m$ moves it from slot $i$ to the required output slot $j$. In slot $j$ the output S stage sends it to the correct outgoing link. Alternative paths = $k$ centre TSIs. **TST working:** the input TSI moves the sample from slot $i$ to a free internal slot $x$; in slot $x$ the space stage connects the input highway to the output highway; the output TSI moves it from slot $x$ to slot $j$. Alternative paths = $l$ internal slots. #### Telephone call example Subscriber A is on slot 3 of highway 1; B is on slot 17 of highway 4. - **TST:** TSI-1 moves slot 3 → internal slot 9; S stage closes (1, 4) in slot 9; TSI-4 moves slot 9 → slot 17. - **STS:** in slot 3, S stage connects highway 1 to centre TSI 2; TSI 2 moves slot 3 → slot 17; in slot 17 the output S stage connects TSI 2 to highway 4. | Point | STS | TST | |---|---|---| | Centre stage | TSIs | Space switch | | Paths | $k$ TSIs | $l$ time slots | | Blocking | Higher | Lower | | Expansion | Space (costly) | Time (cheap) | | Use | Small switches | Large exchanges | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a time switch (TSI) change? A. The highway but not the slot B. The time slot but not the highway C. Both highway and slot D. Neither; it only amplifies Answer: B. A TSI reorders samples in memory, changing the slot on the same highway. (Key point: Space vs time switch) 2. What does a time-multiplexed space switch do? A. Changes the highway, keeping the slot B. Changes the slot, keeping the highway C. Stores samples in a speech memory D. Converts analog to PCM Answer: A. Crosspoints change every slot under a control memory; the slot stays the same. (Key point: Space switch) 3. What is the centre stage of STS and of TST? A. TSIs in STS, space switch in TST B. Space switch in STS, TSIs in TST C. TSIs in both D. Space switches in both Answer: A. STS = S, k TSIs, S; TST = T, space switch, T. (Key point: STS vs TST centre stage) 4. In the STS example, A is on slot 3 of highway 1 and B on slot 17 of highway 4. Where is the slot changed? A. In the input S stage B. In the centre TSI C. In the output S stage D. At the subscriber Answer: B. TSI 2 moves slot 3 to slot 17; the S stages only change highways. (Key point: STS call example) ## Related in Telecommunications - [Multistage switching; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultistage-tst-sts-compare) — 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 - [Time division switching and space switch](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-division-space-switch) — 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 - [TST vs STS; DTMF set pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atst-sts-compare-dtmf) — 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 ## Related in other subjects - [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×