--- title: "Telephone exchange structure; TST vs STS" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:exchange-structure-tst-sts" source: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-structure-tst-sts site: BCT Engineering --- # Telephone exchange structure; TST vs STS Telecommunications · Chapter 4: Digital Switching - Asked in: 2075 Asoj, 4+4 marks - Video: [Telephone Switching||Time Division Switching||Space Division Switching||Switching](https://www.youtube.com/watch?v=c775sTZOewg) — Engineering Basics - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/exchange-structure-tst-sts - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question Describe technical structure of a telephone exchange. Compare TST and STS switch used in digital telephone exchange system. ## Short answer A digital exchange has subscriber line units (BORSCHT), switching network, trunk interfaces, signaling, SPC control, O&M, power and sync. TST uses Time-Space-Time with time expansion, lower blocking, non-blocking if l>=2c-1, for large exchanges; STS uses Space-Time-Space with space expansion, higher blocking, non-blocking if k>=2N-1, for small switches. ## Answer #### Technical structure of a telephone exchange A telephone exchange connects any subscriber line or trunk to any other on demand. It has the following main subsystems: ```text Subscribers Other exchanges | ^ +------------+ +-------------+ +-------------+ | Subscriber |->| Switching |->| Trunk / | | line units |<-| network |<-| junction | | (BORSCHT) | | (T, S, TST)| | interface | +------------+ +-------------+ +-------------+ ^ ^ ^ ^ | +--------+ | +--------+ | | Signal-| | | | | ling |<-->+<-->| Control (SPC | +--------+ | processors) +--------------------> +---------------+ O&M: charging, testing, admin ``` 1. **Subscriber line interface:** each line card does BORSCHT (battery feed, over-voltage protection, ringing, supervision, codec, hybrid, testing). Line concentrators combine many lightly used lines onto fewer switch ports. 2. **Switching network:** makes the speech path. In digital exchanges it is a TST (or TSST) network of time switches and space switches. 3. **Trunk/junction interface:** connects to other exchanges on E1 (2.048 Mbps) PCM links, with frame alignment and clock synchronisation. 4. **Signaling equipment:** subscriber signaling (dial tone, DTMF receivers, ringing, busy tone) and inter-exchange signaling (CAS or CCS/SS7). 5. **Control unit:** SPC processors that detect calls, analyse digits, find paths, set the switch, supervise calls and release them. 6. **Operation and maintenance:** charging (billing) records, traffic measurement, alarms, testing and subscriber administration. 7. **Power and synchronisation:** 48 V DC battery plant and a master clock. #### Comparison of TST and STS switches | Point | TST switch | STS switch | |---|---|---| | Stage order | Time – Space – Time | Space – Time – Space | | Centre stage | Space switch (TMS) | TSIs | | Alternative paths | $l$ internal time slots | $k$ centre TSIs | | Blocking | $[1-(1-p/\alpha)^2]^l$, lower | $[1-(1-p/\beta)^2]^k$, higher | | Expansion method | Time expansion, cheap | Space expansion, costly | | Non-blocking condition | $l \ge 2c - 1$ | $k \ge 2N - 1$ | | Cost for large size | Lower | Higher | | Use | Large digital exchanges (e.g. Ericsson AXE) | Small switches, light load | TST is preferred in practice because, for the same blocking, it needs less hardware and its paths are easy to find by searching time slots. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the role of line concentrators in an exchange? A. Combine many lightly used lines onto fewer switch ports B. Generate ringing tones C. Provide the master clock D. Do charging Answer: A. Subscriber lines carry little traffic, so concentration saves switch ports. (Key point: Line concentrators) 2. Which subsystem detects calls, analyses digits and sets paths? A. SPC control unit B. Trunk interface C. Power plant D. Line card hybrid Answer: A. The SPC processors do call detection, digit analysis, path finding, supervision and release. (Key point: Control unit) 3. The non-blocking conditions for TST and STS are respectively A. l >= 2c - 1 and k >= 2N - 1 B. k >= 2N - 1 and l >= 2c - 1 C. l >= c and k >= N D. both l >= N Answer: A. TST needs l >= 2c - 1 internal slots; STS needs k >= 2N - 1 centre TSIs. (Key point: Non-blocking conditions) 4. Why is TST preferred for large exchanges? A. Time expansion by faster memory is cheaper than space expansion, giving lower hardware for the same blocking B. It has no TSIs C. It has fewer paths D. STS cannot be built Answer: A. For the same blocking TST needs less hardware and its paths are easy to find by searching time slots. (Key point: Why TST preferred) ## Related in Telecommunications - [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 - [Multistage switching; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultistage-tst-sts-compare) — Telecommunications, chapter 4 - [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 - [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — 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 - [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 - [Digital transmission system; analog vs digital](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aanalog-digital-transmission-compare) — Instrumentation II, chapter 5 - [Compare switch and if-else-if ladder](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Aswitch-vs-ifelse-ladder) — Computer Programming, chapter 5, asked 2× - [Digital system, logic level, BCD vs binary](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-system-logic-level-bcd) — Digital Logic, chapter 1 - [Duplex digital communication system vs analog](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-comm-duplex-vs-analog) — Communication Systems, chapter 1