--- title: "TST vs STS; DTMF set pros and cons" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:tst-sts-compare-dtmf" source: https://www.bctengineer.com/q/semester_8/telecommunications/tst-sts-compare-dtmf site: BCT Engineering --- # TST vs STS; DTMF set pros and cons Telecommunications · Chapter 4: Digital Switching - Asked in: 2072 Kartik, 6+4 marks - Video: [Telecommunication Systems (19 - Multi-Stage TST or STS Switch)](https://www.youtube.com/watch?v=ipZmBPa4SdM) — Dr. Samy S. Soliman - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/tst-sts-compare-dtmf - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question Compare between TST and STS switch used in digital telephone exchange system. State the advantages and disadvantages of DTMF telephone set. ## Short answer TST uses time stages outside and space centre, achieving lower cost and blocking via time expansion, suited for large exchanges. STS uses space stages outside and time centre, needing more hardware, used in small switches. DTMF advantages: fast dialling (~10 digit/s), low errors, end-to-end signaling, extra keys. Disadvantages: needs DTMF receivers, prone to talk-off, incompatible with old electromechanical exchanges. ## Answer #### Comparison of TST and STS switches Both are three-stage digital switches that give many alternative paths to reduce the blocking of two-stage ST/TS switches. ```text TST: link-->[TSI]-->[ S ]-->[TSI]-->link STS: link-->[ S ]-->[TSI x k]-->[ S ]-->link ``` | Point | TST | STS | |---|---|---| | Stage order | Time – Space – Time | Space – Time – Space | | Outer stages | TSIs (memories) | Time-multiplexed space switches | | Centre stage | Space switch | $k$ TSIs | | Alternative paths | $l$ internal time slots | $k$ centre TSIs | | Blocking | $[1-(1-p')^2]^{l}$, $p' = pc/l$ | $[1-(1-p')^2]^{k}$, $p' = pN/k$ | | Non-blocking if | $l \ge 2c - 1$ | $k \ge 2N - 1$ | | Expansion | Time expansion: faster memory, cheap | Space expansion: more TSIs and crosspoints, costly | | Cost for large switch | Lower | Higher | | Control | Search for a free internal slot | Search for a free centre TSI | | Typical use | Large exchanges, heavy traffic | Small switches, light traffic | TST gives lower blocking at lower cost, so most large digital exchanges use TST (or TSST). #### DTMF telephone set In **Dual Tone Multi-Frequency** dialling, each key sends two tones at once: one from the low group (697, 770, 852, 941 Hz) and one from the high group (1209, 1336, 1477, 1633 Hz). For example key "5" sends 770 Hz + 1336 Hz. **Advantages** - **Fast dialling:** about 10 digits per second (≈ 50 ms tone + 50 ms gap) compared with about 1 s per digit for rotary pulse dialling. - **Fewer errors:** no pulse distortion; tones are easy to detect. - **End-to-end signaling:** tones pass through the speech path after the call is connected, so they can be used for IVR menus, phone banking, voice mail and remote control. - **Extra keys** (*, #, A–D) allow supplementary services. - **Electronic, no moving parts**, so the set is reliable and compact. - Works well with digital (SPC) exchanges. **Disadvantages** - The exchange needs **DTMF receivers** (more costly than pulse counters), and enough of them for busy-hour traffic. - The set needs electronic tone generators and power from the line. - **Talk-off:** speech or music may imitate a valid tone pair and cause a false digit; receivers need guard times and twist limits. - Older electromechanical exchanges cannot use DTMF without converters. - Tones can be distorted by noise or line loss on poor lines. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which pair is correct? A. TST: k TSIs in centre; STS: space switch in centre B. TST: space switch in centre; STS: k TSIs in centre C. Both have TSIs in centre D. Both have space switches in centre Answer: B. TST = T-S-T with space centre; STS = S-T-S with k TSIs centre. (Key point: Centre stage) 2. Why does TST give lower blocking at lower cost? A. Time expansion needs only faster memory B. It uses more crosspoints C. It uses more TSIs D. It has fewer time slots Answer: A. Time expansion is cheap compared with STS space expansion. (Key point: Cost of expansion) 3. How does a DTMF key send its digit? A. One tone from the high group B. Two tones at once, one from low group and one from high group C. A series of pulses D. A single 1 kHz tone Answer: B. Each key sends one low-group and one high-group tone, e.g. 5 = 770 + 1336 Hz. (Key point: DTMF tones) 4. What is talk-off in DTMF? A. Slow dialling B. Speech or music imitating a valid tone pair and causing a false digit C. Loss of dial tone D. Pulse distortion Answer: B. Receivers need guard times and twist limits to avoid false digits. (Key point: DTMF disadvantage) ## Related in Telecommunications - [Telephone exchange structure; TST vs STS](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aexchange-structure-tst-sts) — 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 - [Roof coverings in Terai, pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-pros-cons) — Building Technology, chapter 4 - [Distribution layouts, pros/cons, improvements](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adistribution-layouts-improvements) — Water Supply Engineering, chapter 7, asked 4× - [Collective intelligence pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Acollective-intelligence-pros-cons) — Information System, chapter 4 - [Brushless excitation pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-pros-cons) — Power Plant Design, chapter 4