Skip to main content

TST vs STS; DTMF set pros and cons

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.

Compare between TST and STS switch used in digital telephone exchange system. State the advantages and disadvantages of DTMF telephone set.

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.

 TST: link-->[TSI]-->[ S ]-->[TSI]-->link
 STS: link-->[ S ]-->[TSI x k]-->[ S ]-->link
PointTSTSTS
Stage orderTime – Space – TimeSpace – Time – Space
Outer stagesTSIs (memories)Time-multiplexed space switches
Centre stageSpace switchkk TSIs
Alternative pathsll internal time slotskk centre TSIs
Blocking[1−(1−p′)2]l[1-(1-p')^2]^{l}, p′=pc/lp' = pc/l[1−(1−p′)2]k[1-(1-p')^2]^{k}, p′=pN/kp' = pN/k
Non-blocking ifl≥2c−1l \ge 2c - 1k≥2N−1k \ge 2N - 1
ExpansionTime expansion: faster memory, cheapSpace expansion: more TSIs and crosspoints, costly
Cost for large switchLowerHigher
ControlSearch for a free internal slotSearch for a free centre TSI
Typical useLarge exchanges, heavy trafficSmall 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Telecommunications chapter 4 questions →