Space and time switches; STS vs TST

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.

 Space switch:  I1 slot5 --> O3 slot5
 Time switch:   slot5 --> slot12 (same highway)

How STS differs from TST

 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.