Short note: ST switch

An ST switch is a two-stage digital switch in which a space (S) stage is followed by a time (T) stage. The S stage is a time-multiplexed space switch that connects incoming TDM highways to outgoing highways; each outgoing highway has a time slot interchanger (TSI) that moves the sample to the required output slot.

 TDM in            S stage           T stage     TDM out
 I1 ------->+----------------+--->[TSI 1]---> O1
 I2 ------->|  time-shared   |--->[TSI 2]---> O2
 ..         |  crosspoint    |     ...         ..
 IN ------->|  matrix (N x N)|--->[TSI N]---> ON
            +----------------+
                  ^
            control memory (per output column)

Working: a sample arriving in slot $i$ on input highway $I_a$ must go to slot $j$ on output highway $O_b$. In slot $i$ the S-stage control memory closes crosspoint $(a,b)$, so the sample enters TSI $b$, which writes it and reads it out in slot $j$.

Drawback (blocking): the space stage must move the sample in the same time slot $i$ in which it arrives. If another call from a different input already uses slot $i$ to reach output highway $b$, the new call is blocked, even though slot $j$ on $O_b$ is free. There is only one path for each connection, so blocking is high under heavy load. TS switches have the same problem the other way round.

Remedy: add a third stage to get several alternative paths, giving STS or TST switches. The TST form is preferred in practice (e.g. in most large digital exchanges) because time expansion is cheaper than space expansion.