Time switch in digital exchange

A time (T) switch, or Time Slot Interchanger (TSI), is the part of a digital exchange that moves a PCM sample from one time slot to another on a TDM highway. Since each subscriber occupies a fixed slot, swapping slots connects one subscriber to another.

 in (slots 0..c-1)                  out
 ------->[ Speech memory c x 8 ]------->
          ^ write       read ^
   [counter]          [Control memory]
                       (set by processor)

Parts

• Speech memory (SM): $c$ locations of 8 bits, one per slot.
• Control memory (CM): $c$ words of $\log_2 c$ bits giving read or write addresses.
• Time-slot counter and control logic.

Working (sequential write, random read)

1. The sample in input slot $i$ is written into SM($i$).
2. In output slot $j$, CM($j$) holds $i$, so SM($i$) is read and sent out in slot $j$.
3. Example: CM(7) = 3 connects input TS3 to output TS7, with a delay of 4 slots.

In the other mode (random write, sequential read), CM($i$) gives the write address and SM is read in order.

Speed limit: each slot needs a write and a read, so $t_{access} \le 125\ \mu s / 2c$. For $c = 32$ this is 1.95 µs; a 50 ns memory can handle up to $125\ \mu s / (2 \times 50\ ns) = 1250$ channels.

Features: non-blocking within its frame, delay under one frame, low cost (just memory). Since it serves only one highway, large exchanges combine T switches with space switches as TST.