Time division switching and space switch

Principle of time division switching

In time division switching, the switching hardware is shared in time: each call is given a short time slot in a repeating frame and the hardware connects that call only during its slot. In PCM telephony the voice is sampled every 125 µs (8 kHz) and coded into 8 bits; 32 slots form one 125 µs frame. A crosspoint or memory location used by one call in slot 3 can be used by another call in slot 4. Two forms exist:

• Time division space switching: a crosspoint matrix is shared in time (changes highway, keeps slot).
• Time division time switching: samples are stored and read in a different order (changes slot, keeps highway).

Time division space switch (TMS)

A time-multiplexed space switch connects $N$ input TDM highways to $M$ output highways. It has an $N \times M$ crosspoint matrix (electronic gates or multiplexers) and a control memory for each output column (output-controlled) or each input row (input-controlled).

            O1      O2     ...   OM
 I1 ------- x ----- x ---------- x
 I2 ------- x ----- x ---------- x
 ...
 IN ------- x ----- x ---------- x
            |       |            |
         [CM 1]  [CM 2]   ...  [CM M]
     c words x log2(N) bits each
             ^
      time-slot counter (0..c-1)

Operation

1. Each column control memory has $c$ words, one per time slot. Word $t$ holds the number of the input highway to connect to that output in slot $t$.
2. A time-slot counter steps through the slots. In slot $t$ it reads word $t$ of every control memory.
3. A decoder for each column closes the one crosspoint named by the word, so the sample on that input highway passes to that output highway during slot $t$.
4. In the next slot new words are read and different crosspoints close. So each crosspoint is shared by up to $c$ calls per frame.
5. The processor writes the control memories during call set-up and clears them at release.

Example: in slot 5, CM2 holds "3", so crosspoint (I3, O2) closes and the sample in slot 5 of I3 goes to slot 5 of O2. In slot 6, CM2 may hold "1", connecting I1 to O2.

Features

• The sample keeps its time slot; only the highway changes. To change slots, a time stage is added (TS, ST, TST).
• Control memory size: $M$ memories of $c \times \lceil\log_2 N\rceil$ bits.
• The crosspoint count is $N \times M$, but each crosspoint serves up to $c$ calls, so the capacity is very large.
• Speed of the gates limits the number of slots per frame.
• On its own it cannot move a call to a different slot, so it is used as the middle stage of TST switches.