Space switch design with connection memory

Assumptions: each 5-bit connection-memory word is a one-hot select for the decode logic, with the bits in the order I1 I2 I3 I4 I5 (left to right). Channel 1 of each trunk arrives first (in $t_0$), channel 2 in $t_1$, and so on, as the channels are written right-to-left ("A4 A3 A2 A1").

Design of the space switch

This is a time-multiplexed space switch with 5 input trunks (rows) and 4 output trunks (columns), with 4 time slots per frame.

          O1    O2    O3    O4
 I1 ----- x --- x --- x --- x
 I2 ----- x --- x --- x --- x
 I3 ----- x --- x --- x --- x
 I4 ----- x --- x --- x --- x
 I5 ----- x --- x --- x --- x
          |     |     |     |
       [CM1] [CM2] [CM3] [CM4]
       decode logic per column
       (4 words x 5 bits each)

• Each output column has its own control memory with 4 words (one per time slot) of 5 bits.
• In each time slot the decoder of a column closes the one crosspoint whose bit is 1, connecting that input trunk to the output trunk for that slot only.
• The switch has $5 \times 4 = 20$ crosspoints and $4 \times 4 = 16$ control words.

Number of virtual paths

Each crosspoint can carry a different connection in each time slot:

$$\text{virtual paths} = 5 \times 4 \times 4 = 80$$

In each time slot at most 4 connections exist (one per output), so in one frame at most $4 \times 4 = 16$ of these paths can be in use. The given connection memory sets up all 16, so the switch must provide 16 virtual paths per frame (out of 80 possible).

Decoding the connection memory

Output channels in $t_0$ to $t_3$

Written in the same style as the input trunks (latest on the left):

• O1: B4 E3 A2 C1
• O2: A4 D3 C2 B1
• O3: D4 C3 B2 A1
• O4: C4 A3 E2 D1

A space switch does not change the time slot of a sample: channel $m$ always leaves in time slot $t_{m-1}$. Changing slots needs a time switch, which is why practical exchanges use TST.