## Time division switching

Time division switching is accomplished by interchanging time slots between digital data streams. You will recall that in digital systems each speech channel is sampled at

8000 times per second with each 8-bit sample placed in a time slot. Time switching involves storage of the data from the various incoming time slots then placing that data in a different sequence in the outgoing time slots. This is shown diagrammatically in Figure 4.9.

Figure 4.9

### Time slot interchange

In this simple example the time division multiplexer on the left samples each of the customers 'a' to 'e' in turn producing the sequence of five time slots 'abcde'. This enters the time switch where the time slot samples are rearranged to emerge as the sequence 'eda-c'. These time slots correspond to customers 'fghij' when demultiplexed on the right of the drawing. As a consequence the time slot data from customer 'a' is sent to customer 'h'. Likewise connections are made between c-j, d-g and e-f. Note that the data from customer 'b' was not sent out of the time switch, and the time slot for customer 7' in this case was empty.

In practical digital switches large numbers of time slots are used within each time switch, and two separate paths through the switches are simultaneously used for the outgoing and incoming parts of each call. The switches are usually designed in multiples of 120 channels which fits well with five groups of 24 channels from the North American (T1) transmission systems and four groups of the 30 channel European (E1) transmission systems. Modules of 120 telephone channels would probably require 128 time slots and if these are switched at a bit rate of 64 kbps then the switch highway speed corresponds to a bit rate of 8.192 Mbps. To increase the number of time slots most manufacturers utilize parallel switching within their time switches, operating the high-speed highways in an 8-bit parallel format. In this way a highway providing 528 time slots and 480 telephone channels can be operated at an effective bit rate of 4.224 Mbps.

Practical digital switches are built up of combinations of time-division (T) and spacedivision (S) switches. This allows the time slots from one highway to be interchanged with slots on a different highway thereby greatly expanding the size of the switch. A switch comprising TSST switching stages is illustrated in Figure 4.10.

The functions of the different stages in this example TSST switch are as follows:

• T1 (time switch 1): Shifts data from one time slot to another on the same highway entering the space switch.

• S1 (space switch 1): Shifts time slots from one of the incoming highways to one of the junctor highways to space switch 2, without changing the time slot positions.

• S2 (space switch 2): Shifts time slots from one of the junctor highways to one of the highways to time switch 2, without changing the time slot positions.

• T2 (time switch 2): Shifts data from one time slot to another on the same highway leaving the space switch.

Figure 4.10

Example of TSST switch

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