The TS switch can be made non-blocking by using an expanding time switch (T to T2 slots) and a concentrating space switch (which is complex).

Implementaion complexity. In general the complexity of the switching is represented interms of number of cross points (N) and its associated cost. The number of cross points in space stage can be easily calculated which is based on the array size. The time stage uses significant amount of memory which adds the cost of the whole system. To take this into account the cost of memory bit is assumed one hundredth of the cost of cross point. Thus,

where NX = Number of space stage cross points

Nb = Number of bits of memory.

The NB not only includes the time stage memory arrays, but also the control memory (store) of the time stage and space stage. Thus,

where NBX = Number of memory bits for the space stage control store

= N x (Number of control words) (number of bits per control word)

Nbt = Number of memory bits in the time stage equal to sum of time slot interchange and the control store bits.

= N x number of channels x number of bits per channel + N x number of control words x number of bits per control world.

Example 5.3. If N = 80, NBX = 13, 440 and NBT = 24,960 for a typical TS switch, calculate the implementation complexity.

IC = NX + Nbx±NBT = 80 x 80 + 13440 + 24960 X 100 100

IC = 6784 equivalent cross points.

As the number of cross points in space array is equal to 6400, the total cost is dominated by the space stage.

5.7.3. STS and TST Switching

The TS structure is of blocking nature. Let A and B are the subscribers using different time slot on the same line want to connect to two subscribers C and D using same time slot on different lines. A and B can be moved to the same time slot but during that time slot, the inlet line can be connected to C's line or D's line but not both. This is the significant limitation of the structure. Moreover, time stage switching is generally less expensive than space stage switching as digital memory is much cheaper than digital cross points (AND gates).

The multiple stages overcomes the limitations of the individual switches and cost savings can also be achieved. TST, STS, TSST, TSSSST and TSTSTSTSTSTSTS are the switching system configurations used in digital switching system. However, the TST structure is the most common.

STS Switching. In STS switching, the time stage is sandwiched between two space arrays. The digital switching system ITS 4/5 of USA (1976) uses the STS switching configuration. It handles 3000 trunks and accomodates 1500 Erlangs of traffic. Fig. 5.23 shows the spacetime-space (S-T-S) switching network for M incoming and outgoing PCM highways.

Establishing a path through an STS switch requires finding a time switch array with an available units access during the incoming time slot and an available read access during the desired outgoing time slot. The input side space stage as well as the output side space stage is free to utilise any free time switch modules. In the diagram shown in Fig. 5.23, the time slot 2 is connected to the TSM 2 where the time slot alloted is 16 and passed to the (M - 1)th line of output space array. Thus the path is provided. This structure is of non-blocking nature.

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