 idle. The blocking probability can be minimized if the number of space-stage time slots is large. A TST switch is strictly nonblocking if = 2c - 1, (6.1) 14 The order is a switch designer's decision. DIGITAL NETWORKS Time stage Space stage Time stage Space switch array m X n Figure 6.20 A time-space-time (TST) switch. TSI = time-slot interchanger. where l is the number of space-stage time slots and C is the number of external TDM time slots (Ref. 3). 6.11.2.5 Space-Time-Space Switch. A space-time-space switch reverses the order architecture of a TST switch. The STS switch consists of a space cross-point matrix at the input followed by an array of time-slot interchangers whose ports feed another cross-point matrix at the output. Such a switch is shown in Figure 6.21. Consider this operational example with an STS. Suppose that an incoming time slot 5 on port No. must be connected to an output slot 12 at outgoing port 4. This can be accomplished by time-slot interchanger No. 1, which would switch it to time slot 12; then the outgoing space stage would place that on outgoing trunk No. 4. Alternatively, time slot 5 could be placed at the input of TSI No. 4 by the incoming space switch, where it would be switched to time slot 12, and then out port No. 4. Space stage Time stage Space stage Space Space switch switch array array (mx n) (nxp) Figure 6.21 A space-time-space switch. 6.11 DIGITAL SWITCHING 6.11.2.6 TST Compared with STS. Both TST and STS switches can be designed with identical call-carrying capacities and blocking probabilities. It can be shown that a direct one-to-one mapping exists between time-division and space-division networks (Ref. 3). The architecture of TST switching is more complex than STS switching with space concentration. The TST switch becomes more cost-effective because time expansion can be achieved at less cost than space expansion. Such expansion is required as link utilization increases because less concentration is acceptable as utilization increases. It would follow, then, that TST swit