he array during any one time-slot interval measured in simultaneous call connections is the smaller value of M or N. For example, if the array is 20 X 20 and a time-slot interchanger is placed on each input (horizontal) line and the interchanger handles 30 time slots, the array then can serve 20 X 30 = 600 different time slots. The reader should note how the TSI multiplies the call-handling capability of the array when compared with its analog counterpart. 6.11.2.4 Time-Space-Time Switch. Digital switches are composed of time and space switches in any order. 14 We use the letter T to designate a time-switching stage and use S to designate a space-switching stage. For instance, a switch that consists of a sequence of a time-switching stage, a space-switching stage, and a time-switching stage is called a TST switch. A switching consisting of a space-switching stage, a time-switching stage, and a space-switching stage is designated an STS switch. There are other com- binations of T and S. As we mentioned earlier, the AT&T No. 4 ESS switch is a good example. It is a TSSSST switch. Figure 6.20 illustrates the time-space-time (TST) concept. The first stage of the switch is the TSI or time stages that interchange time slots (in the time domain) between external incoming digital channels and the subsequent space stage. The space stage provides con- nectivity between time stages at the input and output. It is a multiplier of call-handling capacity. The multiplier is either the value for M or value for N, whichever is smaller. We also saw earlier that space-stage time slots need not have any relation to either external incoming or outgoing time slots regarding number, numbering, or position. For instance, incoming time slot 4 can be connected to outgoing time slot 19 via space network time slot 8. If the space stage of a TST switch is nonblocking, blocking in the overall switch occurs if there is no internal space-stage time slot during which the link from the inlet time stage and the link to the outlet time stage are both