ots into the speech memory can be controlled by a simple time-slot counter and can be sequential (e.g., in the order in which they appear in the incoming bit stream, as in Figure 6.17a). The readout of the speech memory is controlled by the control memory. In this case the readout is random where the time slots are read out in the desired output order. The memory has as many cells as there are time slots. For the DS1 example there would be 24 cells. This time switch, as shown, works well for a single inlet-outlet switch. With just 24 cells, it can handle 23 stations besides the calling subscriber, not an auspicious number. How can we increase a switch's capacity? Enter the space switch (S). Figure 6.18 affords a simple illustration of this concept. For example, time slot B1 on the B trunk is moved to the Z trunk into time slot Z; and time slot Cn is moved to trunk W into time-slot Wn. However, the reader should note that there is no change in the time-slot position. 6.11 DIGITAL SWITCHING Speech memory Control memory Time slot counter Figure 6.17a Time-slot interchange: time switch (T). Sequential write, random read. Speech memory Control memory Time slot counter Figure 6.17b Time-switch, time-slot interchange (T). Random write, sequential read. 6.11.2.3 Space Switch. A typical time-division space switch (S) is shown in Figure 6.19. It consists of a cross-point matrix made up of logic gates that allow the switching of time slots in a spatial domain. These PCM time slot bit streams are organized by the switch into a pattern determined by the required network connectivity. The matrix consists of a number of input horizontals and output verticals with a logic gate at each cross point. The array, as shown in the figure, has M horizontals and N verticals, and DIGITAL NETWORKS Space switch Figure 6.18 Space switch connects time slots in a spatial configuration. Control store Figure 6.19 Time-division space switch cross-point array showing enabling gates. we call it an M x N array. If M = N, the switch is nonblocking; I