ivision switch, which we call a time-slot interchanger (TSI). Connectivity shown is from user C in the incoming slot C to user G in outgoing slot G. 8000 times a second (with different binary values of course). DS1 has 24 time slots in a frame, one for each channel. E1 has 32 time slots. The time duration of an 8-bit time slot in each case is (125 usec)/24= 5.2083 usec for the DS1 case, and (125 usec)/32= 3.906 usec for the E1 case. Time-slot interchanging involves moving the data contained in each time slot from the incoming bit stream to an outgoing bit stream, but with a different time-slot arrangement in the outgoing stream, in accordance with the destination of each time slot. What is done, of course, is to generate a new frame for transmission at the appropriate switch outlet. Obviously, to accomplish this, at least one time slot must be stored in memory (write) and then called out of memory in a changed position (read). The operations must be controlled in some manner, and some of these control actions must be kept in memory together with the software managing such actions. Typical control functions are time- slot "idle" or "busy." Now we can identify three of the basic functional blocks of a time switch: 1. Memory for speech 2. Memory for control and 3. Time-slot counter or processor These three blocks are shown in Figure 6.17. There are two choices in handling a time switch: (1) sequential write, random read, as illustrated in Figure 6.17a, and (2) the reverse, namely, random write, sequential read as shown in Figure 6.17b. In the first case, sequential write, the time slots are written into the speech memory as they appear in the incoming bit stream. They are read out of the memory in the correct order for the outgoing bit stream. For the second case, random write (Figure 6.17b), the incoming time slots are written into memory in the order of appearance in the outgoing bit stream. This means that the incoming time slots are written into memory in the desired output order. The writing of incoming time sl