ermits a more economical replication strategy. needs faster switches and a control speed to match. Although some central control has been used with crossbar switches, it has been more widely applied to reed relay exchanges in the public network 31.5 Digital switching and later to fully electronic exchanges. The reed relays consist of sealed contact units inside an operating 31.5.1 Space division and time division coil. They are interconnected to form switch matrices, similar to switching Figure 31.1, in which each crosspoint has its own reed relay. Being sealed and having no moving parts, they are more reliable than Analogue exchanges such as Strowger, crossbar and reed relay crossbar switches. systems employ space division switching in which a separate physi- A typical arrangement for a reed relay exchange with central cal path is provided for each call and is held continuously for the control is shown in Figure 31.16. This is the system known as TXE4 exclusive use of that call for its whole duration. which is further described in Chapter 32. Digital and other pulse modulated systems employ time division This system shows a return to the concepts of the simple manual switching in which each call is provided with a path only during its exchange. All terminations on the exchange are connected to the allocated time slot in a continuous cycle. Each switching element or Digital switching 31/13 Incoming RECEIVE channel SPEECH number STORE Incoming pcm Outgoing pcm 8-bit speech samples CLOCK EXTRACTION & FRAME ALIGNMENT Read address Write address (random) (cyclic) Synch =o/g channel no. =i/c channel no. TIME SLOT COUNTER CONNECTION CONTROL STORE O/G channel numbers Update exchange control Read address (cyclic) Figure 31.17 Simple time switch crosspoint is shared by several simultaneous calls, each occupying 31.5.2 Digital tandem (transit) exchange a single time slot, in which a sample of the speech is transmitted. This sample may be a single pulse as in pulse amplitude modulated This interconnects transmission systems 