t sequence on the keyboard of the computer workstation replaces strapping procedures. System faults are displayed as they occur, and circuit status may be indicated on the screen periodically. Due to the high speed of the computer, postdial delay is reduced. SPC exchanges permit numerous new service offerings, such as conference calls, abbreviated dialing, "camp-on-busy," call forwarding, voice mail, and call waiting. 4.3.7.2 Basic SPC Functions. There are four basic functional elements of an SPC switching system: 1. Switching matrix 2. Call store (memory) 3. Program store (memory) 4. Central processor (computer) The earlier switching matrices consisted of electromechanical cross-points, such as a crossbar matrix, reed, correed, or ferreed cross-points. Later switching matrices employed solid-state cross-points. 13 A register is a device that receives and stores signals; in this particular case, it receives and stores dialed digits. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Subscriber Switching lines Trunks network Central Program store control store Figure 4.12 A simplified functional diagram of an SPC exchange. The call store is often referred to as the "scratch-pad" memory. This is temporary storage of incoming call information ready for use, on command from the central proces- sor. It also contains availability and status information of lines, trunks, and service circuits under internal switch-circuit conditions. Circuit status information is brought to the mem- ory by a method of scanning. All speech circuits are scanned for a busy/idle condition. The program store provides basic instructions to the controller (central processor). In many installations, translation information is held in this store (memory), such as DN to EN translation and trunk signaling information. A simplified functional diagram of a basic SPC system is shown in Figure 4.12. 4.3.8 Concentrators and Remote Switching In Chapter 5 we discuss the design of a subscriber loop. There we will find that there are very definite length