Stored Program Control

Introduction

Stored program control (SPC) is a broad term designating switches where common control is carried out entirely by computer. In some exchanges, this involves a large, powerful computer. In others, two or more minicomputers may carry out the SPC function. Still with other switches, the basic switch functions are controlled by distributed microprocessors. Software may be hard-wired on one hand or programmable on the other. There is a natural marriage between a binary digital computer and the switch control functions. In most cases these also work in the binary digital domain. The crossbar markers and registers are typical examples.

The conventional crossbar marker requires about half a second to service a call. Up to 40 expensive markers are required on a large exchange. Strapping points on the marker are available to laboriously reconfigure the exchange for subscriber change, new subscribers, changes in traffic patterns, reconfiguration of existing trunks or their interface, and so on.

Replacing register markers with programmable logic—a computer, if you will—permits one device to carry out the work of 40. A simple input 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.

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.

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 processor. 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 memory 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.