Organization of No. 1 ESS Stored Program

The program for the No. 1 electronic switching system (ESS) 1 constitutes the operating intelligence of the system. In fulfilling this function it must meet the stringent operating requirements of a telephone switching office. This article describes the most important of these requirements and outlines the implementation selected as a result. There are six main classes of requirements. First, the system must respond appropriately in real time to demands for service. Second, the system must perform a large variety of actions to provide the many services which are offered and to work compatibly with a wide range of connecting systems. Third, the system must be extremely reliable. In forty years of continuous operation, the total time the central processor may be out of service is measured in minutes. Fourth, the system must work in wire centers which are growing in the amounts of equipment and in the scope of features offered. Fifth, although there will be a large number of installations which may differ from each other in the kind 1923