No. 4 ESS--Evolution of a Digital Switching System

The No. 4 Electronic Switching System, a high capacity, toll and tandem system based on digital time division switching was first placed in service in Chicago on January 17, 1976. It has provided improved system operation and new service features at reduced cost in comparison to the existing electromechanical systems. The flexibility of system architecture has permitted the evolution of the No. 4 Electronic Switching System. Since cutover of the first system, the introduction of improved hardware and software has made possible additional savings in equipment as well as reducing power and space requirements by approximately 60 percent. The introduction of the 700 ns stores has increased the computing power of the processor by 30 percent, Which has made possible the addition of new revenue producing features without reducing switching capacity. With the new features, No. 4 ESS is now being used in the North American domestic toll and tandem as well as international switching networks. In addition, the move toward centralization of maintenance and administration of the system will further reduce maintenance costs. The overall performance of the 21 in-service systems has been excellent.