Functional Design of a Stored-Program Electronic Switching System

In the development of a large system, one of the most important problems is completely specifying its internal organization. The task is a complex one in the case of a telephone switching system because of the multiplicity of logical interactions involved. This paper discusses this problem as applied to an experimental electronic telephone switching system developed by Bell Telephone Laboratories.* During the course of designing the system many techniques new to the telephone field were employed. These include a stored-program con* The system was described in a paper 1 in the previous issue which provides pertinent background material for this article. 1327 1.356 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1958 trol philosophy and separation of the basic system functions, such as memory and logic, into efficient functionally concentrated blocks of equipment. The experimental system, a prototype of a practical telephone office, is a real-time special-purpose machine. One of the ever-present problems was that of effecting a balance between real-time occupancy, amount of memory and other equipment, and complexity of system operation. In this balance good telephone service and over-all economy are the ultimate objectives. Finally, a method of system operation had to be developed in order to process numerous types of telephone calls. An ideal solution is one where there is 110 interdependence between equipment requirements and telephone call functions. This would give rise to a universal system in which the addition or deletion of features or traffic in a telephone office would require no basic equipment changes.