Fault-tolerant design of local ESS processors

The stored program control of Bell System Electronic Switching Systems (ESS) has been under development since 1953. During this period, the No. 1 ESS, the No. 2 ESS, and the No. 3 ESS have been developed and used extensively by Bell System operating companies to provide commercial telephone service. These systems serve all types of telephone offices: The large-capacity No. 1 ESS serves metropolitan offices, the medium-capacity No. 2 ESS was designed for suburban offices, and the No. 3 ESS can be found in many small rural offices. The fault tolerant design of ESS processors provides the same highly dependable telephone service established by the previous electro-mechanical systems. Pertinent processor architecture features used to achieve ESS reliablity objectives are discussed. A detailed discussion of the maintenance design of the 3A Processor is also included.