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No. 1 ESS is a general purpose telephone switching machine for local, tandem, and toll application, both two and four wire. The need for flexibility leads to the use of stored program control. To meet the requirement of absolute dependability, an unusual system organization is used together with automatic duplicate switching and automatic trouble detection and diagnosis. These actions must not interfere with service. Matching and redundant coding are used to detect faults before they can interfere with service. Thus No. 1 ESS is designed as a real-time digital machine without down time. This dependability requirement differs in both kind and degree from that of a general purpose digital computer. The flexibility of stored program control makes possible the economical addition of new telephone services and will allow other services to be readily added in the future. In addition, variations in size, traffic, and features between offices can be handled largely by variations in stored translation tables. This leads to the use of highly standardized hardware and programs. The first installation of No. 1 ESS is operating in the laboratory. It now serves as a permanent program development vehicle after completing its initial function of hardware evaluation. The second installation is located at Succasunna, N. J., and is undergoing tests prior to being put into regular telephone service. The Succasunna office is also currently providing additional machine capacity for program development. Equipment for numerous other installations is now in manufacture.