No. 4 ESS: Prologue

Prologue By A. E. SPENCER, Jr. (Manuscript received February 23, 1977) No. 4 ESS, a high-capacity, toll and tandem switching system, is the largest single system development ever undertaken by the Bell System. It is also the vehicle by which electronic switching was first introduced into the Bell System long distance telecommunications network. During 1976, four No. 4 ESS offices were placed in service--the first in Chicago in January, the second in Kansas City in July, and the third and fourth in Jacksonville and Dallas in December. No. 4 ESS had its origin in the 1950s in both research and planning. Research had provided the fundamental time-division switching techniques on which No. 4 ESS is based, and planning revealed that a very large switching system would be required to cope with expected growth. Preliminary development began in 1968. By 1970, a specific plan was laid out. This plan required development of a high-speed processor, a new device interconnection technology, a digital switching network and associated transmission terminal equipment, a large body of software, new manufacturing facilities, and new installation and operating procedures. A system as large and complex as No. 4 ESS would have been difficult (if not impossible) to achieve without the benefits of the integrated Bell System structure. Building on a background of solid research, the integrated design and manufacturing team worked with systems engineers, with switching, transmission, and device development engineers, with planners from AT&T headquarters, Long Lines, and the operating telephone companies, and with members of the manufacturing, documentation, and training and installation forces over a period of 7 years to introduce No.