No. 4 ESS: Performance Objectives and Service Experience

The No. 4 ESS is a digital time-division toll and tandem switching system first placed in service in Chicago. It was described in the Bell System Technical Journal in 1976.1 Since then, 51 offices with over 1,000,000 terminations have been put into service. The deployment progress is shown in Fig. 1. The average size of the No. 4 ESS is 22,000 terminations with current office sizes ranging from 6,000 to over 60,000 terminations. Detailed statistics demonstrate that the No. 4 ESS provides high-quality service to its customers and that its performance continues to improve as the system matures despite office growth, new generic programs, and evolving hardware. 1203 1.6 t/5 z o z £ 0.8 0.4 i L U 1976 1977 1978 YEAR 1979 1980 1981 0 Fig. 1--No. 4 ESS deployment.