THE EVOLUTION OF THE 4ESS(TM) SWITCH

On January 17, 1976, the first 4ESS(TM) switch was placed into service , culminating the single largest switch development ever undertaken in the Bell System until that time. When it was introduced, the 4ESS swi tch was the largest high-capacity digital switch in existence, capable of terminating 107,520 trunks and completing over 500,000 calls per h our. During the next 18 years, the 4ESS switch played a pivotal role i n enabling the emerging U.S. long distance market to grow into today's $60 billion industry. The hallmark reliability and capacity of the 4E SS switch set the standard for digital switching. Since then, neither technology nor service needs have stood still. Although the classic di gital switch architecture pioneered by the 4ESS switch has passed the test of time, every major subsystem within the switch has gone through one or more technology upgrades to accept new features and reduce cos ts. This paper describes the evolution of the 4ESS switch, how our cus tomers' investment has been protected, and how new features have been added by selectively upgrading 4ESS switch subsystems.