5ESS

5ESS Switching System

The 5ESS Switching System is a telephone electronic switching system developed by Western Electric for the American Telephone and Telegraph Company (AT&T) and the Bell System in the United States.

The 5ESS came to market as the Western Electric No. 5 ESS. It first commenced service in Seneca, Illinois on 25 March 1982, and was destined to replace the Number One Electronic Switching System (1ESS and 1AESS) and other electromechanical systems in the 1980s and 1990s. The 5ESS was also used as a Class 4 telephone switch or as a hybrid Class 4/Class 5 switch in markets too small for the 4ESS. Approximately half of all US central offices are served by 5ESS switches. The 5ESS is also exported internationally, and manufactured outside the US under license.

The 5ESS technology was transferred to the AT&T Network Systems division upon the breakup of the Bell System. The division was divested by AT&T as Lucent Technologies, and after becoming Alcatel-Lucent, it was acquired by Nokia.

The 5ESS switch has three main types of modules: the Administrative Module (AM) contains the central computers; the Communications Module (CM) is the central time-divided switch of the system; and the Switching Module (SM) makes up the majority of the equipment in most exchanges. The SM performs multiplexing, analog and digital coding, and other work to interface with external equipment. Each has a controller, a small computer with duplicated CPUs and memories, like most common equipment of the exchange, for redundancy. Distributed systems lessen the load on the Central Administrative Module (AM) or main computer.