The 5ESS Switching System is a Class 5 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. It came into service in 1982 and the last unit was produced in 2003. History The 5ESS came to market as the Western Electric No. 5 ESS. It 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–2000 version, introduced in the 1990s, increased the capacity of the switching module (SM), with more peripheral modules and more optical links per SM to the communications module (CM). A follow-on version, the 5ESS–R/E, was in development during the late 1990s but did not reach market. Another version was the 5E–XC. The 5ESS technology was transferred to the AT&T Network Systems division upon the 1984 breakup of the Bell System. The division was divested by AT&T in 1996 as Lucent Technologies, and after becoming Alcatel-Lucent in 2006, it was acquired by Nokia in 2016. 5ESS switches in service in 2021 included several operated by the United States Navy. Architecture 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. Power for all circuitry is distributed as –48 VDC (nominal), and converted locally to logic levels or telephone signals. Switching Module Each Switching Module (SM) handles several hundred to a few thousand telephone lines or several hundred trunks or combination thereof. Each has its own processors, also called Module Controllers, which perform most