The No. 4 Electronic Switching System (4ESS) is a class 4 telephone electronic switching system that was the first digital electronic toll switch introduced by Western Electric for long-distance switching. It was introduced in Chicago in January 1976, to replace the 4A crossbar switch. The last of the 145 systems in the AT&T network was installed in 1999 in Atlanta. Approximately half of the switches were manufactured in Lisle, Illinois, and the other half in Oklahoma City, Oklahoma. At the time of the Bell System divestiture, most of the 4ESS switches became assets of AT&T as part of the long-distance network, while others remained in the RBOC networks. Over 140 4ESS switches remained in service in the United States in 2007. System architecture The 4ESS Switch is often considered as a switching network and its controlling processor. The major functional equipment areas include: 1B processor, the 4ESS primary controlling processor 3B computer and associated attached processor system, which provides disk storage and additional functions CNI ring, a group of peripheral processors serially interconnected with each other in a dual ring configuration, and a 3B20D processor that functions in a distributed input/output processing architecture. Terminal equipment provides the interface between metallic trunk facilities in the toll network and the 4ESS Switch. Two-wire and 4-wire voice-frequency trunks terminating at metallic terminal facilities and equipment which provides an interface between digital carrier equipment and the 4ESS switching network. The switching network contains the equipment which actually switches pulse code modulated data from one trunk or service circuit to another. The switching network also contains timing equipment that generates the precise timing signals required to switch traffic. The switching network is also used to connect test equipment, announcements, and various tones to trunks when required. Processor The processor acts as the CPU for the switch. The processor includes a central control, call stores, and program stores. In addition it had access to additional units through the auxiliary unit bus (AUB) and peripheral unit bus (PUB). A master control console (MCC) provides office technicians access to the switch through the processor peripheral interface (PPI). Early versions used the same 1A processor as the contemporaneous improved 1AESS switch. All existing switches have been subsequently upgraded to use the 1B processor. File store and CNI ring The file store provides long term storage (disk storage) of the processor programs (program store) and office data (call store). It