4ESS

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 145 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. Almost 150 4ESS switches remained in service in the United States in 2007.

The 4ESS has three major components: the processor, the file store, which was later known as the attached processor system (4EAPS), and the peripheral units.

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.

The file store provides long term storage (disk storage) of the processor programs (program store) and office data (call store). It was first implemented using disk technology but was replaced by the 4E attached processor system (4EAPS). The 4EAPS is a 3B computer running 4EAPS application software on the DMERT operating system. The 4EAPS interfaces to the 4ESS processor via the attached processor interface (API) units. The "1A file store" became partitions on the 3B computer disks. At first the 4EAPS just provided "file store" but soon it also provided access to the common-network interface ring (CNI ring) to provide common-channel signaling (CCS). The 4EAPS originally used the 3B20D computer. These were all converted to the 3B21D around 1995.

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