4ESS Switch

The 4ESS switch 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 1976 in Chicago, Illinois 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, and the other half were manufactured in Oklahoma City, Oklahoma. At 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 remain in service in the United States in 2007. The 4ESS has 3 major components: the 1A or 1B Processor, the Attached Processor System (4E APS), and the Peripheral Units.

Early versions used the same 1A processor as the contemporaneous improved 1ESS switch. All existing switches have been upgraded to use the 1B Processor. The 1B Processor acts as the CPU for the switch.

The 4E APS provides long term storage (disk) of the 1B Processor programs and office data. It also provides access to the Common Network Interface (CNI) Ring to provide Common Channel Signaling (CCS). The 4E APS originally used the 3B20D Computer. These were all converted to the 3B21D around 1995.

The Peripheral Units include terminating equipment used to connect the switch to the transport network and the Time Slot Interchanges (TSI) and Time Multiplexed Switches (TMS), which actually perform the "time-space-time" switching function. Timing is provided by a high speed, high accuracy Network Clock.