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. Early versions used the same 1A processor as the contemporaneous improved 1ESS switch. All existing switches have been upgraded to a 1B Processor. It was introduced in 1976 in Chicago to replace the 4a crossbar switch and the last of 145 in the AT&T network was installed in 1999 in Atlanta. Almost 150 4ESS switches remain in service in the United States in 2007. It is estimated it may still be in operation until approximately 2015.