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 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 2015Fact|date=October 2007. The 4ESS has 3 major components: the 1A or 1B Processor, the Attached Processor System (4E APS), and the Peripheral Units.

1A/1B Processor

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.

Attached Processor System (4E APS)

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

Peripheral Units

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 switching function. Timing is provided by a high speed, high accuracy Network Clock.

ee also

*1ESS switch
*5ESS switch
*Telephone exchange
*Office classification

References

* Special issue on the 4ESS switch BSTJ, Sept. 1977, Vol. 56, No. 7