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 and 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. 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^{[citation needed]}. The 4ESS has 3 major components: the 1A or 1B Processor, the Attached Processor System (4E APS), and the Peripheral Units.

Contents 1 1A/1B Processor 2 Attached Processor System (4E APS) 3 Peripheral Units 4 Development History 5 See also 6 References

[edit] 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.

[edit] 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 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.

[edit] 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 "time-space-time" switching function. Timing is provided by a high speed, high accuracy Network Clock.

[edit] Development History

4ESS development began in about 1970, mainly in Naperville, IL under the direction of Earl Vaughn. AT&T Long Distance was the primary customer for the switch. Driving development from the customer's perspective was AT&T VP Billy Oliver. Previous switching systems had all switched an analog voice signal; and consequently the decision to switch in a digital voice format was controversial at the time, both from a technical and economic viewpoint. Nevertheless, visionaries like Vaughn and Oliver recognized that the network would eventually become digital and consequently so must switching.

[edit] See also

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

[edit] References

• Special issue on the 4ESS switch BSTJ, Sept. 1977, Vol. 56, No. 7
• http://www.phworld.org/switch/weess.htm
• http://att.elearn.ihost.com/websites/Contingency_Source/TS%20144%20Nodal%20Services%20Delivery/51246880/tf0100-05.pdf - Password required
• ATT Crash

This article related to telecommunications is a stub. You can help Wikipedia by expanding it. v • d • e

v • d • e NANP Telephone Switches Early Automatic & Crossbar Switches Strowger Switch • Panel Switch • Western Electric 1XB • Western Electric 5XB Electronic Switching Systems Western Electric 1ESS/1AESS • Western Electric 4ESS • Western Electric 5ESS • Northern Electric SP1 • Northern Telecom DMS-100 • Stromberg-Carlson/Siemens DCO • Siemens EWSD • Automatic Electric GTD-5 EAX