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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 3B Processor, the Attached Processor System (4E APS), and the Peripheral Units.
Early versions used the same 1A processor as the contemporaneous improved 1AESS switch. All existing switches have been subsequently upgraded to use the 3B Processor. The Processor acts as the CPU for the switch.
The 4E APS provides long term storage (disk) of the 3B 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.
4ESS development began in about 1970, mainly in Naperville, Illinois under the direction of Henry Earle Vaughan. 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 tandem switching systems (primarily the #4 XBar) used analog voice signaling. 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.
The last #4ESS was installed in suburban Atlanta, GA in 1999 as a toll tandem for AT&T. At the peak (1999), there were 145 #4ESS switches in the AT&T long-haul network, with several owned by various Regional Bell Operating Companies (RBOCs).
As time goes on, AT&T is replacing or supplementing their #4ESS toll tandem switches with #5ESS switches, which are of a much advanced design and are used as "edge" switches in the network. Most RBOCs who used #4ESS tandems have replaced them with #5ESS switches and/or tandems of other manufacturers (e.g. Nortel). As of 2008, AT&T still operates and maintains approximately 100 #4ESS switches in the public switched telephone network.
- Special issue on the 4ESS switch BSTJ, Sept. 1977, Vol. 56, No. 7
- Bell System commercial about the 4ESS switch on YouTube
- http://att.elearn.ihost.com/websites/Contingency_Source/TS%20144%20Nodal%20Services%20Delivery/51246880/tf0100-05.pdf - Password required
- ATT Crash