The 4ESS switch is a Class 4

Class 4 telephone switch

A Class 4, or Tandem, telephone switch is a U.S. telephone company central office telephone exchange used to interconnect local exchange carrier offices for long distance communications in the Public Switched Telephone Network....

 telephone Electronic Switching System

Electronic switching system

In telecommunications, an electronic switching system is:* A telephone exchange based on the principles of time-division multiplexing of digitized analog signals. An electronic switching system digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized...

 that was the first digital electronic toll switch introduced by Western Electric

Western Electric

Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 for long distance switching. It was introduced in 1976 in Chicago, Illinois to replace the 4a crossbar switch

Crossbar switch

In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....

. The last of 145 in the AT&T

AT&T

AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...

 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 city

Oklahoma City is the capital and largest city of the U.S. state of Oklahoma.Oklahoma City may also refer to:*Oklahoma City metropolitan area*Downtown Oklahoma City*Uptown Oklahoma City*Oklahoma City bombing*Oklahoma City National Memorial...

, Oklahoma. At divestiture

Bell System divestiture

The Bell System divestiture, or the breakup of AT&T, was initiated by the filing in 1974 by the U.S. Department of Justice of an antitrust lawsuit against AT&T. The case, United States v...

, 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. It is estimated it may still be in operation until approximately 2015. 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

1ESS switch

The Number One Electronic Switching System, the first large-scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, was introduced in Succasunna, New Jersey, in May 1965. The switching fabric was composed of reed matrixes controlled by wire spring relays...

. 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 Network Interface (CNI) Ring to provide Common Channel Signaling

Common Channel Signaling

In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...

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

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.