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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 January, 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, Illinois, 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 Processor, the File Store (later known as the Attached Processor System (4E APS)), and the Peripheral Units.
The Processor acts as the CPU for the switch. The Processor includes a Central Control, Call Stores, and Program Stores. In addition it had access to additional units through the Auxiliary Unit Bus (AUB) and Peripheral Unit Bus (PUB). A Master Control Console (MCC) provides office technicians access to the switch through the Processor Peripheral Interface (PPI).  Early versions used the same 1A Processor as the contemporaneous improved 1AESS switch. All existing switches have been subsequently upgraded to use the 1B Processor.
File Store and CNI Ring
The File Store provides long term storage (disk storage) of the processor programs (Program Store) and office data (Call Store). It was first implemented using disk technology but was replaced by the 4E Attached Processor System (4EAPS). The 4EAPS is a 3B Computer running 4EAPS application software on the DMERT operating system. The 4EAPS interfaces to the 4ESS Processor via the Attached Processor Interface (API) units. The "1A File Store" became partitions on the 3B computer disks. At first the 4EAPS just provided "File Store" but soon it also provided access to the Common Network Interface (CNI) Ring to provide Common Channel Signaling (CCS). The 4EAPS originally used the 3B20D Computer. These were all converted to the 3B21D around 1995.
The Peripheral Units include units that interface to the Central Control over the Peripheral Unit Bus. This includes the Common Channel Interface Signaling (CCIS) Terminal, Signal Processors, Time Slot Interchanges (TSI) and Time Multiplexed Switches (TMS).  It also includes equipment not directly on the PUB including terminating equipment used to connect the switch to the transport network and the TSIs and TMSs, 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).
AT&T replaced or supplemented 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 2014, AT&T still operates and maintains approximately 100 #4ESS switches in the public switched telephone network.
Next Generation 4ESS
The Alcatel-Lucent N4E (New 4ESS) Solution is the ATCA based next generation 4ESS (No. 4 Electronic Switching System) for AT&T. The N4E includes the 4E APS and 4ESS software but replaces the 1B Processor and the Peripheral Units which run in emulated environments on an ATCA blade. The N4E is based on the Alcatel-Lucent Gateway Platform (7520 Media Gateway (MGW), 1310 Operations and Management Console – Plus (OMC-P) and the 5400 Linux Control Platform (LCP) and includes other elements such as MRV console terminal servers.
- Bell System Technical Journal, v56: i7 September 1977, No. 4 ESS: Prologue,Spencer, A.E. Jr.
- Bell System Technical Journal, v56: i2 February 1977, 1A Processor
- Bell System Technical Journal, v56: i7 September 1977, No. 4 ESS: Peripheral Systems
- IEEE biography of Billy Oliver
- 4E Replacement Alcatel-Lucent customer support
- Special issue on the 4ESS switch BSTJ, Sept. 1977, Vol. 56, No. 7
- Bell System commercial about the 4ESS switch on YouTube
- ATT Crash