page_title: Number One Electronic Switching System
section_title: 1AESS
hierarchical_section_title: Number One Electronic Switching System / 1AESS
caption_attribution_description: English: Head on view of 1AESS Master Control Center
context_page_description: The Number One Electronic Switching System was the first large-scale stored program control telephone exchange or electronic switching system in the Bell System. It was manufactured by Western Electric and first placed into service in Succasunna, New Jersey, in May 1965. The switching fabric was composed of a reed relay matrix controlled by wire spring relays which in turn were controlled by a central processing unit.
  The 1AESS central office switch was a plug compatible, higher capacity upgrade from 1ESS with a faster 1A processor that incorporated the existing instruction set for programming compatibility, and used smaller remreed switches, fewer relays, and featured disk storage. It was in service from 1976 to 2017.
context_section_description: The 1AESS version CC (Central Control) had a faster clock, approximately one MHz, and required only one bay of space instead of four. The majority of circuit boards were manufactured from metal for better heat dissipation, and carried TTL SSI chips, usually attached by hybrid packaging. Each finger at the back of the board was not a mere trace on the circuit board, as usual in plug-in boards, but a leaf spring, for greater reliability.
  1AESS used memory with 26-bit words, of which two were for parity checking. The initial version had 32 Kilowords of core mats. Later versions used semiconductor memory. Program stores were arranged to feed two words (52 bits) at a time to the CPU via the Program Store Bus, while Call Stores only gave one word at a time via the Call Store Bus. 1A Program Stores were writable and not fully duplicated but were backed up by the File Stores. They were provided in multiplicity of N+2, i.e. as many as were needed for the size of the office, plus two hot standby units to be loaded from disk as needed.
  In both the original version and 1A, clocks for Program Store and Call Store were operated out of phase, so one would be delivering data while the other was still accepting an address. Instruction decoding and execution were pipelined, to allow overlapping processing of consecutive instructions in a program.
  The original File Stores had four hard drives each. These hard drives were large, fast, expensive and crude, weighing about a hundred pounds (40 kg) with 128 tracks and one head per track as in a drum memory. They contained backups for software and for fixed data (translations) but were not used in call processing. These file stores, a high maintenance item with pneumatic valves and other mechanical parts, were replaced in the 1980s with the 1A Attached Processor System (1AAPS) using the 3B20D Computer to provide access to the "1A File Store". The 1AAPS "1A File Store" is just a disk partition in the 3B20D Computer.
  When the Common Network Interface (CNI) Ring became available it was added to the 1AAPS to provide Common Channel Signaling (CCS).
  The 1AESS tape drives had approximately four times the density of the original ones in 1ESS, and were used for some of the same purposes as in other mainframe computers, including program updates and loading special programs.
  Most of the thousands of 1ESS and 1AESS offices in the USA were replaced in the 1990s by DMS-100, 5ESS Switch and other digital switches, and since 2010 also by packet switches. As of late 2014, just over 20 1AESS installations remained in the North American network, which were located mostly in AT&T's legacy BellSouth and AT&T's legacy Southwestern Bell states, especially in the Atlanta GA metro area, the Saint Louis MO metro area, and in the Dallas/Fort Worth TX metro area. In 2015, AT&T did not renew a support contract with Alcatel-Lucent (now Nokia) for the 1AESS systems still in operation and notified Alcatel-Lucent of its intent to remove them all from service by 2017. As a result, Alcatel-Lucent dismantled the last 1AESS lab at the Naperville Bell Labs location in 2015, and announced the discontinuation of support for the 1AESS.  In 2017, AT&T completed the removal of remaining 1AESS systems by moving customers to other newer technology switches, typically with Genband switches with TDM trunking only.
  The last known 1AESS switch was in Odessa, TX (Odessa Lincoln Federal wirecenter ODSSTXLI). It was disconnected from service around June 3, 2017 and cut over to a Genband G5/G6 packet switch.
caption: Head on view of 1AESS Master Control Center
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