1A ESS
The 1AESS version CC (Central Control) had a faster clock, approximately one MHz, and took only one bay instead of four. The majority of circuit boards were 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 stores (memory units) with 26 bit words, of which two were for parity check. 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 as 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 partitions 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 2012, just under 40 1AESSes remain in the North American PSTN. These are 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, although there are still a few remaining 1AESS switches in other parts of BellSouth's and Southwestern Bell's legacy territory. AT&T does plan on replacement of several more existing 1AESS switches, with new packet switches, during 2013.
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