1ESS switch

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The Number One Electronic Switching System, the first large-scale stored program control (SPC) telephone exchange or electronic switching system in the Bell System, was introduced in Succasunna, New Jersey, in May 1965.[1] The switching fabric was composed of reed matrixes controlled by wire spring relays which in turn were controlled by a central processing unit (CPU). The 1AESS Switch incorporating the 1A Processor of the late 1970s was a plug compatible upgrade, using smaller remreed switches, fewer relays, a faster CPU, and disk storage.

Switching fabric[edit]


For a switch with 1000 input customers and 1000 output customers, a full connection would require a matrix of 1000x1000, or 1 million, physical switches for full interconnection possibility. When one considers that a large telephone system can have many more than 1000 x 1000 customers, the hardware to establish a full interconnection can grow rapidly and exceed practical implementations. Agner Krarup Erlang first theorized a compromise which is based upon the concept that not all telephones lines are connected at the same time. From statistical theory, it is possible to design hardware that can connect most of the calls, in the sense of a high percentage, and block others as exceeding the design capacity. These are commonly referred to as blocking switches and are the most common in modern telephone exchanges. They are generally implemented as smaller switch fabrics in cascade. In many, a randomizer is used to select the start of a path through the multistage fabric so that the statistical properties predicted by the theory can be gained.

Line and trunk networks[edit]






The original 1ESS Ferreed[2][3][4] switching fabric was packaged as separate 8x8 switches or other sizes, tied into the rest of the speech fabric and control circuitry by wire wrap connections. The transmit/receive path of the analog voice signal is through a series of magnetic-latching reed switches (very similar to latching relays).[5]


Fabric error[edit]






Supervision and trunk signalling were the responsibility of trunk circuits. The most common kinds (reverse battery one-way trunks) were in plug-in trunk packs, two trunks per pack, 128 packs per Trunk Frame (originally) on 16 shelves. Each trunk pack was originally about 3x5x8 inches (8x12x20 cm) with edge connector in the back. The later 1AESS were made with shorter wire spring relays, making them less than half as wide, with more complex leaf spring connector. Trunk Frames were in pairs, the even numbered one having the Signal Distributor to control the relays in both. Most trunks had three wire spring relays and two scan points. They could supply regular battery or reverse battery to a line, and on-hook or off-hook supervision to the distant end, or be put into a bypass state allowing all functions (usually sending and receiving address signals) to be performed by common control circuits such as digit transmitters and receivers. Slightly more complex trunks, for example those going to TSPS offices for operator control, were packaged as only one per plugin unit.

Junctor Circuits were installed in similar frames, but were simpler, with only two relays. They were used only in Line to Line junctors. Large offices, in addition to these Junctor Circuits, had Intraoffice Trunks, which were of similar design but fit into the same Universal Trunk Frames as interoffice trunks. They carried overflow traffic when the small Junctor Groups of an office with many LN could not cope. Digit transmitters, receivers, other complex service circuits, and some complex trunks including those using E&M signaling, were permanently mounted in relay racks similar to those of 5XB rather than plug-in frames.

Scan and distribute[edit]







1ESS computer[edit]





Memory card for 128 words of 44 bits

Memory had a 44 bit word length for Program Stores, of which 6 were for Hamming error correction and one for an additional parity check. This left 37 bits for the instruction, of which usually 22 bits were for the address. This was an unusually wide instruction word for the time.








1A ESS[edit]

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 1AESS installations remain in the North American network. 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 plans on replacement of several 1AESS switches with new packet switches during 2013.

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