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. 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
The voice switching fabric plan approximately followed that of the earlier 5XB switch in being bidirectional and in using the call-back principle. The largest full access matrix switches in the system, however, were 8x8 rather than 10x10 or 20x16. Thus they required eight stages rather than four to achieve large enough junctor groups in a large office. Crosspoints being more expensive in the new system but switches cheaper, system cost was minimized with fewer crosspoints organized into more switches. The fabric was divided into Line Networks and Trunk Networks of four stages, and partially folded to allow connecting line-to-line or trunk-to-trunk without exceeding eight stages of switching.
Although the following discussion is technically detailed, a brief review of the principles can aid in understanding. Suppose we have the need to connect 1000 input customers to 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 input customers and 1000 output customers, the hardware to establish a full interconnection can grow rapidly and exceed practical implementations. There is a reasonable compromise first theorized by Agner Krarup Erlang which is based upon the concept that not all calls need to be 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
Each four stage Line Network (LN) or Trunk Network (TN) was divided into Junctor Switch Frames (JSF) and either Line Switch Frames (LSF) in the case of a Line Network, or Trunk Switch Frames (TSF) in the case of a Trunk Network. Links were designated A, B, C, and J for Junctor. A Links were internal to the LST or TSF; B Links connected LSF or TSF to JSF, C were internal to JSF, and J links or Junctors connected to another net.
All JSFs had a unity concentration ratio, that is the number of B links within the network equalled the number of junctors to other networks. Most LSFs had a 4:1 Line Concentration Ratio (LCR); that is the lines were four times as numerous as the B links. In some urban areas 2:1 LSF were used. The B links were often multipled to make a higher LCR, such as 3:1 or (especially in suburban 1ESS) 5:1. Line Networks always had 1024 Junctors, arranged in 16 grids that each switched 64 junctors to 64 B links. Four grids were grouped for control purposes in each of four LJFs.
TSF had a unity concentration, but a TN could have more TSFs than JSFs. Thus their B links were usually multipled to make a Trunk Concentration Ratio (TCR) of 1.25:1 or 1.5:1, the latter being especially common in 1A offices. TSFs and JSFs were identical except for their position in the fabric and the presence of a ninth test access level or no-test level in the JSF. Each JSF or TSF was divided into 4 two-stage grids.
Early TNs had four JSF, for a total of 16 grids, 1024 J links and the same number of B links, with four B links from each Trunk Junctor grid to each Trunk Switch grid. Starting in the mid 1970s, larger offices had their B links wired differently, with only two B links from each Trunk Junctor Grid to each Trunk Switch Grid. This allowed a larger TN, with 8 JSF containing 32 grids, connecting 2048 junctors and 2048 B links. Thus the junctor groups could be larger and more efficient. These TN had eight TSF, giving the TN a unity trunk concentration ratio.
Within each LN or TN, the A, B, C and J links were counted from the outer termination to the inner. That is, for a trunk, the trunk Stage 0 switch could connect each trunk to any of eight A links, which in turn were wired to Stage 1 switches to connect them to B links. Trunk Junctor grids also had Stage 0 and Stage 1 switches, the former to connect B links to C links, and the latter to connect C to J links also called Junctors. Junctors were gathered into cables, 16 twisted pairs per cable constituting a Junctor Subgroup, running to the Junctor Grouping Frame where they were plugged into cables to other networks. Each network had 64 or 128 subgroups, and was connected to each other network by one or (usually) several subgroups.
The original 1ESS Ferreed 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. 1AESS crosspoints were packaged as grid boxes of four principal types. Type 10A Junctor Grids and 11A Trunk Grids were a box about 16x16x5 inches (40x40x12 cm) with sixteen 8x8 switches inside. Type 12A Line Grids with 2:1 LCR were only about 5 inches (12 cm) wide, with eight 4x4 Stage 0 line switches with ferrods and cutoff contacts for 32 lines, connected internally to four 4x8 Stage 1 switches connecting to B-links. Type 14A Line Grids with 4:1 LCR were about 16x12x5 inches (40x30x12 cm) with 64 lines, 32 A-links and 16 B-links. The boxes were connected to the rest of the fabric and control circuitry by slide-in connectors. Thus the worker had to handle a much bigger, heavier piece of equipment, but didn't have to unwrap and rewrap dozens of wires.
Fabric error
The two controllers in each Junctor Frame had no-test access to their Junctors via their F-switch, a ninth level in the Stage 1 switches which could be opened or closed independently of the crosspoints in the grid. When setting up each call through the fabric, but before connecting the fabric to the line and/or trunk, the controller could connect a test scan point to the talk wires in order to detect potentials. Current flowing through the scan point would be reported to the maintenance software, resulting in a "False Cross and Ground" (FCG) teleprinter message listing the path. Then the maintenance software would tell the call completion software to try again with a different junctor.
With a clean FCG test, the call completion software told the "A" relay in the trunk circuit to operate, connecting its transmission and test hardware to the switching fabric and thus to the line. Then, for an outgoing call, the trunk's scan point would scan for the presence of an off hook line. If the short was not detected, the software would command the printing of a "Supervsion Failure" (SUPF) and try again with a different junctor. A similar supervision check was performed when an incoming call was answered. Any of these tests could alert for the presence of a bad crosspoint.
Staff could study a mass of printouts to find which links and crosspoints (out of, in some offices, a million crosspoints) were causing calls to fail on first tries. In the late 1970s, teleprinter channels were gathered together in Switching Control Centers (SCC), later Switching Control Center System, each serving a dozen or more 1ESS exchanges and using their own computers to analyze these and other kinds of failure reports. They generated a so-called histogram (actually a scatterplot) of parts of the fabric where failures were particularly numerous, usually pointing to a particular bad crosspoint, even if it failed sporadically rather than consistently. Local workers could then busy out the appropriate switch or grid and replace it.
When a test access crosspoint itself was stuck closed, it would cause sporadic FCG failures all over both grids that were tested by that controller. Since the J links were externally connected, switchroom staff discovered that such failures could be found by making busy both grids, grounding the controller's test leads, and then testing all 128 J links, 256 wires, for a ground.
Peripherals
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 pe
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