Number One Electronic Switching System
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.
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.
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.
This allowed a larger TN, with 8 JSF containing 32 grids, connecting 2048 junctors and 2048 B links.
[3][4][5] The transmit/receive path of the analog voice signal is through a series of magnetic-latching reed switches (very similar to latching relays).
[6] The much smaller Remreed crosspoints, introduced at about the same time as 1AESS, were packaged as grid boxes of four principal types.
Thus the worker had to handle a much bigger, heavier piece of equipment, but did not have to unwrap and rewrap dozens of wires.
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.
If the short was not detected, the software would command the printing of a "Supervision Failure" (SUPF) and try again with a different junctor.
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.
Though detected, the system was designed to connect the calling party to the wrong person rather than a disconnect, intercept, etc.
[7] The computer received input from peripherals via magnetic scanners, composed of ferrod sensors, similar in principle to magnetic core memory except that the output was controlled by control windings analogous to the windings of a relay.
Ferrods were mounted in pairs, usually with different control windings, so one could supervise a switchward side of a trunk and the other the distant office.
Odd numbered line equipment could not be made ground start, their ferrods being inaccessible.
SD were originally contact trees of 30-contact wire spring relays, each driven by a flipflop.
Each magnetic latching relay had one transfer contact dedicated to sending a pulse back to the SD, on each operate and release.
The pulser in the SD detected this pulse to determine that the action had occurred, or else alerted the maintenance software to print a FSCAN report.
One hard plastic card commonly held the components necessary to implement, for example, two gates or a flipflop.
Multiple card failures were not uncommon and the success rate for first time repair dropped rapidly.
Having this function as an atomic instruction (rather than implementing as a subroutine) dramatically sped scanning for service requests or idle circuits.
Memory had a 44-bit word length for program stores, of which six bits were for Hamming error correction and one was used for an additional parity check.
All memory frames, all busses, and all software and data were fully dual modular redundant.
The dual CCs operated in lockstep and the detection of a mismatch triggered an automatic sequencer to change the combination of CC, busses and memory modules until a configuration was reached that could pass a sanity check.
This resulted in further overlapping, thus higher program execution speed than might be expected from the slow clock rate.
Bugs that previously went unnoticed became prominent when 1ESS was brought to big cities with heavy telephone traffic, and delayed the full adoption of the system for a few years.
Temporary fixes included the Service Link Network (SLN), which did approximately the job of the Incoming Register Link and Ringing Selection Switch of the 5XB switch, thus diminishing CPU load and decreasing response times for incoming calls, and a Signal Processor (SP) or peripheral computer of only one bay, to handle simple but time-consuming tasks such as the timing and counting of Dial Pulses.
The Basic Generic program included constant "audits" to correct errors in the call registers and other data.
For Central office staff this could be a scary time as seconds and then perhaps minutes passed while they knew subscribers who picked up their phones would get dead silence until the phase was over and the processor regained "sanity" and resumed connecting calls.
If the automated phases failed to restore system sanity, there were manual procedures to identify and isolate bad hardware or buses.
It was created to respond to a growing need for rapid and economical delivery of data and printed copy.
