[[File:Volunteers æstrid and Colin working on the 3ESS.png|thumb|'''Volunteers æstrid and Colin working on the 3ESSm using an oscilloscope and a logic analyzer.''']]
{{About|specific switch in the Connections Museum|the switch in general|Number Three Electronic Switching System}}
The '''3ESS (Electronic Switching System)''' is a electronic switching system. It was installed in 1978 as a Community Dial Office (CDO) in the small town of Crosby, WA. It contains a custom tape drive emulator created by volunteers at the museum after the original tapes began to fail.
[[File:3ESS at the Connections Museum.jpg|thumb|The 3ESS at the Connections Museum while running]]
[[File:Volunteers æstrid and Colin working on the 3ESS.png|thumb|Volunteers æstrid and Colin working on the 3ESS using an oscilloscope and a logic analyzer.]]
== History ==
The '''Number Three Electronic Switching System (3ESS)''' is an electronic switching system on the second floor of the Connections Museum. It was originally installed in 1978 as a Community Dial Office (CDO) in the village of [[wikipedia:Crosby,_Washington|Crosby, Washington]]. It contains a custom [[wikipedia:Tape_drive|tape drive]] emulator created by volunteers at the museum after the original tapes began to fail.
== Technology ==
The system makes use of a control complex consisting of two custom-designed minicomputers running in an active/standby configuration. It uses a stored program, known as a ''generic'', to control the basic functions of the switching network. The generic also includes maintenance and administration routines, such as self-tests, fault recovery, man-machine interface handling, and job execution control. The generic is loaded at boot from a tape drive, along with ''translations'' that contain the data specific to this particular machine. This data includes variables such as line and trunk assignments, etc.
The switching network is a space-division, 5-stage, folded design, which utilizes remreeds (remnant-reeds) as crosspoints. Essentially, this means that even though the machine is computer controlled, it still switches physical circuits like the older electromechanical switches. A ''folded'' network is one where all trunks, lines, and service circuits appear on only one side of the network, in stage 1. A call enters the network at stage 1, and proceeds through the remaining stages before folding back and reaching its destination again at stage 1. This is in contrast to a step-by-step or panel switch, which does not use a folded network. Calls always enter on one end, and leave on the other end.
The 3ESS at the Connections Museum while runningVolunteers æstrid and Colin working on the 3ESS using an oscilloscope and a logic analyzer.
The Number Three Electronic Switching System (3ESS) is an electronic switching system on the second floor of the Connections Museum. It was originally installed in 1978 as a Community Dial Office (CDO) in the village of Crosby, Washington. It contains a custom tape drive emulator created by volunteers at the museum after the original tapes began to fail.
