CMS:Number Three Electronic Switching System

Volunteers æstrid and Colin working on the 3ESSm using an oscilloscope and a logic analyzer.

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.

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.