Number Three Electronic Switching System

This article is about the switch in general. For specific switch in the Connections Museum, see CMS:Number Three Electronic Switching System.

The last operational 3ESS at the Connections Museum in Seattle.

The Number Three Electronic Switching System (3ESS) is an electronic switching system that was manufactured by Western Electric.

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.