Previous part (Reed Electronic (Matrix) Switching)

Stored Program (Software) Controlled switching

When was it in use? 1965-Present day

So far all the control logic of the switching systems we've looked at has been hard wired logic. Changes to the instruction sequence or routing translations has to be done by re-wiring connections or replacing units with new ones wired differently

In May 1965 AT&T introduced the Electronic Switching System No. 1 (1ESS) in Succasunna New Jersey. This system, designed by AT&Ts Bell Labs was the first application of software to control telephone switching. The actual switching was done using matrixes of reed relay crosspoints as described on the previous page, but unlike other reed-relay systems the routing and control logic was done in software making changes quick and easy. Bell Labs would go on to develop a range of ESS exchanges over the next few years culminating in the 5ESS which is a solid state digital switch still in use throughout the world today.

In the UK the worlds first digital exchange, a field trial called Empress (described in more detail here) did not use Stored Program Control (SPC); in fact, despite the pioneering advances in the actual switching & transmission technology by UK scientists, British interest in SPC was initially somewhat lacking.

The first use of SPC in the UK was another prototype field-trial at Moorgage exchange London brought into use in March 1971. This system was wholly designed and built by STC. Like Empress it was a wholly digital switch acting as a tandem switch (routing traffic between other exchanges). Traffic between Bishopsgate, Mile End & Shoreditch exchanges was routed through the prototype SPC exchange and traffic gradually increased. On seven occasions the system failed due to software bugs and component failure in the control system but the experience gained provided valuable input to the later development of other SPC systems - the TXE4 and System X.