Development History
4ESS development began in about 1970, mainly in Naperville, Illinois under the direction of Henry Earle Vaughan. AT&T Long Distance was the primary customer for the switch. Driving development from the customer's perspective was AT&T VP Billy Oliver. Previous switching systems had all switched an analog voice signal; and consequently the decision to switch in a digital voice format was controversial at the time, both from a technical and economic viewpoint. Nevertheless, visionaries like Vaughn and Oliver recognized that the network would eventually become digital and consequently so must switching.
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