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The No.1 Electronic Switching Systems (ESS) use stored program control to perform the telephone switching function. Central processor units, including memory, are duplicated for reliability. New features are generally provided by program changes only and their inclusion in a precutover office poses no new problems. However, to include new features in an operational office via program changes, without affecting service, is a unique task. This task was faced in the development of a new route control feature for the AUTOVON network.