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A central office crossbar switching system with functionally separate electronic common control units is described. The common control subsystem is divided into routing control, trunk supervision control, and data storage functions. Routing control is performed by a stored program controlled electronic processor. Trunk supervision control is accomplished on an optional basis either by a common wired logic electronic control unit or by logic in individual trunk circuits which can have access to the routing control processor for trunk identifying purposes. Data storage is provided in electrically alterable ferrite core memory banks. All common control subsystems are redundant with automatic switchover in case of failure. Crossbar switches are used in the switching matrix. Marker logic and simple switching functions in trunk and receiver/sender circuits are designed with conventional relay techniques. The switching system incorporates new custom calling and Centrex features. It provides for improved administrative capabilities and can be used as an integral part to expand an existing electromechanical crossbar system in an evolutionary fashion.