ELECTRONIC COMMUNICATION SWITCHING SYSTEM

A stored program controlled electronic communication switching system provided with central control units having access to a memory consisting of high speed memory devices and low speed memory devices, such as magnetic drums. The low speed devices co-operate with said high speed devices so that data can be transferred therebetween to reduce the cost ratio of the memory necessary to meet the service specification of the system. The system further comprises means for minimizing the access time to the low speed memory devices. The central control units and the low speed memory devices are of duplicated construction in order to provide high reliability. Furthermore facilities are provided to enable the system to operate in a fallback mode, should faults occur, by changing the allocation of the content of memories to thereby modify the processing mode of the system without altering the processing program itself.