any other systems which can plan for regular downtime for are compared with the stored routing data (translation table) to maintenance, telephone switching systems are required to provide enable the switch to be controlled via the connection control stores virtually uninterrupted service 24 hours a day, seven days a week, (Figures 31.17 to 31.19). Exchange staff can interrogate the status year in - year out, to tens of thousands of customers (and junctions). of the processor and update the routing and class of service tables At the same time they must be adaptable to growth and operational through the Man Machine Interface (MMI). requirements. Occasional faults are, of course, inevitable and the system design must allow for this, SO that a fault does not result in loss of service to any significant number of customers. It is better Connection Operations control for a fault to temporarily reduce the grade of service to all customers stores maintenance rather than to deny service entirely to a smaller number. As a result, Billing Line interfaces centre various system architectures have been evolved. In electromechanical systems, each call follows a path wholly HARDWARE exclusive to itself, sufficient paths being provided to carry the predicted traffic. Failure in any path, therefore, reduces the grade of service to all terminations normally having access to it, but since SOFTWARE other paths are available, service is not wholly cut off to any one. Electromechanical systems are, therefore, inherently fault-tolerant Signalling as regards path switching and, for example, Strowger type selectors Charging incorporate their own local control which reacts to loop-disconnect signalling over the path concerned. In electronic (common control) systems, security is achieved by APPLICATION PROGRAMS building in redundancy and by the use of inbuilt check circuits and fault detection which will both route calls round a faulty condition Routing machine and also indicate the presence of the fault SO that maintenance action interface