Improving the efficiency of supervision by software through state aggregation

Software supervision is an approach to the real-time detection of failures. A software supervisor is a unit which monitors both the inputs and outputs of a real-time system and reports discrepancies between observed and expected behaviors. The major diiculty with software supervision is the running-time complexity of the supervisor. A signiicant source of running-time complexity was determined to be the total number of signals at the inputs queues of speciication processes for which no ordering information is known. This paper describes an approach to reduce the running-time complexity of a software supervisor. The approach makes use of a transformed speciication in which several state transitions are grouped into a single transition or aggregate state. This eeectively reduces the number of signals at the input queues of processes. In this paper, the case where the system under supervision is speciied in the ITU Speciication and Description Language (SDL) 6] is considered. An algorithm to translate SDL processes into aggregate-state representations is described. Use of the algorithm is exempliied with the aid of an SDL speciication of a small private branch telephone exchange.