Report Number: CSL-TR-71-24
Institution: Stanford University, Computer Systems Laboratory
Title: An improved reliability model for NMR
Author: Siewiorek, Daniel P.
Date: December 1971
Abstract: The classical reliability model for N-modular redundancy (NMR) assumes the network to be failed when a majority of modules which drive the same voter fail. It has long been known that this model is pessimistic since there are instances, termed compensating module failures, where a majority of the modules fail but the network is nonfailed. A different module reliability model based on lead reliability is proposed which has the classical NMR reliability model as a special case. It is shown that the standard procedure for altering the classical model to take compensating module failures into account may predict a network reliability which is too low in some cases and too high in others. It is also demonstrated that the improved model can increase the predicted mission time (the time the system is to operate at or above a given reliability) by 50% over the classical model prediction for a simple network.