Report Number: CSL-TR-74-75
Institution: Stanford University, Computer Systems Laboratory
Title: Influence of fault-detection and switchinig mechanisms on the reliability of stand-by systems
Author: Losq, Jacques
Date: July 1975
Abstract: This paper concerns the reliability of stand-by systems when switch reliability is taken into account. It is assumed that failures obey a Poisson distribution for modules and switches. A very detailed method is given to model stand-by systems. Several cases are investigated: ideal systems, real systems with fault-detection mechanisms that can detect any module error and systems for which the fault-detection mechanisms detect only some of the module errors. The reliability versus time curves are determined for each value of the number of spares. It is shown that the best number of spares increases as the length of the mission increases. Systems with extremely short mission time have the best reliability when they have only one spare. The limit when the number of spares increases is the reliability obtained with simplex systems. Whatever the number of spares is, the reliability of stand-by systems goes to zero as time goes to infinity. For a given mission time, it is possible to determine the best number of spares and the best possible reliability. For a given reliability, it is possible to compute the number of spares that gives the longest mission time. These models can be used to determine whether or not there exists a stand-by system that meets the requirements of a given reliability and a given mission time. If such stand-by system exists, its characteristics (minimum number of spares and reliability) can be derived.