Performability and reliability modeling of N version fault tolerant software in real time systems

The paper presents a hierarchical modeling approach of the N version programming in a real time environment. The model is constructed in three layers. At the first layer we distinguish the NVP structure from its operational environment. The NVP structure submodel considers both failures of functionality and failures of performance. The operational environment submodel is based on the concept of the operational profile. The second layer consists of a per run reliability and performance submodels. The first considers per run failure probabilities, while the second is responsible for modeling the series of successive runs over a mission. The information contributed by the second layer constitutes third layer models which support the evaluation of a performability and reliability over mission. The work presented, generalizes our previous work as it considers general distributions of the versions´ time to failure and execution time. Also, in addition to the performability model, the third layer includes a model aimed at reliability assessment over a mission period.