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 here 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. 1

N Version Programming: An Unified Modeling Approach

This paper presents an unified approach aimed at modeling the joint behavior of the N version system and its operational environment. Our objective is to develop reliability model that considers both functional and performance requirements which is particularly important for real – time applications. The model is constructed in two steps. First, the Markov model of N version failure and execution behavior is developed. Next, we develop the user – oriented model of the operational environment. In accounting for dependence we use the idea that the influence of the operational environment on versions failures and execution times induces correlation. The model addresses a number of basic issues and yet yields closed – form solutions that provide considerable insight into how reliability is affected by both versions characteristics and the operational environment. 1

Hierarchical decomposition for estimating reliability of fault-tolerant software in mission-critical systems

This paper presents a hierarchical modeling approach aimed at reliability assessment over a mission period of the software fault tolerance technique based on N version programming. The model is constructed in three layers wherein submodels represent different parts of an object system and time scale distinctions. Our modeling approach is systematic as opposed to the ad hoc methods used in related works. Moreover, it permits modifications to be flexibly made at a specific level or in a specific submodel. Thus, the work presented here generalizes our previous work as it allows to consider general distributions of the versions time to failure and execution time at the first level. Also, at the third level, instead of the performability model in our previous work, we develop a new model aimed at reliability assessment over a mission period which supports the evaluation of a reliability over mission period in terms of the functional and timeliness requirements