Reliability Of Fault-tolerant Systems Research Articles

Reasoning about the Reliability of Diverse Two-Channel Systems in Which One Channel Is "Possibly Perfect"

This paper refines and extends an earlier one by the first author [1]. It considers the problem of reasoning about the reliability of fault-tolerant systems with two “channels” (i.e., components) of which one, A, because it is conventionally engineered and presumed to contain faults, supports only a claim of reliability, while the other, B, by virtue of extreme simplicity and extensive analysis, supports a plausible claim of “perfection.” We begin with the case where either channel can bring the system to a safe state. The reasoning about system probability of failure on demand (pfd) is divided into two steps. The first concerns aleatory uncertainty about 1) whether channel A will fail on a randomly selected demand and 2) whether channel B is imperfect. It is shown that, conditional upon knowing pA (the probability that A fails on a randomly selected demand) and pB (the probability that channel B is imperfect), a conservative bound on the probability that the system fails on a randomly selected demand is simply pA X pB. That is, there is conditional independence between the events “A fails” and “B is imperfect.” The second step of the reasoning involves epistemic uncertainty, represented by assessors' beliefs about the distribution of (pA, pB), and it is here that dependence may arise. However, we show that under quite plausible assumptions, a conservative bound on system pfd can be constructed from point estimates for just three parameters. We discuss the feasibility of establishing credible estimates for these parameters. We extend our analysis from faults of omission to those of commission, and then combine these to yield an analysis for monitored architectures of a kind proposed for aircraft.

An integrated methodology for the dynamic performance and reliability evaluation of fault-tolerant systems

We propose an integrated methodology for the reliability and dynamic performance analysis of fault-tolerant systems. This methodology uses a behavioral model of the system dynamics, similar to the ones used by control engineers to design the control system, but also incorporates artifacts to model the failure behavior of each component. These artifacts include component failure modes (and associated failure rates) and how those failure modes affect the dynamic behavior of the component. The methodology bases the system evaluation on the analysis of the dynamics of the different configurations the system can reach after component failures occur. For each of the possible system configurations, a performance evaluation of its dynamic behavior is carried out to check whether its properties, e.g., accuracy, overshoot, or settling time, which are called performance metrics, meet system requirements. Markov chains are used to model the stochastic process associated with the different configurations that a system can adopt when failures occur. This methodology not only enables an integrated framework for evaluating dynamic performance and reliability of fault-tolerant systems, but also enables a method for guiding the system design process, and further optimization. To illustrate the methodology, we present a case-study of a lateral-directional flight control system for a fighter aircraft.

Reliability of fault-tolerant systems with parallel task processing

The paper considers performance and reliability of fault-tolerant software running on a hardware system that consists of multiple processing units. The software consists of functionally equivalent but independently developed versions that start execution simultaneously. The computational complexity and reliability of different versions are different. The system completes the task execution when the outputs of a pre-specified number of versions coincide. The processing units are characterized by different availability and processing speed. It is assumed that they are able to share the computational burden perfectly and that execution of each version can be fully parallelized. The algorithm based on the universal generating function technique is used for determining the distribution of system task execution time. This algorithm allows analysts to evaluate complex hardware–software reliability and performance indices such as expected task execution time and probability that the task is completed within a given time. Illustrative examples are also presented.

Reliability analysis based on binary decision diagrams

A binary decision diagram (BDD) is a representation of Boolean functions that uses the notion of two‐way branching. It has long been used in the synthesis, simulation and testing of Boolean circuits, and has recently been adopted to solve fault tree models for both quantitative and qualitative reliability analyses. In this paper, the concept of binary decision diagram is first introduced. Then, a new method is proposed to analyze the reliability of fault tolerant systems using binary decision diagrams. Traditionally, such analyses are tackled by using fault trees based on cutsets. For complex models, an algorithm based on binary decision diagrams can shorten solution time dramatically. Experimental results are also presented to demonstrate the practicality and benefits of applying the proposed method in reliability analysis.

Analysis of fault tolerant systems operating under different stress levels during a mission

Derives expressions for performance availability and performance reliability of fault tolerant systems operating under various levels of stress during a mission using stochastic reward models (SRM). Develops a mathematical model using semi‐regenerative reward model. Obtains a system of convolution integral equations satisfied by various state probabilities corresponding to different initial conditions. As a particular case, analyses a 3‐unit parallel system under three different stress levels. Uses an iterative numerical method to solve the system of integral equations satisfied by various state probabilities to study the effect of parameters on various system performance.

Calculation of Coverage Parameter

Programs for calculating the reliability of fault-tolerant systems do not explicitly take into account the effect of faults in the hardware recovery mechanism. This paper shows via an example how to incorporate these failures into the fault-handling (coverage) model of CARE III. A simple fault-tolerant system is described. The required coverage parameters are determined and the reliability is calculated using the models in CARE III.