Life-critical Applications Research Articles

Importance Sampling to Evaluate Real-time System Reliability: A Case Study

Real-time distributed computers are often used in life- critical applications. However, the complexity of such systems calls for extensive simulation studies to validate their performance and reliability before a design can be accepted and a prototype constructed. A simulator testbed has been built to model a variety of such systems quickly from a few basic building blocks. Life-critical applications require reliability levels so high that brute-force simula tion to validate these levels would take weeks of computer time. In this paper, we present studies we have conducted into the use of importance sampling in simulating real- time systems. This paper presents a interesting case- study of the use of importance sampling in an increas ingly important branch of computer engineering. Impor tance sampling may not work for all cases and over all parameter ranges. In this paper, we are interested in finding out whether (and how well) this scheme works for the case of distributed real-time systems and also the range of failure bias values for which it works well. Specifically, we look at the implementation of two heuris tics called 'forcing' and failure biasing' in the testbed. This was validated by comparing the reliability estimates with that of normal (very long) simulation. The effect of the failure bias on the dynamics of the scheme is also investigated to provide readers with some guidance on choosing appropriate bias values.

Verification of a safety-critical railway interlocking system with real-time constraints

Ensuring the correctness of computer systems used in life-critical applications is very difficult. The most commonly used verification methods, simulation and testing, are not exhaustive and can miss errors. This work describes an alternative verification technique based on symbolic model checking that can automatically and exhaustively search the state space of the system and verify if properties are satisfied or not. The method also provides useful quantitative timing information about the behavior of the system. We have applied this technique using the Verus tool to a complex safety-critical system designed to control medium and large-size railway stations. We have identified some anomalous behaviors in the model with serious potential consequences in the actual implementation. The fact that errors can be identified before a safety-critical system is deployed in the field not only eliminates sources of very serious problems, but also makes it significantly less expensive to debug the system.

Task-scheduling strategies for reliable TMR controllers using task grouping and assignment

Real-time computers are often used in embedded, life-critical applications where high reliability is important. A common approach to make such systems dependable is to vote on redundant processors executing multiple copies of the same task. The most popular redundant structure is triple modular redundancy (TMR). The processors that make up such systems are subject not only to independently occurring permanent and transient faults, but to correlated transient faults, such as electromagnetic interference (EMI) caused by the operating environment. This paper proposes two new scheduling strategies for TMR computer-controllers. Both strategies can tolerate correlated faults as well as independent faults. These strategies, TMR-R (TMR with rotated task group) and TMR-Q (TMR with quintuple computation), are developed using task grouping and assignment. To evaluate the reliability of these strategies, a discrete-time Markov model for control systems is devised. Reliability equations for the TMR-R and TMR-Q are derived from state transitions of sampling intervals based on the Markov model. The reliability of these TMR is proved by comparing them with a conventional TMR, using numerical analysis. These proposed strategies are anticipated to be useful for control systems operating in harsh environments, such as controllers of airplanes or nuclear power plants.

Safety issues in the comparative analysis of dependable architectures

This paper illustrates the value of analytical techniques for the safety analysis of dependable architectures at the system level. Its important contributions are: (1) comparative analysis of five common hardware architectures for life-critical applications; (2) demonstration of the effect of various coverage parameters on system safety; and (3) illustration of important metrics in evaluating system safety. Discrete space, CTMC (continuous time Markov chains) are used to model the five architectures at the building block level: a simplex architecture; two gracefully degrading architectures with and without repair; and two hard-failing architectures.

Geographical information systems and tropical diseases

Geographical information systems and tropical diseases

Reliability evaluation of a limited-flow network in terms of minimal cutsets

Body Sensor Networks (BSNs) have been developed to provide wearable, real-time health monitoring systems for many life-critical applications that require a high level of reliability. Therefore it is significant to analyze the reliability attribute of BSNs, contributing to their reliable designs and operations. This paper models reliability of BSNs subject to probabilistic competing failures. Specifically, in a BSN system, biomedical sensors sense physiological information that is then transmitted through a relay node to a sink device used by decision makers. When the relay fails, these sensors may be isolated in transmission with certain probabilities, depending on whether the remaining power can enable a long-range, direct transmission to the sink. This isolation effect prevents the system from being compromised by further failures of those sensors. However, biomedical sensors may experience propagated failures. If any of the propagated failures occurs before the relay failure, the entire system can fail. Therefore, there exists a competition in time domain between probabilistic failure isolation and propagation effects. This paper considers such probabilistic competing effects and different statistical relationships between local and propagated failures of sensors in reliability analysis of BSNs. A case study is given to illustrate application and advantages of the proposed combinatorial method.

Reliability of checkpointed real-time systems using time redundancy

Real-time computers are often used in embedded, life-critical applications where high reliability is important. A common approach to making such systems dependable is to vote on redundant processors executing multiple copies of the same task is described. The processors which make up such voted systems are subjected not only to independently occurring permanent and transient failure, but also to correlated transients brought about by electromagnetic interference from the operating environment. To counteract these transients, checkpointing and time redundancy are required, in addition to processor redundancy. This work analyzes the use of time and device redundancy in systems subject to correlated failure. The tradeoffs in checkpoint placement in such a system are found to be considerably different from those for non-redundant systems without real-time constraints. The authors compare fault-tolerant designs and without a rollback capability, accounting for the increased hardware-failure rate due to processor duplication when faults are detected in hardware, and the doubled execution times when detection is implemented in software. >

Estimating the probability of failure when testing reveals no failures

Formulas for estimating the probability of failure when testing reveals no errors are introduced. These formulas incorporate random testing results, information about the input distribution; and prior assumptions about the probability of failure of the software. The formulas are not restricted to equally likely input distributions, and the probability of failure estimate can be adjusted when assumptions about the input distribution change. The formulas are based on a discrete sample space statistical model of software and include Bayesian prior assumptions. Reusable software and software in life-critical applications are particularly appropriate candidates for this type of analysis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A insegurança do computador e a vulnerabilidade social

Muitos têm argumentado que as sociedades industriais estão se tornando cada vez mais dependentes da tecnologia e, conseqüentemente, mais vulneráveis às falhas tecnológicas. Não obstante a difusão da tecnologia computacional, pouco é conhecido a respeito das falhas do computador, exceto, talvez, as que são muito comuns. Este artigo analisa as fontes de insegurança do computador e revê a extensão e o custo dos computadores inconfiáveis. Ao contrário dos articulistas anteriores, os autores argumentam que os computadores digitais são inerentemente não confiáveis por duas razões: primeira, são mais propensos a falhas totais, ao invés de parciais; e segunda, sua enorme complexidade significa que jamais poderão ser completamente testados antes de serem colocados em uso. Os autores descrevem em seguida várias tentativas institucionais para melhorar a confiabilidade, bem como possíveis soluções propostas pelos cientistas da computação, mas concluem que até agora nenhuma foi adequada. Em conseqüência, recomendam que os computadores não devam ser utilizados em aplicações onde haja risco de vida.

Computer unreliability and social vulnerability

Many have argued that industrial societies are becoming more technology-dependent and are thus more vulnerable to technology failures. Despite the pervasiveness of computer technology, little is known about computer failures, except perhaps that they are all too common. This article analyses the sources of computer unreliability and reviews the extent and cost of unreliable computers. Unlike previous writers, the authors argue that digital computers are inherently unreliable for two reasons: first, they are prone to total rather than partial failure; and second, their enormous complexity means that they can never be thoroughly tested before use. The authors then describe various institutional attempts to improve reliability and possible solutions proposed by computer scientists, but they conclude that as yet none is adequate. Accordingly, they recommend that computers should not be used in life-critical applications.

Experiments in software reliability: Life-critical applications

Digital computers are being used more frequently for process control applications in which the cost of system failure is high. Consideration of the potentially life-threatening risk, resulting from the high degree of functionality being ascribed to the software components of these systems, has stimulated the recommendation of various designs for tolerating software faults. The author discusses four reliability data gathering experiments which were conducted using a small sample of programs for two problems having ultrareliability requirements: <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> -version programming for fault detection, and repetitive run modeling for failure and fault rate estimation. The experimental results agree with those of M. Nagel and J.A. Skrivan (1982) in that the program error rates suggest an approximate log-linear pattern and the individual faults occurred with significantly different error rates.

Safety And Reliability In Computer-basedTraffic Managment: A Probabilistic ApproachUsing Petri Nets

Real time computer systems, already employed in several industrial fields, as far as system management concerns, are today applied, thanks to their high flexibility, reliability and compactness, also in the transport area, particularly in life-critical applications, where electromechanical devices are state of the art. In such applications safety and reliability must be the keywords of the problem: safety to preserve human lives from dangerous events and reliability to keep the system operational as far as possible. One of the most flexible approaches is the statistical approach based on the high modelling capabilities of Petri nets. In particular, the stochastic extension of Petri nets is today, probably, the best modelling tool to perform safety and reliability analysis of real time computer systems. In this paper the authors present a safety and reliability analysis, based on a Petri net model, of a life-critical system; a central apparatus for veichle routing and peripheral management for a railway station traffic control, is modelled and statistical results are presented.