A contribution to the evaluation and optimization of networks reliability

L’évaluation de la fiabilité des réseaux est un problème combinatoire très complexe qui nécessite des moyens de calcul très puissants. Plusieurs méthodes ont été proposées dans la littérature pour apporter des solutions. Certaines ont été programmées dont notamment les méthodes d’énumération des ensembles minimaux et la factorisation, et d’autres sont restées à l’état de simples théories. Cette thèse traite le cas de l’évaluation et l’optimisation de la fiabilité des réseaux. Plusieurs problèmes ont été abordés dont notamment la mise au point d’une méthodologie pour la modélisation des réseaux en vue de l’évaluation de leur fiabilités. Cette méthodologie a été validée dans le cadre d’un réseau de radio communication étendu implanté récemment pour couvrir les besoins de toute la province québécoise. Plusieurs algorithmes ont aussi été établis pour générer les chemins et les coupes minimales pour un réseau donné. La génération des chemins et des coupes constitue une contribution importante dans le processus d’évaluation et d’optimisation de la fiabilité. Ces algorithmes ont permis de traiter de manière rapide et efficace plusieurs réseaux tests ainsi que le réseau de radio communication provincial. Ils ont été par la suite exploités pour évaluer la fiabilité grâce à une méthode basée sur les diagrammes de décision binaire. Plusieurs contributions théoriques ont aussi permis de mettre en place une solution exacte de la fiabilité des réseaux stochastiques imparfaits dans le cadre des méthodes de factorisation. A partir de cette recherche plusieurs outils ont été programmés pour évaluer et optimiser la fiabilité des réseaux. Les résultats obtenus montrent clairement un gain significatif en temps d’exécution et en espace de mémoire utilisé par rapport à beaucoup d’autres implémentations. Mots-clés: Fiabilité, réseaux, optimisation, diagrammes de décision binaire, ensembles des chemins et coupes minimales, algorithmes, indicateur de Birnbaum, systèmes de radio télécommunication, programmes.Efficient computation of systems reliability is required in many sensitive networks. Despite the increased efficiency of computers and the proliferation of algorithms, the problem of finding good and quickly solutions in the case of large systems remains open. Recently, efficient computation techniques have been recognized as significant advances to solve the problem during a reasonable period of time. However, they are applicable to a special category of networks and more efforts still necessary to generalize a unified method giving exact solution. Assessing the reliability of networks is a very complex combinatorial problem which requires powerful computing resources. Several methods have been proposed in the literature. Some have been implemented including minimal sets enumeration and factoring methods, and others remained as simple theories. This thesis treats the case of networks reliability evaluation and optimization. Several issues were discussed including the development of a methodology for modeling networks and evaluating their reliabilities. This methodology was validated as part of a radio communication network project. In this work, some algorithms have been developed to generate minimal paths and cuts for a given network. The generation of paths and cuts is an important contribution in the process of networks reliability and optimization. These algorithms have been subsequently used to assess reliability by a method based on binary decision diagrams. Several theoretical contributions have been proposed and helped to establish an exact solution of the stochastic networks reliability in which edges and nodes are subject to failure using factoring decomposition theorem. From this research activity, several tools have been implemented and results clearly show a significant gain in time execution and memory space used by comparison to many other implementations. Key-words: Reliability, Networks, optimization, binary decision diagrams, minimal paths set and cuts set, algorithms, Birnbaum performance index, Networks, radio-telecommunication systems, programs

EVMDD-based analysis and diagnosis methods of multi-state systems with multi-state components

A multi-state system with multi-state components is a model of systems, where performance, capacity, or reliability levels of the systems are represented as states. It usually has more than two states, and thus can be considered as a multi-valued function, called a structure function. Since many structure functions are monotone increasing, their multi-state systems can be represented compactly by edge-valued multi-valued decision diagrams (EVMDDs). This paper presents an analysis method of multi-state systems with multi-state components using EVMDDs. Experimental results show that, by using EVMDDs, structure functions can be represented more compactly than existing methods using ordinary MDDs. Further, EVMDDs yield comparable computation time for system analysis. This paper also proposes a new diagnosis method using EVMDDs, and shows that the proposed method can infer the most probable causes for system failures more efficiently than conventional methods based on Bayesian networks.Japan Society for the Promotion of ScienceMinistry of Education, Culture, Sports, Science and Technology (MEXT)Hiroshima City UniversityGrant-in Aid No. 2500050 (MEXT)Grant no. 0206 (HCU)Grant in Aid for Scientific Research (JSPS

An overview of decision table literature 1982-1995.

This report gives an overview of the literature on decision tables over the past 15 years. As much as possible, for each reference, an author supplied abstract, a number of keywords and a classification are provided. In some cases own comments are added. The purpose of these comments is to show where, how and why decision tables are used. The literature is classified according to application area, theoretical versus practical character, year of publication, country or origin (not necessarily country of publication) and the language of the document. After a description of the scope of the interview, classification results and the classification by topic are presented. The main body of the paper is the ordered list of publications with abstract, classification and comments.

Some extensions to reliability modeling and optimization of networked systems

Ph.DDOCTOR OF PHILOSOPH

EVMDD-Based Analysis and Diagnosis Methods of Multi-State Systems with Multi-State Components *

A multi-state system with multi-state components is a model of systems, where performance, capacity, or reliability levels of the systems are represented as states. It usually has more than two states, and thus can be considered as a multi-valued function, called a structure function. Since many structure functions are monotone increasing, their multi-state systems can be represented compactly by edge-valued multivalued decision diagrams (EVMDDs). This paper presents an analysis method of multi-state systems with multi-state components using EVMDDs. Experimental results show that, by using EVMDDs, structure functions can be represented more compactly than existing methods using ordinary MDDs. Further, EVMDDs yield comparable computation time for system analysis. This paper also proposes a new diagnosis method using EVMDDs, and shows that the proposed method can infer the most probable causes for system failures more efficiently than conventional methods based on Bayesian networks

Inclusion-exclusion principle for belief functions

International audienceThe inclusion-exclusion principle is a well-known property in probability theory, and is instrumental in some computational problems such as the evaluation of system reliability or the calculation of the probability of a Boolean formula in diagnosis. However, in the setting of uncertainty theories more general than probability theory, this principle no longer holds in general. It is therefore useful to know for which families of events it continues to hold. This paper investigates this question in the setting of belief functions. After exhibiting original sufficient and necessary conditions for the principle to hold, we illustrate its use on the uncertainty analysis of Boolean and non-Boolean systems in reliability

Addressing Complexity and Intelligence in Systems Dependability Evaluation

Engineering and computing systems are increasingly complex, intelligent, and open adaptive. When it comes to the dependability evaluation of such systems, there are certain challenges posed by the characteristics of “complexity” and “intelligence”. The first aspect of complexity is the dependability modelling of large systems with many interconnected components and dynamic behaviours such as Priority, Sequencing and Repairs. To address this, the thesis proposes a novel hierarchical solution to dynamic fault tree analysis using Semi-Markov Processes. A second aspect of complexity is the environmental conditions that may impact dependability and their modelling. For instance, weather and logistics can influence maintenance actions and hence dependability of an offshore wind farm. The thesis proposes a semi-Markov-based maintenance model called “Butterfly Maintenance Model (BMM)” to model this complexity and accommodate it in dependability evaluation. A third aspect of complexity is the open nature of system of systems like swarms of drones which makes complete design-time dependability analysis infeasible. To address this aspect, the thesis proposes a dynamic dependability evaluation method using Fault Trees and Markov-Models at runtime.The challenge of “intelligence” arises because Machine Learning (ML) components do not exhibit programmed behaviour; their behaviour is learned from data. However, in traditional dependability analysis, systems are assumed to be programmed or designed. When a system has learned from data, then a distributional shift of operational data from training data may cause ML to behave incorrectly, e.g., misclassify objects. To address this, a new approach called SafeML is developed that uses statistical distance measures for monitoring the performance of ML against such distributional shifts. The thesis develops the proposed models, and evaluates them on case studies, highlighting improvements to the state-of-the-art, limitations and future work

Computing system reliability modeling, analysis, and optimization

Ph.DDOCTOR OF PHILOSOPH

Conceptual Models for Assessment & Assurance of Dependability, Security and Privacy in the Eternal CONNECTed World

This is the first deliverable of WP5, which covers Conceptual Models for Assessment & Assurance of Dependability, Security and Privacy in the Eternal CONNECTed World. As described in the project DOW, in this document we cover the following topics: • Metrics definition • Identification of limitations of current V&V approaches and exploration of extensions/refinements/ new developments • Identification of security, privacy and trust models WP5 focus is on dependability concerning the peculiar aspects of the project, i.e., the threats deriving from on-the-fly synthesis of CONNECTors. We explore appropriate means for assessing/guaranteeing that the CONNECTed System yields acceptable levels for non-functional properties, such as reliability (e.g., the CONNECTor will ensure continued communication without interruption), security and privacy (e.g., the transactions do not disclose confidential data), trust (e.g., Networked Systems are put in communication only with parties they trust). After defining a conceptual framework for metrics definition, we present the approaches to dependability in CONNECT, which cover: i) Model-based V&V, ii) Security enforcement and iii) Trust management. The approaches are centered around monitoring, to allow for on-line analysis. Monitoring is performed alongside the functionalities of the CONNECTed System and is used to detect conditions that are deemed relevant by its clients (i.e., the other CONNECT Enablers). A unified lifecycle encompassing dependability analysis, security enforcement and trust management is outlined, spanning over discovery time, synthesis time and execution time

Hazard elimination using backwards reachability techniques in discrete and hybrid models

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, February 2002.Includes bibliographical references (leaves 173-181).One of the most important steps in hazard analysis is determining whether a particular design can reach a hazardous state and, if it could, how to change the design to ensure that it does not. In most cases, this is done through testing or simulation or even less rigorous processes--none of which provide much confidence for complex systems. Because state spaces for software can be enormous (which is why testing is not an effective way to accomplish the goal), the innovative Hazard Automaton Reduction Algorithm (HARA) involves starting at a hypothetical unsafe state and using backwards reachability techniques to obtain enough information to determine how to design in order to ensure that state cannot be reached. State machine models are very powerful, but also present greater challenges in terms of reachability, including the backwards reachability needed to implement the Hazard Automaton Reduction Algorithm. The key to solving the backwards reachability problem lies in converting the state machine model into a controls state space formulation and creating a state transition matrix. Each successive step backward from the hazardous state then involves only one n by n matrix manipulation. Therefore, only a finite number of matrix manipulations is necessary to determine whether or not a state is reachable from another state, thus providing the same information that could be obtained from a complete backwards reachability graph of the state machine model. Unlike model checking, the computational cost does not increase as greatly with the number of backward states that need to be visited to obtain the information necessary to ensure that the design is safe or to redesign it to be safe. The functionality and optimality of this approach is proved in both discrete and hybrid cases.(cont.) The new approach of the Hazard Automaton Reduction Algorithm combined with backwards reachability controls techniques was demonstrated on a blackbox model of a real aircraft altitude switch. The algorithm is being implemented in a commercial specification language (SpecTRM-RL). SpecTRM-RL is formally extended to include continuous and hybrid models. An analysis of the safety of a medium term conflict detection algorithm (MTCD) for aircraft, that is being developed and tested by Eurocontrol for use in European Air Traffic Control, is performed. Attempts to validate such conflict detection algorithms is currently challenging researchers world wide. Model checking is unsatisfactory in general for this problem because of the lack of a termination guarantee in backwards reachability using model checking. The new state-space controls approach does not encounter this problem.by Natasha Anita Neogi.Ph.D