Continuous-time Markov chains (CTMCs) have been widely used to determine system performance and dependability characteristics. Their analysis most often concerns the computation of steady-state and transient-state probabilities. This paper introduces a branching temporal logic for expressing real-time probabilistic properties on CTMCs and presents approximate model checking algorithms for this logic. The logic, an extension of the continuous stochastic logic CSL of Aziz et al. (1995, 2000), contains a time-bounded until operator to express probabilistic timing properties over paths as well as an operator to express steady-state probabilities. We show that the model checking problem for this logic reduces to a system of linear equations (for unbounded until and the steady-state operator) and a Volterra integral equation system (for time-bounded until). We then show that the problem of model-checking time-bounded until properties can be reduced to the problem of computing transient state probabilities for CTMCs. This allows the verification of probabilistic timing properties by efficient techniques for transient analysis for CTMCs such as uniformization. Finally, we show that a variant of lumping equivalence (bisimulation), a well-known notion for aggregating CTMCs, preserves the validity of all formulas in the logic.

Model-Checking Algorithms for Continuous-Time Markov Chains

Model checking temporisé

Evaluation de performance de protocoles dans l’internet des objets à base de méthodes formelles

Bien que concus pour repondre a de fortes exigences de fiabilite et de disponibilite malgre un environnement spatial naturel hostile, les systemes spatiaux de teledetection traditionnels, monolithiques et teleoperes depuis le sol, demeurent vulnerables face a un nombre croissant de menaces emergentes issues de l’environnement spatial artificiel (armes antisatellites, debris). Plutot que de chercher a proteger physiquement les satellites, nous proposons d’adopter une strategie fondee sur le concept de resilience, qui traduit la capacite d’un systeme a poursuivre sa mission face a des aleas imprevisibles, fut-ce en mode degrade.En nous appuyant sur de recentes innovations dans les technologies spatiales, nous nous sommes interesses a la conception et a l’evaluation d’architectures systeme fondees sur la mise en reseau de constellations de microsatellites heterogenes, autonomes et communicants.Afin d’etudier de telles architectures, appelees reseaux de constellations autonomes (RCA), nous proposons une approche de modelisation ainsi qu’un outil de simulation a base de reseaux de Petri imbriques. Grâce a des metriques issues des reseaux de telecommunication ainsi que des systemes multiagents, nous avons evalue les RCA au travers de leurs performances operationnelles et de leurs capacites de communication, nominales puis dans divers modes degrades. Du point de vue de la resilience, les resultats presentes mettent en evidence l’interet de disposer de reseaux de communication denses et de modules de reconfiguration autonomes embarques au sein meme des satellites.

/pdf/amelioration-de-la-resilience-de-systemes-spatiaux-soumis-a-284464b7ka.pdf

Amélioration de la résilience de systèmes spatiaux soumis à des menaces : vers des réseaux de satellites autonomes

b this paper we present a case for using new power-aware metn.cs for determining routes in wireless ad hoc networks. We present five ~erent metriw based on battery power consumption at nodw. We show that using th=e metrics in a shortest-cost routing algorithm reduces the cost/packet of routing packets by 5-30% over shortwt-hop routing (this cost reduction is on top of a 40-70% reduction in energy consumption obtained by using PAMAS, our MAC layer prtocol). Furthermore, using these new metrics ensures that the mean time to node failure is increased si~cantly. An interesting property of using shortest-cost routing is that packet delays do not increase. Fintiy, we note that our new metrim can be used in most tradition routing protocols for ad hoc networks.

Power Aware Routing in Mobile Ad Hoc Networks

Nous nous interessons a l'analyse des exigences de performabilite des systemes communicants mobiles par model checking. Nous modelisons ces systemes a l'aide d'un formalisme de haut niveau issu du π-calcul, permettant de considerer des comportements stochastiques, temporels, deterministes, ou indeterministes. Cependant, dans le π-calcul, la primitive de communication de base des systemes est la communication en point-a-point synchrone. Or, les systemes mobiles, qui utilisent des reseaux sans fil, communiquent essentiellement par diffusion locale. C'est pourquoi, dans un premier temps, nous definissons la communication par diffusion dans le π-calcul, afin de mieux modeliser les systemes que nous etudions. Nous proposons d'utiliser des versions probabilistes et stochastiques de l'algebre que nous avons defini, pour permettre des etudes de performance. Nous en definissons une version temporelle permettant de considerer le temps dans les modeles. Mais l'absence d'outils d'analyse des proprietes sur des modeles specifies en une algebre issue du π-calcul est un obstacle majeur a notre travail. La definition de regles de traduction en langage PRISM, nous permet de traduire nos modeles, en modeles de bas niveau supports du model checking, a savoir des chaines de Markov a temps discret, a temps continu, des automates temporises, ou des automates temporises probabilistes. Nous avons choisi l'outil PRISM car, a notre connaissance, dans sa derniere version, il est le seul outil a supporter les formalismes de bas niveau que nous venons de citer, et ainsi il permet de realiser des etudes de performabilite completes. Cette facon de proceder nous permet de pallier a l'absence d'outils d'analyse pour nos modeles. Par la suite, nous appliquons ces concepts theoriques aux reseaux de capteurs sans fil mobiles.

/pdf/approches-formelles-pour-l-analyse-de-la-performabilite-des-48pf7m6dht.pdf

Approches formelles pour l'analyse de la performabilité des systèmes communicants mobiles : Applications aux réseaux de capteurs sans fil

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Approches formelles pour l'analyse de la performabilité des systèmes communicants mobiles : Applications aux réseaux de capteurs sans fil