Abstract not found

We consider probabilistic timed automata of [13], an extension of the timed automata model of [2] with discrete probability distributions. In contrast to timed automata, which model real-time systems purely in terms of nondeterminism, our model allows to express the likelihood of the system making certain transitions, and is thus appropriate for modelling fault-tolerance and probabilistic failures. We present a symbolic model checking algorithm for the existential fragment of the logic PTCTL of [13] based on backward reachability as in [12]. The logic allows us to specify properties such as \with probability 0.99 or greater, it is possible to correctly deliver a data packet within 5 time units", or \with probability 0.87 or greater, the system never enters an error state".

Abstract. We consider the problem of automatically verifying realtime systems with continuously distributed random delays. We generalise probabilistic timed automata introduced in [19], an extension of the timed automata model of [4], with clock resets made according to continuous probability distributions. Thus, our model exhibits nondeterministic and probabilistic choice, the latter being made according to both discrete and continuous probability distributions. To facilitate algorithmic verification, we modify the standard region graph construction by subdividing the unit intervals in order to approximate the probability to within an interval. We then develop a model checking method for continuous probabilistic timed automata, taking as our specification language Probabilistic Timed Computation Tree Logic (PTCTL). Our method improves on the previously known techniques in that it allows the verification of quantitative probability bounds, as opposed to qualitative properties which can only refer to bounds of probability 0 or 1. 1

The increasing dependence of businesses on distributed architectures and computer networking places heavy demands on the speed and reliability of data exchange, leading to the emergence of sophisticated protocols which involve both real-time and randomization, for example FireWire IEEE1394. Automatic verification techniques such as model checking have been adapted to this class of probabilistic, timed systems [1, 9, 3, 14]. This abstract considers an application of such techniques to the IEEE1394 (FireWire) root contention protocol, in which the interplay between timed and probabilistic aspects is used to break the symmetry which may arise during the leader election process. Here, the properties of interest concern the election of a leader within a certain deadline, with a certain probability or greater. Our specification formalism is that of probabilistic timed automata [14], a variant of timed automa...

We report on the automatic veri cation of timed probabilistic properties of the IEEE 1394 root contention protocol combining two existing tools: the real-time modelchecker Kronos and the probabilistic model-checker Prism. The system is modelled as a probabilistic timed automaton. We rst use Kronos to perform a symbolic forward reachability analysis to generate the set of states that are reachable with non-zero probability from the initial state, and before the deadline expires. We then encode this information as a Markov decision process to be analyzed with Prism. We apply this technique to compute the minimal probability of a leader being elected before a deadline, for dierent deadlines, and study the inuence of using a biased coin on this minimal probability.

Abstract not found

We study the maximal reachability probability problem for infinite-state systems featuring both nondeterministic and probabilistic choice. The problem involves the computation of the maximal probability of reaching a given set of states, and underlies decision procedures for the automatic verification of probabilistic systems. We extend the framework of symbolic transition systems, which equips an infinite-state system with an algebra of symbolic operators on its state space, with a symbolic encoding of probabilistic transitions to obtain a model for an infinite-state probabilistic system called a symbolic probabilistic system.

The IEEE 1394 Root Contention Protocol is an industrial leader election algorithm for two processes in which probability, real{time and parameters play an important role. This protocol has been analysed in various case studies, using a variety of veri cation and analysis methods. In this paper, we survey and compare several of these case studies.

Abstract. This paper describes work in progess performed as part of an ongoing project aimed at the development of theoretical foundations and model checking algorithms for the verification of soft deadlines in timed systems, that is, properties such as “there is a 90 % chance that the message will be delivered within 5 time units”. The research is focussed on the probabilistic timed automata model [11], an extension of timed automata [3], and includes: model checking of discrete-probabilistic automata based on the region graph construction [11]; symbolic methods based on forwards and backwards reachability [10,13]; and the continuous probabilistic timed automata [12]. 1

this document is its second year report.