Giacomo Bucci

Compositional validation of time-critical systems using communicating time Petri nets

An extended Petri net model which considers modular partitioning along with timing restrictions and environment models is presented. Module constructs permit the specification of a complex system as a set of message passing modules with the timing semantics of time Petri nets. The state space of each individual module can be separately enumerated and assessed under the assumption of a partial specification of the intended module operation environment. State spaces of individual modules can be recursively integrated, to permit the assessment of module clusters and of the overall model, and to check the satisfaction of the assumptions made in the separate analysis of elementary component modules. In the intermediate stages between subsequent integration steps, the state spaces of module and module clusters can be projected onto reduced representations concealing local events that are not essential to the purposes of the analysis. The joint use of incremental enumeration and intermediate concealment of local events allows for a flexible management of state explosion, and permits a scalable approach to the validation of complex systems.

Timed state space analysis of real-time preemptive systems

A modeling notation is introduced which extends time Petri nets with an additional mechanism of resource assignment making the progress of timed transitions be dependent on the availability of a set of preemptable resources. The resulting notation, which we call preemptive time Petri nets, permits natural description of complex real-time systems running under preemptive scheduling, with periodic, sporadic, and one-shot processes, with nondeterministic execution times, with semaphore synchronizations and precedence relations deriving from internal task sequentialization and from interprocess communication, running on multiple processors. A state space analysis technique is presented which supports the validation of preemptive time Petri net models, combining tight schedulability analysis with exhaustive verification of the correctness of logical sequencing. The analysis technique partitions the state space in equivalence classes in which timing constraints are represented in the form of difference bounds matrixes. This permits it to maintain a polynomial complexity in the representation and derivation of state classes, but it does not tightly encompass the constraints deriving from preemptive behavior, thus producing an enlarged representation of the state space. False behaviors deriving from the approximation can be cleaned-up through an algorithm which provides a necessary and sufficient condition for the feasibility of a behavior along with a tight estimation of its timing profile.

Oris: a tool for modeling, verification and evaluation of real-time systems

Oris is a tool for qualitative verification and quantitative evaluation of reactive timed systems, which supports modeling and analysis of various classes of timed extensions of Petri Nets. As most characterizing features, Oris implements symbolic state space analysis of preemptive Time Petri Nets, which enable schedulability analysis of real-time systems running under priority preemptive scheduling; and stochastic Time Petri Nets, which enable an integrated approach to qualitative verification and quantitative evaluation. In this paper, we present the current version of the tool and we illustrate its application to two different case studies in the areas of qualitative verification and quantitative evaluation, respectively.

Correctness verification and performance analysis of real-time systems using stochastic preemptive time Petri nets

Time Petri nets describe the state of a timed system through a marking and a set of clocks. If clocks take values in a dense domain, state space analysis must rely on equivalence classes. These support verification of logical sequencing and quantitative timing of events, but they are hard to be enriched with a stochastic characterization of nondeterminism necessary for performance and dependability evaluation. Casting clocks into a discrete domain overcomes the limitation, but raises a number of problems deriving from the intertwined effects of concurrency and timing. We present a discrete-time variant of time Petri nets, called stochastic preemptive time Petri nets, which provides a unified solution for the above problems through the adoption of a maximal step semantics in which the logical location evolves through the concurrent firing of transition sets. We propose an analysis technique, which integrates the enumeration of a succession relation among sets of timed states with the calculus of their probability distribution. This enables a joint approach to the evaluation of performance and dependability indexes as well as to the verification of sequencing and timeliness correctness. Expressive and analysis capabilities of the model are demonstrated with reference to a real-time digital control system.

Modeling flexible real time systems with preemptive time Petri nets

Preemptive Time Petri nets are obtained by extending Time Petri nets with an additional mechanism of resource assignment which makes the progress of timed transitions be dependent on the availability of a set of preemptable resources, and with the capability to make transition times and priorities be dependent on the marking. The combination of these capabilities supports description and verification of flexible real time systems running under preemptive scheduling, with periodic, sporadic and one shot processes, with non-deterministic execution times, with semaphore synchronizations and precedence relations deriving from internal task sequentialization and from interprocess communication. The expressive capabilities of the model and the type of results that can be derived through symbolic enumeration of its dense-timed state space are illustrated with reference to a flexible system mixing dynamic acceptance and performance polymorphism.

Integrating content-based retrieval in a medical image reference database

Image reference databases (IRDBs) are a recurrent research topic in medical imaging. Most IRDBs are designed to help experienced physicians in diagnostic tasks and require that users have prior extensive knowledge of the field for their use to be fruitful. Therefore, the educational potential of such image collections cannot be exploited thoroughly. In this paper we propose an image-indexing method to extend the functionalities of an existing medical IRDB and allow for its use in educational applications, as well as in computer-assisted diagnosis. Our method, based on the Kahrunen-Leève transform, has been used to develop a content-based search engine for tomographic image databases on which we are presently experimenting and which we aim to integrate into a working radiological IRDB installed at the University of Florence. Results achieved in our preliminary tests are also reported.

Introducing probability within state class analysis of dense-time-dependent systems

Several techniques have been proposed for symbolic enumeration and analysis of the state space of reactive systems with nondeterministic temporal parameters taking values within a dense domain. In a large part of these techniques, the state space is covered by collecting states within equivalence classes each comprised of a discrete logical location and a dense variety of clock valuations encoded as a difference bounds matrix (DBM). The reachability relation among such classes enables qualitative verification of properties pertaining the ordering of events along critical runs and the satisfaction of stimulus/response deadlines. However, up to now, no results have been proposed which extend state class enumeration so as to combine the verification of the possibility of critical behaviors with a quantitative evaluation of their probability. In this paper, we extend the concept of equivalence classes based on DBM encoding with a density function which provides a measure for the probability associated with individual states collected in the class itself. To this end, we extend the formalism of time Petri nets by associating the static firing interval of each transition with a probability density function. We then expound how this probabilistic information determines a probability for the states collected within a class and how this probability evolves in the enumeration of the reachability relation among state classes. This opens the way to characterizing the possibility of critical behaviors with a quantitative measure of probability.

ORIS: a tool for state-space analysis of real-time preemptive systems

Formal methods based on state-space enumeration, such as timed automata and time Petri nets (TPN), have been proposed for designing and validating reactive real-time systems. The great expressiveness of these methods is counterbalanced by the increased complexity of the analysis, which may grow exponentially. Furthermore, the enumerated state-space needs to be inspected to identify critical behaviors with respect to sequencing and timing requirements. This naturally leads to the implementation of tools supporting the different stages of the development process. In this paper we present Oris, an environment for building, simulating, analyzing and validating complex real time systems specified in terms of an extended TPN formalism, named preemptive time Petri nets. Oris includes not only the state-space enumeration engine, but also a number of modules which ease user interaction, and make it usable also by a designer with no specific experience in formal modelling.

An object-oriented dual language for specifying reactive systems

Descriptive software specification techniques are based on mathematical formalism and produce precise, rigorous specifications which are in general to be preferred for the design of reactive systems with respect to operational techniques. Recently, dual languages which tend to integrate these aspects have been investigated. An object-oriented specification dual language, named TROL is presented, it consists in an executable formal specification model which can be used for validation of reactive systems. TROL has the capability to describe the system behavior, its functionality and structural aspects. It allows one to describe the system at different levels of structural abstractions and specification details without boundaries among the specification steps. At each specification level, TROL helps the user in the verification of consistency, thus allowing incremental specification. TROL has a visual representation which has been supported by a CASE tool named TOOMS.<<ETX>>

Ontologies and Bayesian Networks in Medical Diagnosis

The amount of information that must be taken into ac count in medical diagnosis is huge and subject to evolution. Ontologies are a means for formalizing the concepts of the domain of interest. Open, interoperable ontologies already exist for the biomedical field, enabling scientists to communicate with minimum ambiguity. Unfortunately, reasoners acting upon ontologies operate in a deterministic manner, which is unsuitable for the medical domain, where uncertainty must also be taken into account. Bayesian networks (BNs) offer a coherent and intuitive representation of uncertain domain knowledge. This paper presents an approach to the use of ontologies and BNs in medical diagnosis. The approach is based on the adoption of predefined structures for the BNs. These lead to reduced extensions to the domain ontology, yet allowing probabilistic analysis.