Sistemas Informáticos y Computación

The term Ubiquitous Computing was coined by Mark Weiser almost two decades ago. Despite all the time that has passed since Weiser's vision, ubiquitous computing still has a long way ahead to become a pervasive reality. One of the reasons for this may be the lack of widely accepted formal models capable of capturing and analyzing the complexity of the

Abstract. Bisimulation semantics are a very pleasant way to define the semantics of systems, mainly because the simplicity of their definitions and their nice coalgebraic properties. However, they also have some dis-advantages: they are based on a sequential ...

(edition of Annex A) 4 A formal definition of the notion of upward compatibility is given in section 2.5.8 Definition of Ci Ci(tm,stop) = true Ci(tm,i{t in d-..d + };P) = tm d + Ci(tm,gd 1 …d n {t in d-..d + }[SP];P) = true Ci(tm,exit(d 1 ,…d n){t in d-..d + }) = true Ci(tm,Wait(d);P) = { Ci(tm-d,P) if tm>d true

We prove several decidability and undecidability results for ν-PN, an exten-sion of P/T nets with pure name creation and name management. We give a simple proof of undecidability of reachability, by reducing reachability in nets with inhibitor arcs to it. Thus, the expressive ...

Bisimulation can be defined in a simple way using coinductive methods, and has rather pleasant properties. Ready similarity was proposed by Meyer et al. as a way to weakening the bisimulation equivalence thus getting a semantics defined in a similar way, but supported for more reasonable (weaker) observational properties. Global bisimulations were introduced by Frutos et al. in order to study different variants of non-determinism getting, in particular, a semantics under which the internal choice operator becomes associative. Global bisimulations are defined as plain bisimulations but allowing the use of new moves, called global transitions, that can change the processes not only locally in its head, but anywhere. Now we are continuing the study of global bisimulation but focusing on the way different semantics can be characterised as global bisimulation semantics. In particular, we have studied ready similarity, on the one hand because it was proposed as the strongest reasonable semantics weaker than bisimulation; on the other hand, because ready similarity was not directly defined as an equivalence relation but as the nucleus of an order relation, and this open the question whether it is also possible to define it as a symmetric bisimulation-like semantics. We have got a simple and elegant characterisation of ready similarity as a global bisimulation semantics, that provides a direct symmetric characterisation of it as an equivalence relation, without using any order as intermediate concept. Besides, we have found that it is not necessary to start from a simulation based semantics to get an equivalent global bisimulation. What has proved to be very useful is the axiomatic characterisation of the semantics. Following these ideas we have got also global bisimulation for several semantics, including refusals and traces. That provides a general framework that allows to relate both intensional and extensional semantics.

Abstract. Covariant-contravariant simulation and conformance simulation generalize plain simulation and try to capture the fact that it is not always the case that “the larger the number of behaviors, the better”. We have previously studied some of their properties, showing ...

In recent papers we have introduced Mobile Synchronizing Petri Nets, a new model for mobility based on coloured Petri Nets. It allows the description of systems composed of a collection of (possibly mobile) hardware devices and mobile agents, both modelled in a homogenous way and abstracting from middleware details. Our basic model introduced a colour to describe localities, but still lacked appropriate primitives to deal with security, and in fact it was equivalent to P/T nets. Then, we introduced the primitives to cope with security: a new colour for identifiers, basically corresponding to the natural numbers, that are created by means of a special transition. This mechanism allows us to deal with authentication issues. In this paper we discuss the expressiveness of the extended model with the authentication primitives. More specifically, we study several instances of the classical reachability and coverability problems. Finally, we also study a more abstract version of the mechanism to create identifiers, using abstract names, close to those in the πcalculus or the Ambient Calculus. We have proved that both models are strictly in between P/T nets and Turing machines.

There are two ways to define a semantics for process algebras: either directly by means of an equivalence relation or by means of a preorder whose kernel is the desired equivalence. We are interested in the relationship between these two presentations. Using our characterisation of the behaviour preorders by means of simulations up-to we were able to generate the canonical preorders corresponding to each behaviour equivalence. The axiomatizations of these preorders can be obtained by adding to the axioms of the equivalence that of the appropriate simulation. Aceto, Fokkink and Ingólfsdóttir have presented an algorithm that goes in the opposite direction, constructing an axiomatization of the induced equivalence from that of a given preorder. Following a different path we were able to get a correct proof and an enhanced algorithm. In this paper we present an shorter and simpler proof of this result, based on our coinductive characterisations of the behaviour preorders, and in particular in the existence of the canonical preorders. More important, we also present further generalisations of the result, since our coinductive characterisations are not only valid for the semantics coarser than the ready simulation. By means of these new proofs and results we hope to contribute to a better knowledge of the semantics of processes and to better understand the tight relations between preorders and equivalences that define them.

Both the Ambient Calculus by L. Cardelli and the Elementary Object Systems by R. Valk model the behaviour of mobile systems. The Ambient Calculus is based on the concept of ambient, which is an environment with a given name that is delimited by a boundary, where some internal processes are executed. The main property of these ambients is that they can be moved to a new location thus modeling mobility. Elementary Object Systems are two-level net systems composed of a system net and one or more object nets, which can be seen as high-level token objects of the system net modeling the execution of mobile processes. This paper intends to contribute to the relationship between both frameworks by defining a multilevel extension of Elementary Object Systems, which will be used to provide a denotational semantics of a new process algebra called APBC (Ambient Petri Box Calculus). Such process algebra is an extension of the Petri Box Calculus that includes both ambients and their mobility capabilities, which conversely can be also interpreted as an extension of the Ambient Calculus with the main operations from the PBC.

Nested simulations define an interesting hierarchy of semantic preorders and equivalences in which every semantics refines the previous one and it is refined by the following. This nested nature provides a fruitful framework for the study of the formal meaning and the properties of concurrent processes. In this paper we present the notion of constrained simulation that, although rather simple, allows us to find general results for a wide family of semantics. In particular, we provide an axiomatization for both the preorder and the equivalence induced by any constrained simulation. Nested simulations are constrained simulations and therefore our results can be instantiated directly to them. Besides, constrained simulations suggest the definition of a new family of semantics, generalised nested simulation semantics, constructed over the base of any order relation, instead of plain simulation. Finally, we conclude the study of the (generalised) nested semantics defining a generalisation of bisimulation relations, counting bisimulation, that allows us to define a characterisation of nested semantics in terms of a bisimulation-like game.

TIC is a timed algebraic calculus which combines ideas from asynchronous and synchronous calculi Time is introduced by assigning explicit time restrictions to the events of an asynchronous calculus The semantics is defined in an operational way. Interleaving of behaviours is defined-in.such a way that a ptop"i metg" of events in time is achieved. Weak timed bisimulation is also dehned. Examples are presented to show the applicability of the calculus to the study of timed behaviours. * This work was partially supporled by CICYT u¡der the TIC program (MEDAS p¡oject) Correspondence nia o¡print ,"qu"rt, to, Juan Quemada.

Bisimulations up-to Simulations up-to Canonical preorder Linear time-branching time spectrum We define (bi)simulations up-to a preorder and show how we can use them to provide a coinductive, (bi)simulation-like, characterisation of semantic (equivalences) preorders for processes. In particular, we can apply our results to all the semantics in the linear timebranching time spectrum that are defined by preorders coarser than the ready simulation preorder. The relation between bisimulations up-to and simulations up-to allows us to find some new relations between the equivalences that define the semantics and the corresponding preorders. In particular, we have shown that the simulation up-to an equivalence relation is a canonical preorder whose kernel is the given equivalence relation. Since all of these canonical preorders are defined in an homogeneous way, we can prove properties for them in a generic way. As an illustrative example of this technique, we generate an axiomatic characterisation of each of these canonical preorders, that is obtained simply by adding a single axiom to the axiomatization of the original equivalence relation. Thus we provide an alternative axiomatization for any axiomatizable preorder in the linear time-branching time spectrum, whose correctness and completeness can be proved once and for all. Although we first prove, by induction, our results for finite processes, then we see, by using continuity arguments, that they are also valid for infinite (finitary) processes.

Van Glabbeek's linear time-branching time spectrum is one of the most relevant work on comparative study on process semantics, in which semantics are partially ordered by their discrimination power. In this paper we bring forward a refinement of this classification and show how the process semantics can be dealt with in a uniform way: based on the very natural concept of constrained simulation we show how we can classify the spectrum in layers; for the families lying in the same layer we show how to obtain in a generic way equational, observational, logical and operational characterizations; relations among layers are also very natural and differences just stem from the constraint imposed on the simulations that rule the layers. Our methodology also shows how to achieve a uniform treatment of semantic preorders and equivalences.

This paper studies the relationships between three notions of behavioural preorder that have been proposed in the literature: refinement over modal transition systems, and the covariant-contravariant simulation and the partial bisimulation preorders over labelled transition systems. It is shown that there are mutual translations between modal transition systems and labelled transition systems that preserve, and reflect, refinement and the covariant-contravariant simulation preorder. The translations are also shown to preserve the modal properties that can be expressed in the logics that characterize those preorders. A translation from labelled transition systems modulo the partial bisimulation preorder into the same model modulo the covariant-contravariant simulation preorder is also offered, together with some evidence that the former model is less expressive than the latter. In order to gain more insight into the relationships between modal transition systems modulo refinement and labelled transition systems modulo the covariant-contravariant simulation preorder, their connections are also phrased and studied in the context of institutions.

We prove several decidability and undecidability results for ν-PN, an extension of P/T nets with pure name creation and name management. We give a simple proof of undecidability of reachability, by reducing reachability in nets with inhibitor arcs to it. Thus, the expressive power of ν-PN strictly surpasses that of P/T nets. We encode ν-PN into Petri Data Nets, so that coverability, termination and boundedness are decidable. Moreover, we obtain Ackermann-hardness results for all our decidable decision problems. Then we consider two properties, width-boundedness and depth-boundedness, that factorize boundedness. Width-boundedness has already been proven to be decidable. Here we prove that its complexity is also non primitive recursive. Then we prove undecidability of depth-boundedness. Finally, we prove that the corresponding "place version" of all the boundedness problems are undecidable for ν-PN. These results carry over to Petri Data Nets.

This paper is devoted to the study of the (in)equational theory of the largest (pre)congruences over the language BCCSP induced by variations on the classic simulation preorder and equivalence that abstract from internal steps in process behaviours. In particular, the article focuses on the (pre)congruences associated with the weak simulation, the weak complete simulation and the weak ready simulation preorders. For each of these behavioural semantics, results on the (non)existence of finite (ground-)complete (in)equational axiomatizations are given. The axiomatization of those semantics using conditional equations is also discussed in some detail.

Covariant-contravariant simulation and conformance simulation are two generalizations of the simple notion of simulation which aim at capturing the fact that it is not always the case that "the larger the number of behaviors, the better". Therefore, they can be considered to be more adequate to express the fact that a system is a correct implementation of some specification. We have previously shown that these two more elaborated notions fit well within the categorical framework developed to study the notion of simulation in a generic way. Now we show that their behaviors have also simple and natural logical characterizations, though more elaborated than those for the plain simulation semantics.

Recently, we have presented operational and denotational definitions for distances between processes corresponding to any semantics in the ltbt-spectrum. In this paper, we develop a general algebraic framework to define distances between terms from any arbitrary signature. We apply this framework obtaining a new algebraic characterization of our previous distances. Moreover, we prove the generality of our approach developing an algebraic characterization of the distances based on the (bi)simulation game by other authors.

Covariant-contravariant simulation and conformance simulation generalize plain simulation and try to capture the fact that it is not always the case that "the larger the number of behaviors, the better". We have previously studied their logical characterizations and in this paper we present the axiomatizations of the preorders defined by the new simulation relations and their induced equivalences. The interest of our results lies in the fact that the axiomatizations help us to know the new simulations better, understanding in particular the role of the contravariant characteristics and their interplay with the covariant ones; moreover, the axiomatizations provide us with a powerful tool to (algebraically) prove results of the corresponding semantics. But we also consider our results interesting from a metatheoretical point of view: the fact that the covariant-contravariant simulation equivalence is indeed ground axiomatizable when there is no action that exhibits both a covariant and a contravariant behaviour, but becomes non-axiomatizable whenever we have together actions of that kind and either covariant or contravariant actions, offers us a new subtle example of the narrow border separating axiomatizable and non-axiomatizable semantics. We expect that by studying these examples we will be able to develop a general theory separating axiomatizable and non-axiomatizable semantics.