Real-time Discrete Event Systems Research Articles

Intrusion detection system for PS-Poll DoS attack in 802.11 networks using real time discrete event system

Wi-Fi devices have limited battery life because of which conserving battery life is imperative. The 802.11 Wi-Fi standard provides power management feature that allows stations U+0028 STAs U+0029 to enter into sleep state to preserve energy without any frame losses. After the STA wakes up, it sends a null data or PS-Poll frame to retrieve frame U+0028 s U+0029 buffered by the access point U+0028 AP U+0029, if any during its sleep period. An attacker can launch a power save denial of service U+0028 PS-DoS U+0029 attack on the sleeping STA U+0028 s U+0029 by transmitting a spoofed null data or PS-Poll frame U+0028 s U+0029 to retrieve the buffered frame U+0028 s U+0029 of the sleeping STA U+0028 s U+0029 from the AP causing frame losses for the targeted STA U+0028 s U+0029. Current approaches to prevent or detect the PS-DoS attack require encryption, change in protocol or installation of proprietary hardware. These solutions suffer from expensive setup, maintenance, scalability and deployment issues. The PS-DoS attack does not differ in semantics or statistics under normal and attack circumstances. So signature and anomaly based intrusion detection system U+0028 IDS U+0029 are unfit to detect the PS-DoS attack. In this paper we propose a timed IDS based on real time discrete event system U+0028 RTDES U+0029 for detecting PS-DoS attack. The proposed DES based IDS overcomes the drawbacks of existing systems and detects the PS-DoS attack with high accuracy and detection rate. The correctness of the RTDES based IDS is proved by experimenting all possible attack scenarios.

State feedback control of real-time discrete event systems with infinite states

In this paper, we study a state feedback supervisory control of timed discrete event systems (TDESs) with infinite number of states modelled as timed automata. To this end, we represent a timed automaton with infinite number of untimed states (called locations) by a finite set of conditional assignment statements. Predicates and predicate transformers are employed to finitely represent the behaviour and specification of a TDES with infinite number of locations. In addition, the notion of clock regions in timed automata is used to identify the reachable states of a TDES with an infinite time space. For a real-time specification described as a predicate, we present the controllability condition for the existence of a state feedback supervisor that restricts the behaviour of the controlled TDES within the specification.

Supervision localization of timed discrete-event systems

We study supervisor localization for real-time discrete-event systems (DES) in the Brandin–Wonham framework of timed supervisory control. We view a real-time DES as comprised of asynchronous agents which are coupled through imposed logical and temporal specifications; the essence of supervisor localization is the decomposition of monolithic (global) control action into local control strategies for these individual agents. This study extends our previous work on supervisor localization for untimed DES, in that monolithic timed control action typically includes not only disabling action as in the untimed case, but also “clock preempting” action which enforces prescribed temporal behavior. The latter action is executed by a class of special events, called “forcible” events; accordingly, we localize monolithic preemptive action with respect to these events. We demonstrate the new features of timed supervisor localization with a manufacturing cell case study and discuss a distributed control implementation.

SetExp: a method of transformation of timed automata into finite state automata

Real-time discrete event systems are discrete event systems with timing constraints, and can be modeled by timed automata. The latter are convenient for modeling real-time discrete event systems. However, due to their infinite state space, timed automata are not suitable for studying real-time discrete event systems. On the other hand, finite state automata, as the name suggests, are convenient for modeling and studying non-real time discrete event systems. To take into account the advantages of finite state automata, an approach for studying real-time discrete event systems is to transform, by abstraction, the timed automata modeling them into finite state automata which describe the same behaviors. Then, studies are performed on the finite state automata model by adapting methods designed for non real-time discrete event systems. In this paper, we present a method for transforming timed automata into special finite state automata called Set-Exp automata. The method, called SetExp, models the passing of time as real events in two types: Set events which correspond to resets with programming of clocks, and Exp events which correspond to the expiration of clocks. These events allow to express the timing constraints as events order constraints. SetExp limits the state space explosion problem in comparison to other transformation methods of timed automata, notably when the magnitude of the constants used to express the timing constraints are high. Moreover, SetExp is suitable, for example, in supervisory control and conformance testing of real-time discrete event systems.

Diagnosability of delay-deadline failures in fair real time discrete event models

In this article a method for failure diagnosis of real time discrete event systems (RTDES) with ‘fairness of traces’ has been developed. Discrete event system (DES) modelling framework with provision for associating timing information with the transitions are required for handling real time systems. RTDES models and timed DES (TDES) models are examples of such modelling frameworks. Failure diagnosis in untimed DES models enables only the study of diagnosability of failures resulting in a change in the logical behaviour of the failed system. In addition to logical failures, failure diagnosis in RTDES and TDES models also enables diagnosability of failures that change the timing behaviour of the system but maintain the logical behaviour. Many systems exhibit fairness of traces with respect to transitions in the sense that any trace that visits a state infinitely often has infinitely many occurrences of all the transitions that emanate from that state. The abstraction employed in obtaining their (timed) DES models often obliterates this property. The RTDES and TDES diagnosability conditions, proposed in the literature and which do not consider fairness, are shown to be inadequate in this article. A new diagnosability condition is achieved by taking into account this fairness property in the RTDES models and shown to be necessary and sufficient for such systems. An analysis of time complexity for analysing the diagnosability of systems with fairness of traces is presented.

Fuzzy Assessment Algorithm for Ladder Programming

This paper* proposes an assessment algorithm to measure the quality in ladder programming for programmable logic controllers (PLCs). The proposed quality metrics involve the criteria of simplicity, reconfigurability, reliability, and flexibility. The value of each metric criterion is normalized between 0 and 1. A fuzzy assessment algorithm is developed to select the best controller design among a set of developed ladder programs for the same application. A single tone membership function is used to represent the metric criterion for each controller between 0 and I. The fitness of each controller is evaluated as the minimum value of all the applied metric criteria. Thereafter, the decision making to select the best logic design is obtained as the maximum value of all fitness values. The developed fuzzy assessment algorithm is applied to real time discrete event systems. These systems are a pneumatic cylinder and a conveyor belt where each of them performs a repeated sequence. The obtained results affirmed the potential of the proposed algorithm to assess the quality of the designed ladder programs. The main contribution of this paper is to define a methodology to measure the quality for programs written in ladder diagrams.

A new method for centralised and modular supervisory control of real-time discrete event systems

This article deals with the problem of controlling a plant described as a real-time discrete event system (RTDES). In particular, automata-based supervisory control of RTDES is addressed. The aim of supervisory control is to restrict the behaviour (using a supervisor) of an uncontrolled plant in order to conform to a given specification. First, we propose a centralised method for the synthesis of a supervisor that forces a given plant to conform to a given specification. Then, we extend this centralised method to the modular case, that is, for the synthesis of n supervisors that force the plant to conform to n given specifications, respectively. Timed automata (TA) with invariants is the model used to describe the plant and the specification(s). The synthesis approach is based on the transformation of the control problem into a non-real-time form, using a transformation of TA into equivalent particular finite state automata called Set-Exp-Automata. This transformation allows to adapt the theory of Ramadge and Wonham, and is justified by the fact that it reduces the state space explosion problem compared to other transformation methods such as the transformation of TA into region automata. Moreover, the Set-Exp-Automata model provides a suitable control architecture for implementation. The proposed approach allows to obtain the solution to both the centralised and modular supervisory control problem, by identifying the solvability conditions and giving a step-by-step computation procedure of the solution.

The real-time supervisory control of an experimental manufacturing system based on a hybrid method

In this paper, the real-time supervisory control of an experimental manufacturing system is reported based on a recently proposed hybrid (mixed PN/automaton) approach. Assuming that an uncontrolled bounded Petri net (PN) model of a (plant) discrete event system (DES) and a set of forbidden state specifications are given, the proposed approach computes a maximally permissive and nonblocking closed-loop hybrid model. The method is straightforward logically, graphically and technologically. This paper particularly shows the applicability of a hybrid (mixed PN/automaton) approach to low-level real-time DES control. To do this, programmable logic controller (PLC) based real-time control of an experimental manufacturing system is considered.

Diagnosis of Faults in Real-Time Discrete Event System

This paper deals with diagnosis, whose objective is to observe a plant and detect any behavior which does not conform to a specification. We study the diagnosis of real-time discrete event systems (RTDES) where the plant and the specification are modeled by timed automata (TA). Our diagnosis method is based on expressing the real-time diagnosis problem into a non-real-time form, using a transformation of TA into finite state automata where the passing of time is represented by two types of events: Set and Exp events that correspond to activation and expiring of clocks, respectively. We develop a diagnosis procedure that checks diagnosability and synthesizes a diagnoser. An advantage of the proposed procedure is that it reduces the state explosion problem in comparison with other methods.

Diagnosis of delay–deadline failures in real time discrete event models

In this paper a method for fault detection and diagnosis (FDD) of real time systems has been developed. A modeling framework termed as real time discrete event system (RTDES) model is presented and a mechanism for FDD of the same has been developed. The use of RTDES framework for FDD is an extension of the works reported in the discrete event system (DES) literature, which are based on finite state machines (FSM). FDD of RTDES models are suited for real time systems because of their capability of representing timing faults leading to failures in terms of erroneous delays and deadlines, which FSM-based ones cannot address. The concept of measurement restriction of variables is introduced for RTDES and the consequent equivalence of states and indistinguishability of transitions have been characterized. Faults are modeled in terms of an unmeasurable condition variable in the state map. Diagnosability is defined and the procedure of constructing a diagnoser is provided. A checkable property of the diagnoser is shown to be a necessary and sufficient condition for diagnosability. The methodology is illustrated with an example of a hydraulic cylinder.

Non-blocking supervisory control under bounded time constraints based on non-deterministic timed transition models

A non-blocking supervisory control problem to meet bounded time constraints for non-deterministic real-time discrete event systems (DESs) is addressed. To this end, notions of trace controllability and time controllability are introduced for the non-deterministic real-time DESs with respect to a timed language specification representing a bounded time constraint. The existence conditions of a non-blocking supervisor to achieve the timed language specification are presented on the basis of the proposed notions, and these are further illustrated by an assembly work station example.

Grid automata and supervisory control of dense real-time discrete event systems

We present a generalization of the classical supervisory control theory for discrete event systems to a setting of dense real-time systems modeled by Alur and Dill timed automata. The main problem involved is that in general the state space of a timed automaton is (uncountably) infinite. The solution is to reduce the dense time transition system to an appropriate finite discrete subautomaton, the grid automaton, which contains enough information to deal with the timed supervisory control problem (TSCP). The plant and the specifications region graphs are sampled for a granularity defined in a way that each state has an outgoing transition labeled with the same time amount. We redefine the controllability concept in the context of grid automata, and we provide necessary and sufficient solvability conditions under which the optimal solution to centralized supervisory control problems in timed discrete event systems under full observation can be obtained. The enhanced setting admits subsystem composition and the concept of forcible event. A simple example illustrates how the new method can be used to solve the TSCP.

A Supervisory Control Method for Ensuring the Conformance of Real-Time Discrete Event Systems

In this article, we study the problem of controlling a plant described as a real-time discrete event system. The aimed objective is to ensure a conformance relation denoted tioco between the plant and the formal specification of the system, by means of a supervisor. We adopt a two-step approach. In Step 1, we express the problem into a non-real-time form, by using a transformation of timed automata (TA) into particular finite state automata called Set-Exp-Automata (SEA). The latter use two additional types of events, Set and Exp. And in Step 2, we propose a non-real-time control method suitable for SEA. We also propose a control architecture.

Supervisory control for the conformance of real-time discrete event systems ,

We study the problem of controlling a plant described as a real-time discrete event system by means of a supervisor, in order to ensure a conformance relation denoted tioco between the plant and the formal specification of the system.

Supervisory control of real-time discrete event systems under bounded time constraints

An analytical framework for supervisory control of real-time discrete event systems (DESs) under bounded time constraints is presented. In order to address the bounded time constraints of the systems, timed languages based on timed transition models are introduced. Using eligible time bounds, the notions of trace-controllability and time-controllability of timed languages are proposed. Based on these notions, necessary and sufficient conditions for the existence of a supervisor for a real-time DES to achieve the given timed language specification are presented. The proposed approach shows that an exhaustive enumeration of the language generated in timed transition models is not needed to verify such existence conditions of the supervisor.

Control synthesis of timed discrete event systems based on predicate invariance

In this paper, arc-timed Petri nets are used to model controlled real-time discrete event systems, and the control synthesis problem that designs a controller for a system to satisfy its given closed-loop behavior specification is addressed. For the problem with the closed-loop behavior specified by a state predicate, real-time control-invariant predicates are introduced, and a fixpoint algorithm to compute the unique extremal control-invariant subpredicate of a given predicate, key to the control synthesis, is presented. For the problem with the behavior specified by a labeled arc-timed Petri net, it is shown that the control synthesis problem can be transformed into one that synthesizes a controller for an induced arc-timed Petri net with a state predicate specification. The problem can then be solved by using the fixpoint algorithm as well. The algorithm involves conjunction and disjunction operations of polyhedral sets and can be algorithmically implemented, making automatic synthesis of controllers for real-time discrete event systems possible.

Control synthesis of timed discrete event systems: Part one

A new class of arc-timed Petri nets that covers timed automata and timed transition model is introduced as a control model for real-time discrete event systems, and the control synthesis problem that designs a controller to satisfy a specification given by a state predicate is addressed. Real time control-invariant predicates are defined and a fixpoint algorithm to compute extremal control-invariant predicates, key to the control synthesis, is proposed. The algorithm involves conjunction and disjunction operations of polyhedra, which can be symbolically implemented.

A Max-Plus Algebra of Signals for the Supervisory Control of Real-Time Discrete Event Systems

This paper defines a max-plus algebra of signals for the evaluation of timing behavior of discrete event systems modeled by timed event graphs. The scope of this work is limited to infinite. periodic sequences for whhich finite representations called signals can be Computed. The implementation of this Inax-plus algebra allows a user to Compute supremal controllers for real-time. discrete event systems

Supervisor Synthesis for Real-Time Discrete Event Systems

This paper introduces a formal framework to logically analyze and control real-time discrete event systems (RTDESs). Time Petri nets are extended to controlled time Petri nets (CtlTPNs) to model the dynamics of RTDESs that can be controlled by real-time supervisors. The logical behaviors of CtlTPNs are represented by control class graphs (CCGs) which are untimed automata with timing and control information in their state transition labels. We prove that the CCG corresponding to a CtlTPN expresses completely the logical behavior of the CtlTPN. The real-time supervisor is based on a nondeterministic logical supervisor for the CCG, including the delay for control computations to ensure the supervisor is acceptable in a true real-time environment. We prove the existence of a unique maximal controllable sublanguage of a given specification language and present an algorithm to construct the sublanguage. We also prove that the real-time supervisor meets the prespecified real-time behavior and present an online control algorithm to implement real-time supervisors. The concepts and algorithms are illustrated for an example of packet reception processes in a communication network.

A visual toolset for the design of real-time discrete-event systems

StateTime is a prototype toolset that supports the design of verified real-time discrete-event systems using executable visual state descriptions (the Build tool). Visual state descriptions allow the designer to browse and understand the structure of the system. A timing hierarchy of spontaneous, just, and forced timed events, and a variety of computational notions such as concurrency, hierarchy, nondeterminism, process interaction, and communication can be represented. The combination of model-checking (the Verify tool) and theorem proving allows for the treatment of finite and infinite state systems. The toolset is illustrated with a shutdown controller of a reactor, as taken from an actual industrial requirements document in which the system is described informally using a mixture of English descriptions, timing diagrams, and pseudocode. Using StateTime, a unified precise description of the shutdown reactor is obtained, which can then be checked automatically for conformance with its requirements.