DISCRETE EVENT SYSTEMS

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INTRODUCTION

A discrete event system (DES) can be defined as a dynamic system in which the states change in response to the occurrence of events. The discrete events take place at possibly irregular or unknown points in time (i.e., asynchronously and nondeterministically) but are the result of interactions within the system itself. The acronym DES, or frequently DEDS (for discrete event dynamic systems), has been used extensively in different fields of mathematics and applications to designate apparently widely different systems


2. MODELS OF DISCRETE EVENT SYSTEMS

The increased complexity of human-made systems, especially as an effect of the widespread application of information technology, has made the development of more detailed formal methods necessary to describe, analyze, and control processes observed in environments such as digital communication networks and manufacturing units. As opposed to the continuous time-driven evolution of a CVDS [Fig. 1(a)], the evolution of a DES is piecewise-constant and event-driven [Fig. 1(b)].


UNTIMED DISCRETE EVENT SYSTEMS

Untimed or logical DES models ignore time as a variable that specifies the moments when the events occur. Only the order of the events is relevant for these models. The untimed DES models have been used for the deterministic qualitative analysis of control issues such as the reachability of states (18,58,59) or deadlock avoidance (23,60). Finite-state machine and Petri nets are the formal mechanisms mostly used for the representation of untimed DESs. Other untimed frameworks, such as the trace theory, have also been explored