Discrete Event Modelling and Simulation CS522 Fall Term 2001

For a class of formalisms labelled discrete-event, system models are described at an abstraction level where the time base is continuous (), but during a bounded time-span, only a nite number of relevant events occurs. These events can cause the state of the system to change. In between events, the state of the system does not change. This is unlike continuous models in which the state of the system may change continuously over time. Discrete-event formalisms are clearly at a high level of abstraction. This abstraction is often appropriate for realistic representation of a system's behaviour. Furthermore, as in between events, the state of the system does not change, a discrete-event simulator need not explicitly represent the state of the system at non-event times. This allows for highly efcient simulation as compared to continuous simulation, where in principle, state information must be represented at each point in continuous time. The high level of abstraction may however introduce simulation artifacts which do not pertain to real-world behaviour. In particular, event simultaneity whereby multiple distinct events occur at exactly the same time may be due to an insuf-ciently detailed discrete-event model. The DEVS formalism and its derivatives rigourously describe the semantics of such event collisions. A detailed presentation of the semantics of pinnacles and mythical states, which occur when events are used for respectively time-scale abstraction and parameter abstraction of continuous phenomena in hybrid models, is given by Mosterman and Biswas in [MB01]