Chapter 3 - DEVS FORMALISM AND DEVS-SCHEME

Abstract

This chapter discusses the discrete event system specification (DEVS) formalism and its implementation in DEVS-Scheme. Its basic features help to understand how hierarchical, modular simulation models are specified in DEVS-Scheme. The chapter describes that DEVS formalism provides a means of specifying a mathematical object called a system. Basically, a system has a time base, inputs, states, and outputs, and functions for determining next states and outputs given current states and inputs. The insight provided by the DEVS formalism is that it characterizes the way discrete event simulation languages specify discrete event system parameters. It is more than just a means of constructing simulation models. It provides a formal representation of discrete event systems capable of mathematical manipulation just as differential equations serve this role for continuous systems. Such manipulation includes behavioral analysis, whereby properties of the behavior of a system are deduced by examining its structure. In the DEVS-Scheme realization, the DEVS formalism is refined so that both the input and output sets X, Y consist of pairs of the form (port, value). Thus, an external input event of the form x = (p, v) signals the fact that a value v has been received at an input port p. Similarly, y = (p, v) represents the sending of a value v to output port p.