Introduction

Engineers design systems with many different kinds of models including mental models, diagrams and schematics, scaled physical models, mathematical models, and computational models. All are representations that enable understanding, design, and building of processes or systems. Many models are static representations where time plays no role, such as a scale model of a building, an equation for estimating the cost of an airplane, or a Bayesian network of contributing factors to an industrial accident. The most common discrete system is the queue/server system. Queue/server systems exist whenever a line forms in front of some processing mechanism. The line is called a queue and the processing mechanism is called the server. An important measure of system performance is a measure of how busy the server is. The utilization of the server is the proportion of time during a simulation that the server is busy.