Simulation generally involves some kind of model or simplified representation. During the course of a simulation, the model mimics important elements of what is being simulated. A simulation model may be a physical model, a mental conception, a mathematical model, a computer model, or some combination of all of these. Computer simulation is currently used in a wide range of applications, since physical models are often relatively expensive to build and unwieldy to move. Queuing systems are used to characterize a broad class of processes sharing a common mathematical structure but which may include many diverse physical realizations. All these processes are stochastic processes with three characteristics, which can be used to describe the system: an input process, a service mechanism, and a queue discipline. The presence of uncertainty is the thing which makes these systems challenging to analyze and design, since randomness in one or more of these three characteristics is the rule rather than the exception. Furthermore, the distributions used to describe these events may be nonstationary, which adds further complication to their study