Discrete models deal with system whose behavior changes only at given instants. A typical example occurs in waiting lines where we are interested in estimating such measures as the average waiting time or the length of the waiting line. Such measures occur only when the customer enters or leaves the system. The instants at which changes in the system occurs identify the model’s events, e.g. arrival and departure of the customers. The arrival events are separated by the ‘interarrival time’ (the interval between successive arrivals), and the departure events are specified by the service time in the facility. The fact that these events occur at discrete points is known as “Discrete-event Simulation.” (Taha, 1997)