SIMULATION

SIMULATION

1. Simulation, according to Shannon (1975), is “the process of designing a model of a real system and conducting experiments with this model for the purpose either of understanding the behavior of the system or of evaluating various strategies (within the limits imposed by a criterion or set of criteria) for the operation of the system.”

   • DISCRETE-EVENT SIMULATION STRUCTURE
     • Entities
       • The best way to understand the function of an entity is understand that entities cause changes in the state of the simulation. Without entities, nothing would happen in a simulation. As a matter of fact, one stopping condition for a simulation model is the condition where there are no active entities in the system
     • Activities and Events
       • Activities are processes and logic in the simulation. Events are conditions that occur at a point in time which cause a change in the state of the system. An entity interacts with activities. Entities interacting with activities create events.
     • Resources
       • In a simulation, resources represent anything that has a restricted (or constrained) capacity. Common examples of resources include workers, machines, nodes in a communication network, traffic intersections, etc. In our example, we have three different resources.
     • Global Variables
       • If you are a programmer, then the idea of having global variables is nothing new. A global variable is a variable that is available to the entire model at all times. A global variable can track just about anything that is of interest to the entire simulation. In our model we have four global variables, two of which help us configure the problem and two of which collect revenue information.
     • Random Number Generator
       • Every simulation package has a random number generator. The random number generator (technically called a pseudo-random number generator) is a software routine that generates a random number between 0 and 1 that is used in sampling random distributions. For example, let us assume that you have determined that a given process delay is uniformly distributed between 10 minutes and 20 minutes.
     • System State Variables
       • Depending on the simulation package, there can be several system state variables, but the one system state variable that every simulation package has is the current time of the simulation. In order to keep from offending any simulation vendors, we will choose a different name for our simulation time variable. Our name of the current time of the simulation is CurrentTime.
     • Statistics Collectors
       • Statistics collectors are a part of the simulation that collects statistics on certain states (such as the state of a resources), or the value of global variables, or certain performance statistics based on attributes of the entity