Components and Organization of Discrete-event Simulation Model

1. Components and Organization of Discrete-event Simulation Model • System state: The collection of state variables necessary to describe the system at a particular time. • Simulation clock: A variable giving the current value of simulated time. • Event list: A list containing all information about all scheduled events like time when each type of event will occur. • Initialization routine: A subprogram to initialize the simulation model at time zero. • Statistical counters: Variables used for storing statistical information about system performance.

2. Examples of Statistical counters • The number of parts produced so far • The total of the waiting times in queue so far • The number of parts that have passed through the queue so far. • The longest time spent in queue we’ve seen so far • The total of the time spent in the system by all parts that have departed so far • The longest time in system we’ve seen so far • The area so far under the queue-length curve Q(t) • The highest level that Q(t) has so far attained • The area so far under the server-busy function B(t)

3. Timing routine: A subprogram that determines the next event from the event list and then advances the simulation clock to the time when that event is to occur. • Event routine: A subprogram that updates the system state when a particular type of event occurs (there is one event routine for each event type). • Library routines: A set of subprograms used to generate random observations from probability distributions that were determined as part of the simulation model.

4. Report generator: A subprogram that • computes estimates (from the statistical counters) of the desired measures of performance • produces a report when the simulation ends. • Main program: A subprogram that • invokes the timing routine to determine the next event • transfers control to the corresponding event routine to update the system state appropriately. • check for termination and invoke the report generator when the simulation is over.

6. Start 0 Initializing routine Timing routine Main Program 1 i 2 Generate Random numbers 1.Update the system state 2.Update statistics counters 3.Generate future events and Add to event list Event routine i Is Simulation over No Yes Report routine Stop

7. Some other Pieces of Simulation Model • Entities: these are dynamic objects in the simulation model. They are created, move around for a while, then disposed. • Examples: parts in a manufacture, customers in a bank, jobs in a processors, messages … etc. • Attributes: A common characteristics of all entities but with a specific value that can differ from entity to another. • They can be thought as a tag or label attached to each entity. Or local variables • Example: priority of an entity, due date, arrival time of an entity, … etc.

8. Resources: Things that provide service for entities like personnel, equipments, or a space in a storage. • They can represent a group of several individual servers each is called a unit of that resource • Queues: When an entity cannot move on because the resource is used by another entity, this entity will be placed in a waiting line (Queue).