Discrete-Event Simulation

Discrete-Event Simulation - Discrete-Event Simulation...

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Unformatted text preview: Discrete-Event Simulation Theresa M. Roeder Spring 2011 2 What is Discrete-Event Simulation? Discrete-event simulation (DES) is a simulation model of systems where The state space is discrete or The system state changes at discrete points in time or The system changes rules at discrete points in time Two defining characteristics of dynamic models: they model time and uncertainty 3 Definitions System Collection of entities that interact with a common purpose according to sets of laws and policies Defined by purpose or function, not membership or boundaries (e.g., transportation system, communication system) Avoid tying design to status quo Entities Real or logical Described by (numerical) attributes 4 Definitions, contd Laws: rules not under our control Policies Rules under our control Distinguished by parameters Model System used a surrogate for another system Most important modeling question: WHY? 5 Modeling Process 1. State purpose of system. Be aware that there might be several (conflicting) purposes. 2. State objectives of study. 3. Design, at least qualitatively, the experiments you might want to run with your simulation. 4. Identify resident and transient entities in your system and their important attributes. Assign names to the attributes. 5. Identify the dynamic attributes and the circumstances that cause their values to change these will be the events. Develop your own style: EXPERTS DO NOT AGREE ON HOW ITS DONE! 6 Elements of a Simulation Variables that describe the state Events that change the state Relationships between the events A A B 7 Event Graph Models Graphical approach to simulation modeling Events represented as nodes Relationships between events represented by arrows Scheduling and cancelling Conditions Time delays 8 A B t (i) Whenever event A occurs, if condition (i) is true, then event B will immediately be scheduled to occur after a delay of t. Event Graph Modeling 9 Single Resource Model Variables Q : number of jobs waiting for service R : number of available resources (R = 0: resource busy; R > 0: resource idle) Timing Data ta : interarrival time ts : service time Events Jobs ENTER the system every ta time units START servicing the job, taking ts time units Jobs LEAVE the system 10 Verbal Event Graph Initialize the RUN Customer ENTER System Customer LEAVEs System Customer STARTs Service ( Are Customers Waiting ?) ( Is Server Idle?) ~ t a t s 11 Main Event Scheduling Algorithm Cancel Events Initialize Advance Clock to Time of Next Event Execute Event Remove Event From List State Event List Change State Schedule Events Summary Statistics Stop Yes No 12...
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This note was uploaded on 01/26/2012 for the course DS 408 taught by Professor Theresam.roeder during the Spring '12 term at S.F. State.

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Discrete-Event Simulation - Discrete-Event Simulation...

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