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Presentation on How discrete-event simulation software works and why it matters

What is discrete-event simulation ? A discrete-event simulation (DES) models is the operation of a system as a discrete sequence of events in time. Each event occurs at a particular instant in time and marks a change of state in the system. Between consecutive events, no change in the system is assumed to occur; thus the simulation can directly jump in time from one event to the next.

Nature of Discrete-Event Simulation  Event time: discrete; state: usually discrete  Transaction-flow world view: discrete units of traffic that move from point to point while competing for scarce resources  Two or more events often have to be manipulated at the same time point. How to order them?  Event time: discrete; state: usually discrete  Transaction-flow world view: discrete units of traffic that move from point to point while competing for scarce resources  Two or more events often have to be manipulated at the same time point. How to order them?  

Discrete-Event Modeling Languages  There are different implementations, which aim to provide convenience for modelers.  Arena for example, provides graphical user interface for the users to input the simulation model.  There are different implementations, which aim to provide convenience for modelers.  Arena for example, provides graphical user interface for the users to input the simulation model.

Basic Simulation Constructs Topics includes -  Entity – a unit of traffic  Resources – service machine  Control Elements – support delay or logical alternatives  Operations - an action for an entity Topics includes -  Entity – a unit of traffic  Resources – service machine  Control Elements – support delay or logical alternatives  Operations - an action for an entity

Model Execution  The simulation advances with the simulation clock, which advances when an event happens  Carry out all possible actions at the current simulated time -> advance the simulated clock;  and repeat;  The simulation advances with the simulation clock, which advances when an event happens  Carry out all possible actions at the current simulated time -> advance the simulated clock;  and repeat; initialize event list get next (nearest future) event from event list time = event time process event (change state values, add/delete future events from event list update statistics done?

Entity States  Active State: current moving state  Ready State: ready but wait due to limited resource  Time-Delayed State: entity waiting for a known future time so they can enter ready state  Condition-Delayed State: remove automatically when conditions permit  Dormant State: modelers set it  Active State: current moving state  Ready State: ready but wait due to limited resource  Time-Delayed State: entity waiting for a known future time so they can enter ready state  Condition-Delayed State: remove automatically when conditions permit  Dormant State: modelers set it

Entity Management Structure  Active State – kept in a list  Ready State – in current event list  Time-Delayed State – in future event list  Conditional-Delayed State – delay list  Dormant State – user-managed list  All these lists are used to store data and provide easy access to their elements  Active State – kept in a list  Ready State – in current event list  Time-Delayed State – in future event list  Conditional-Delayed State – delay list  Dormant State – user-managed list  All these lists are used to store data and provide easy access to their elements

Different Implementation in Tools  The paper together with its WSC96’ version gives 6 representative implementations. Two examples here -  AutoMod, version 9 (Phillips 1997)  Siman, the core of Arena  The paper together with its WSC96’ version gives 6 representative implementations. Two examples here -  AutoMod, version 9 (Phillips 1997)  Siman, the core of Arena

AutoMod Language Generic TermAutoMod Equivalent External EntityLoad Internal EntityLogical Load ResourceResource; Queue; Block Control ElementCounter; Process Traffic Limit OperationAction Current Events ListCurrent Event List Future Events ListFuture Event List Delay ListDelay List; Condition Delay List; Load Ready List User-Managed ListOrder List

Why It Matters  Trying to re-capture a resource immediately  The first in line is still delayed  Yielding control  Conditions involving the clock  Mix Mode Waiting  Trying to re-capture a resource immediately  The first in line is still delayed  Yielding control  Conditions involving the clock  Mix Mode Waiting

Group Members  Rawnat Jahan Anika [ ]  Subho Mrong [ ]  Rawnat Jahan Anika [ ]  Subho Mrong [ ]

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