Slide - 1 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation Simulation of a bank: Three tasks or processes: 1.Customer arrives 2.Customer served 3.Customer leaves Customer Arrives Customer Served Customer Leaves

Slide - 2 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation Let’s describe task 1 Customer Arrives Arrival rate – rate that customers arrive at bank 1 customer every 3 minutes or so i.e. Inter-arrival rate IAT = 3 Time period customers can arrive Bank Hours: 9 to 5 Customer Arrives

Slide - 3 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation Task 2 Customer Served Time to serve a customer Approx 6 minutes per customer Select sample from for example a normal distribution mean = 6 and standard deviation = 2 Choice of distribution ? Customer Served

Slide - 4 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation Task 3 Customer Leaves Record the time customer leaves No further interest in customer Customer Leaves

Slide - 5 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation What else affects this flow of customers? Number of bank staff on duty Lunchtime crowd Customer Arrives Customer Served Customer Leaves

Slide - 6 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation What information would be useful to collect? –Employee utilisation –Waiting times –Length of queue Is there a way to get this information? Customer Arrives Customer Served Customer Leaves

Slide - 7 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design An example of a Simulation To performing this analysis without micro- simulation: –Only use mean times or averages rates or –Run the experiment in real life. –Using micro-simulation every individual is computed taking account of stochastic behaviour – hence more realistic analysis possible - e.g maximum delay not obtained from an average Customer Arrives Customer Served Customer Leaves

Slide - 8 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Essentials of Discrete-Event Micro-Simulation Tasks Events – start and end of tasks Entities Entity flow Resources Queues Clock Scenario events

Slide - 9 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Tasks or processes The building block of any discrete-event simulation A task represents a single step in the process that makes up the discrete event simulation A task may require a certain amount of time in order to execute Tasks & Events have three main parts: –Pre-condition or Release condition –Start condition or Beginning effect –End condition or Ending effect Events trigger subsequent actions

Slide - 10 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Conditions Release condition –The conditions that need to be satisfied so this task may begin Beginning effect –Things that take place when this task begins execution Ending effect –Things that take place this task completes execution You are responsible for writing code to achieve these logical conditions

Slide - 11 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Task Flow For every discrete-event simulation, there is a basic entity flow Here is an example of the flow of a bank: When one task completes the following can happen to the entity at same simulation time: –Nothing –Join a queue –Another activity starts –Several activities start simultaneously Customer Arrives Customer Served Customer Leaves

Slide - 12 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Resources In any process some tasks require a resource to be available in order to begin execution –Example: A task may require the availability of a trained operator in order for that task to begin Many processes are constrained by limited resources –Without resources, steps in the process may be held up until those resources are available –Without resources, the flow of the entities might change

Slide - 13 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Entities Entities flow through a simulation and are involved in the execution of any task –Examples of entities: Customers in a bank Cars going through a toll both Parts being manufactured When an entity arrives at a task the system: –Checks the release condition and if accepted –Executes the beginning effect –Waits the appropriate amount of time (simulation time or clock time) –Executes the ending effect

Slide - 14 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Queues What happens when the release condition of a task is not met (such as when all bank staff are busy)? –The entity must wait in a queue until the release condition is met Sometimes, the behavior of the queue is important –FIFO (e.g. queue for bank staff) –LIFO (e.g. stocked shelves) –Sorted based on priority (e.g. A&E Department) Queue Customer Served

Slide - 15 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design The Simulation Clock Since we are interested in time based behaviour, a clock is needed to keep the system in synchronism As events in your simulation occur, the simulation clock will advance based upon how long each task or process takes – not fixed small steps as in continuous dynamic systems

Slide - 16 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design How a Simulation is Executed As a simulation executes, all scheduled events are kept in an event queue or time line The simulation begins by taking the first event that is scheduled to occur from the event queue (e.g. start the first task) That event may place other events later in the event queue, ordered of time When that event is completed, the simulation takes the next event from the queue, updates the clock, and processes that event

Slide - 17 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Running a Simulation Let’s step through a simulation of a bank. Using the task rules we defined for customer served Here are other rules to follow –Bank is open for 8 hours (480 minutes) –Customers arrive about every 3 minutes –It takes around 6 minutes to serve a customer –We have 2 bank staff on duty

Slide - 18 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Running a Simulation Using this diagram below let’s step through this simulation TimeTaskStaff People Waiting 0 Customer Arrives 2 0 Customer Arrives Customer Leaves Queue Customer Served

Slide - 19 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Simulation What else affects this flow of customers? –Number of bank staff on duty –Lunchtime crowd Customer Arrives Customer Served Customer Leaves

Slide - 20 Dr Terry Hinton 6/9/05UniS - Based on Slides by Micro Analysis & Design Scenario Events What happens if we want to: – Model customers arriving faster at lunchtime? –Put another bank teller on duty at lunchtime? Events that don’t necessarily fall into any one task can be defined as a scenario events Scenario events are defined by –The time when the scenario event occurs –What happens