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# Advantages of simulation - PowerPoint PPT Presentation

Advantages of simulation. New policies, operating procedures, information flows and son on can be explored without disrupting ongoing operation of the real system .

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Presentation Transcript

• New policies, operating procedures, information flows and son on can be explored without disrupting ongoing operation of the real system.

• New hardware designs, physical layouts, transportation systems and … can be tested without committing resources for their acquisition.

• Time can be compressed or expanded to allow for a speed-up or slow-down of the phenomenon( clock is self-control).

• Insight can be obtained about interaction of variables and important variables to the performance.

• Bottleneck analysis can be performed to discover where work in process, the system is delayed.

• A simulation study can help in understanding how the system operates.

• “What if” questions can be answered.

• Model building requires special training.

• Simulation results can be difficult to interpret.

• Simulation modeling and analysis can be time consuming and expensive.

• Many simulation software have output-analysis.

• A discrete system is one in which the state variables change only at a discrete set of points in time : Bank example

• A continues system is one in which the state variables change continuously over time: Head of water behind the dam

### Queuing Theory

• System: is a collection of entities which are logically related and which are of interest to a particular application.

• EntityAn object of interest in the system : Machines in factory

• AttributeThe property of an entity : speed, capacity

• ActivityA time period of specified length :welding, stamping

• StateA collection of variables that describe the system in any time : status of machine (busy, idle, down,…)

• EventA instantaneous occurrence that might change the state of the system

• A queuing system consists of one or more servers that provide service of some sort to arriving customers.

• Customers who arrive to find all servers busy generally join one or more queues (lines) in front of the servers, hence the name queuing systems.

• There are several everyday examples that can be described as queuing systems, such as bank-teller service, computer systems, manufacturing systems, maintenance systems, communications systems and so on.

• 1- Population of Customers can be considered either limited (closed systems) or unlimited (open systems).

• Unlimited population represents a theoretical model of systems with a large number of possible customers (a bank on a busy street, a motorway petrol station).

• Example of a limited population may be a number of processes to be run (served) by a computer or a certain number of machines to be repaired by a service man

• Customers may be people, machines of various nature, computer processes, telephone calls,

• 2- Arrival defines the way customers enter the system. Mostly the arrivals are random with random intervals between two adjacent arrivals. Typically the arrival is described by a random distribution of intervals also called Arrival Pattern.

• 3- Queue represents a certain number of customers waiting for service (of course the queue may be empty).

• The customer being served is considered not to be in the queue.

• There are two important properties of a queue: Maximum Size and Queuing Discipline

• Maximum Queue Size (also called System capacity) is the maximum number of customers that may wait in the queue (plus the one(s) being served).

• Queue is always limited, but some theoretical models assume an unlimited queue length. If the queue length is limited, some customers are forced to renounce without being served

• Service represents some activity that takes time and that the customers are waiting for.

• Output represents the way customers leave the system. Output is mostly ignored by theoretical models, but sometimes the customers leaving the server enter the queue again

• Queuing Discipline represents the way the queue is organized (rules of inserting and removing customers to/from the queue).

• The ways are these ways:

1) FIFO (First In First Out) also called FCFS (First Come First Serve) - orderly queue.

2) LIFO (Last In First Out) also called LCFS (Last Come First Serve) - stack.

3) SIRO (Serve In Random Order).

4) Priority Queue, that may be viewed as a number of queues for various priorities.