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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.
Discrete and Continues Systems
• A discrete system is one in which the state variables change only at a discrete set of points in time : Bank example
Discrete and Continues Systems (cont.)
• A continues system is one in which the state variables change continuously over time: Head of water behind the dam

### Queuing Theory

Definitions
• 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
INTRODUCTION
• 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.
Components of a Queuing System:
• 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,
Components of a Queuing System:
• 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.
Components of a Queuing System:
• 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
Components of a Queuing System:
• 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
Components of a Queuing System:
• 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
Components of a Queuing System:
• 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.