الفهرس 
AcknowledgementOnly 14 pages are availabe for public view
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Abstract
Computer systems are sins wider areas of applications every day.Presently they serve large groups of users having different interests.One common interest of computers is to have some good type of service.However that service should not be exceedingly expensive.To satisfy computer users requirements computer owners try to have the best computer systems that satisfy certain budget constraints.On the other hand they try to have the cheapest computer systems that satisfy certain service standards.People who already own computer systems may have to choose among the above alternatives.That happens when they have to improve the quality of the service that they deliver to their users or when they manage to extend their service to larger groups of users.