A model is a representation of a system. In our case of computer systems, two types of models may be used : Functional models and Performance Models.
Functional Models are used to represent the operation of the system while Performance Models are used to represent the behaviour of the system in terms of its performance
Performance Models are useful for predicting the values of performance measures of a system from a set of values of workload, software and hardware parameters.
Several types of models may be used for performance modelling, depending on the characteristics of the system and its parameters.
Models are classified depending on the following criteria.
(1) Number of classes - Single Class,Multiple Class
(2) Nature of transactions - Open,Closed,Mixed
(3) Solution Techniques - Simulation,Analytical