Suppose that an investigation of the database management system log reveals that individual transactions submitted to the database server have significantly different characteristics. However, also suppose that the analyst notes that these transactions can be grouped into three distinct groups of fairly similar transactions, as indicated in Table 2.1: trivial, medium, and complex. These transaction groups differ in the average CPU time and average number of I/Os per transaction. Therefore, it would not be appropriate to characterize the transactions submitted to the database server as a single group. If they were combined into a single group the resulting model may be too approximate and with large errors. Thus, when describing a QN model, one has to also specify the classes of customers that use the resources of the QN, the workload intensity of each class, and the service demands at each resource per class.
A multiclass QN model should be used in the following cases: