In this chapter queuing network models with multiple classes have been analyzed. These models are appropriate for describing workloads of actual systems. Usually, "what if" questions that appear in capacity planning are associated with individual classes of customers. Multi-class models provide performance results for individual class analysis. The discussion has concentrated on techniques for solving a special category of multiclass models, known as product-form, separable, or BCMP networks. The techniques are grouped into sections. One section refers to the exact and approximate solutions of closed models, where the load intensity is assumed to be constant. Another section shows how to solve open multiclass models, where external arrivals and departures of customers are allowed. Mixed models with both closed and open classes are solved by a combination of the two previous techniques.
This chapter focused on queuing models with load-independent (LI) devices. The algorithms for solving open multiclass QNs are in the MS Excel OpenQN.XLS workbook and the approximate multiclass MVA solution technique is implemented in the MS Excel ClosedQN.XLS workbook. Some computer systems may have components that behave as load-dependent devices (LD), at which the service rate varies with the number of customers present in its queue. The next chapter presents techniques to analyze the performance of queuing models with load-dependent devices.