Approximation methods for two-layer queueing models

We discuss methods to calculate performance measures of queueing models with simultaneous resource possession in which customers who possess different types of passive resources compete with other customers for the same active resources. In this paper, we propose a two-layer queueing model which can adequately model such systems and two extended flow-equivalent methods for approximating the performance measures of two-layer models. We give an example of the model, and we describe the results of a study of the accuracy of the approximation methods.