Non-product form equilibrium probabilities in a class of two-station closed reentrant queueing networks

While many single station queues possess explicit forms for their equilibrium probabilities, queueing networks are more problematic. Outside of the class of product form networks (e.g., Jackson, Kelly and BCMP networks), one must resort to bounds, simulation, asymptotic studies or approximations. By focusing on a class of two-station closed reentrant queueing networks under the last buffer first served (LBFS) policy, we show that non-product form equilibrium probabilities can be obtained. When the number of customer classes in the network is five or less, explicit solutions can be obtained. Otherwise, we require the roots of a characteristic polynomial and a matrix inversion that depend only on the network topology. The approach relies on two key points. First, under LBFS, the state space can be reduced to four dimensions independent of the number of buffers in the system. Second, there is a sense of spatial causality in the global balance equations that can then be exploited. To our knowledge, these two-station closed reentrant queueing networks under LBFS represent the first class of queueing networks for which explicit non-product form equilibrium probabilities can be constructed (for five customer classes or less), the generic form of the equilibrium probabilities can This work was supported in part by Korea Research Foundation (KRF) Grant 20110005696. Woo-sung Kim KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Korea Fax: +82-42-350-3110 Tel.: +82-42-350-3167 *James R. Morrison E-mail: [email protected], http://xs3d.kaist.edu Corresponding author KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Korea Fax: +82-42-350-3110 Tel.: +82-42-350-3127 2 Woo-sung Kim, James R. Morrison* be deduced and matrix analytic approaches can be applied. As discussed via example, there may be other networks for which related observations can be exploited.