Y. Kirsal and O. Gemikonakli Middlesex University, London, UK Y.X.Kirsal, O.Gemikonakli@mdx.ac.uk Abstract— Most wireless communication and mobile computing systems are expected to be operational 24 hours a day, 7 days a week. However, wireless communication systems encounter failures. In wireless cellular systems, handoff and system failure factors cause the degradation of the overall system performance. It is essential to consider handoff phenomena with system failures and repairs. In this paper, a handoff scheme in wireless cellular networks with failures and repairs are modelled. Various approaches such as product form solutions (for pure performance evaluation), approximate Markov reward model solution and the exact spectral expansion solution approaches are considered and discussed in detail. Performance measures, such as blocking probability and mean queue length are presented in order to illustrate the effects of failures, and accuracies of approximate methods. I. INTRODUCTION The development of wireless terminals and mobility has facilitated the rapid growth of wireless communication and mobile computing. Today, wireless cellular systems have played an important role in telecommunication infrastructure. Mobility is one of the major issues in the performance characterization in mobile and wireless communication systems. In wireless cellular systems, it is important to support services when mobile subscribers (MSs) move from one base station (BS) coverage area to another. As a MS moves across from one cell to another, the ongoing call transfers to a new BS. The process of switching data transmission or transferring an ongoing call from the current BS to the new BS is called handoff. Customer expectations from wireless networks are the same as their expectations from traditional wired networks in the sense of availability and performance [5], [8], [11], [15]. Wireless and mobile communication systems are prone to failures. Due to various reasons wireless cellular systems encounter failures caused by software, hardware, human error, or a combination of these factors [14], [15]. Failure of a system depends on the system’s nature and it has different impacts on the system performance. In wireless cellular systems, handoff failures cause the degradation of the system performance. The system may not support the active MSs efficiently. Hence, the system performance may degrade. In order to overcome this problem, availability and performance issues of these systems should be considered together. From the designer and operators point of view, it is necessary to take failure and repair factors into account. Traditional pure performance models ignore failures and recoveries and such an approach is optimistic since it assumes that underlying structure never fails. The system availability model, which is associated with failures and repairs, should be taken into account in order to obtain more realistic performance measures of the systems. The proper modelling methodology which considers both performance and availability models together is called performability modelling and evaluation [4], [5], [6]. The high expectation of performance measures makes the perfromability modelling and evaluation of wireless cellular systems a challenging task. Handoff and unavailability of an idle channel are main factors that affect the wireless cellular systems’ performances. The concept of performability modelling of handoff schemes, analysis of wireless cellular networks, model construction, solution approaches applications, and some general framework studies are considered in [4], [5], [6], [13], [14] and [15]. In [8], pure performance modelling of wireless cellular systems with handoff phenomena is considered. The approach assumes that the system is available at any time. The failures and repairs are not taken into account. Queuing theory is commonly used in performance evaluation. In the queuing theory, under certain assumptions, simple, exact product form solutions are available for the performance evaluation of such systems [2]. It is more difficult to derive product form solutions for performability evaluation because of the irregularities caused by the failures of the servers [2]. One widely applied approach, for performability modelling is to use hierarchical modelling techniques, where the performance and availability models are Approaches to Modelling and Analysis for Performability Evaluation of Handoff Schemes in Wireless Cellular Networks ISBN: 978-1-902560-22-9 © 2009 PGNet