this paper, we focus on methods of fault tolerance, and investigate the differences between hardware fault tolerance and software fault tolerance. 1.2 Fault, Error and Failure

Abstract: Problem statement: The next generation wireless systems should be designed to support heterogeneous traffic with seamless mobility. A single network alone cannot cope up with such heterogeneous requirements. Hence it is desirable to interoperate between diverse and complementary Radio Access Technologies (RATs). In such system, user will switch between different Radio Access Technologies (RATs) to satisfy the User/application requirement and this process is known as Vertical Handoff (VHO). The process of network switching had three phases, network discovery, handoff decision and execution. The decision phase played a crucial role in resource utilization and user/application Quality of Service (QoS) requirement. Hence it was essential to model and evaluate the handoff decision system along with VHO system model. Approach: The traditional performance models were optimistic models and would evaluate the system performance under ideal condition by ignoring failures and recovery in the system. The availability models were conservative models and would assess the availability/reliability of the system. The performabality models were combined models of performance and reliability. The performablity models were more realistic models of the system due to the simultaneous consideration of performance and reliability. Here the VHO process of a next generation wireless system was modeled and evaluated by an analytic performablity model and