QoS provisioning in handoff algorithms for wireless LAN

The performance of the handoff algorithm is the primary concern for supporting mobile devices with roaming capability in wireless communications. The IEEE 802.11 standard for wireless local area networks (LANs) recommends carrier sense multiple access with collision avoidance (CSMA/CA) as the medium access control (MAC) protocol. The contention-based CSMA/CA access mechanism is sensitive to the traffic load whereas the assignment-based access mechanism used by wireless telephone networks is not. Heavy traffic load in wireless LANs causes significant degradation of network performance. Thus conventional handoff algorithms employed in wireless telephone networks are not applicable to wireless LANs. In this paper, after examining existing handoff algorithms for wireless communications, we propose a handoff algorithm for wireless LANs. This algorithm considers traffic load as an important factor for initiating handoff. In normal traffic load, signal strength measurements from various access points are used the same as conventional handoff algorithms. However, in heavy traffic load, the hysteresis margin and the absolute threshold of traffic load are taken into account. A simulation model is developed to study the performance of the proposed algorithm. Simulation results indicate that the proposed algorithm significantly reduces the call dropping probability. Moreover, the relationship between QoS parameters and handoff initiation criteria is analyzed. Analytical results show how handoff initiation criteria might be set in accordance with the quality of services requested by users.