A handoff method for use with mobile devices based on active polling with IEEE 802.11b

Mobile devices have become popular in the last few years. One of the main problems in wireless networks is mobility: when a user moves around in a wireless network, they go out of range of old base stations and come into range of new base stations. This means that the user has to attach to different base stations as the user moves. This procedure is called a handoff. Many different methods of how to find and attach to new base stations have been developed during the years. However, as new standards are introduced, new methods are needed in order to take full advantage of the new standards. The purpose with this thesis is to develop, and evaluate, a handoff algorithm for IEEE 802.11 networks based on active polling. An algorithm was developed which uses results from active scanning to predict the best access point. The access point is selected based on the change rate of the signal strength and the actual signal strength. This allows a prediction about which access point will continue to be the best one until the mobile node has passed it. Simulations were done for ICMP echo requests and replies as well as TCP traffic in a IPv6 network with support for Mobile IPv6. It was not possible to simulate UDP due to limitations on the simulation software. The simulations shows that the packet loss caused by the active scanning is higher in total compared to running the same simulations without performing any active polling. The algorithm leads to a lower packet loss during the actual handoff phase since there will not be a timeout before a new access point is selected. The continuous packet loss caused by the active scanning also causes TCP to trigger a restart after every scan which leads to a severe loss of throughput.Validerat; 20101217 (root