The Research On Handoff Schemes In Wireless LANs

With the popularity of 3G applications, the WLAN which complements the 3G technology at the access rate and the adaption to the environment develops rapidly, it has become a new generation of high-speed wireless access network, and comes to the fore in the 3G's supplementary technologies. At the same time, With the development of the multimedia technology, the application of multimedia is spreading widely, and people become desiredly demanding on multimedia information which including image, video and etc al. The handoff schemes employed in the WLAN based on 802.11 protocol can no longer meet the strict delay constraints of multimedia applications, so the study on handoff mechanism which meet to the needs of the multimedia has become a very key issue in providing seamless mobile services.In the thesis, we firstly reviewed the development and state-of-art of the handoff in the WLAN based on 802.11 protocol, and made performance evaluation and comparison on the exsiting handoff schemes. These two theories provided foundation stone of researches behind. Then we researched on the smooth handoff algorithm. The algorithm reduces the packet loss rate during the handoff and meet he needs of multimedia applications. However, its showcoming is not adapting to the varied network environment. In order to improve the algorithm, we proposed a self-adaptive and prediction-based smooth handoff(APBSH) algorithm. The key idea of the algorithm is using the varying recent network environment parameters to predict the next time′s situation and then self-adaptively choosing the suitable handoff strategy according to the prediction results to accomplish the handoff before losing the connection of the current AP and improve the quality of data service.Next, with the inspiration from the fast handoff scheme with multiple radios, combined with the features of most existing WLAN which mobile host (STA) is distribution-intensive and with small numble of data exchange, we proposed a substituted scan-based fast handoff (SSBFH) algorithm, and implemented it in the 802.11 protocol. The algorithm uses the spare capacity of the STA instead of STA which need to handoff to complete the scan work, which almost completely eliminates the scanning stage, reduces the handoff delay greatly.Finally, with depth study of network simulation tool NCTUns, we added the adaptive predictive-based smooth handoff module and the substituted scan-based fast handoff module into NCTUns network simulator and separately evaluated the performance of two algorithm in NCTUns simulations. The experiment showed that APBSH algorithm could accommodate with different network environments, assure the optimization of the handoff′s performance and better than standard 802.11 handoff algorithm and smooth handoff algorithm, while the SSBFH algorithm which is applicable to the network which is dense-distributed and has some unbusy STAs, could not only reduce the handoff delay, but also make good use of the network resources.