Enabling And Implementing Fast Handoffs In AP-dense 802.11 WLANs

The technology of Wireless Local Networks (WLAN) was given birth at the end of the 20th century when computer networking and wireless telecommunications came together. It makes use of radio waves and wireless channels to enable mobile, personal and multimedia communications through wireless access services. It is also one of the most efficient ways for broadband wireless connections.So far the 802.11 protocol family has successfully evolved into the de facto standards of high speed WLANs and enjoyed wide applications. 802.11-based WLANs are made up of many access points (APs). Due to limited transmission power, every AP can only cover a small vicinity, normally under 100m in radius. However, wireless clients may freely roam in the area covered by the WLAN. So all APs should cooperate to provide the client with seamless access services. Unfortunately clients'mobility was not taken into consideration when 802.11 protocols were first designed. So a large handoff delay is to be perceived, which may greatly compromise such real-time sessions as streaming or VoIP. In this light, it's of great significance to reduce the handoff delay to an acceptable level.On the other hand, a trend of high density of APs occurs in recent 802.11 WLANs. AP-dense 802.11 WLANs show many different features than traditional 802.11 WLANs. Especially it's the high density of APs that enables high quality handoffs. Nevertheless, high AP density also leads to many challenging problems for smooth handoffs, such as high physcial interference, intense MAC contentions and hard AP selections. In this paper, the author has made a thorough study of the handoff problem through extensive reading of relevant literature and an analysis of real data. 4 technqiues to accelerate the handoff process in 802.11 WLANs, especially in AP-dense ones, are then proposed or enhanced: Multiple-AP -connecion reduces the handoff delay by establishing backup wireless connections in the background; Sniff-Scan and Privileged Probe Reponse Scan greatly enhance the efficiency in obtaining useful information of nearby APs; sub-atomic activities enable parallel actions and processes in the MAC layer. Moreover, in an effort to better ensure the continuity of IP layer communications, an improvement of the fast IP acquisition first introduced in [1] is also proposed.In order to evaluate the performance of the proposed techniques, the author and his team have implemented them on either Linux or Windows platform. The test results demonstrate that all the techniques have lived up to their original design objectives, and their synergy has successfully resulted in a smooth handoff in the IP layer and below.