802.11g Mac Layer Fast Handoff Algorithm Design And Experimental Research

Combining the high rate of computer LAN and the convient of wireless communications, WLAN (wireless LAN) has been grown quickly from the late 1990s. The series standards of IEEE 802.11 have been occupied the main position of the WLAN, because they are simple, normal, good compatible, etc. As the strong needs of WLAN, it needs to construct wireless networks by arranging many APs (access points) to creat an omnipresent mobile environment. Mobile user may move between APs, in which a technology problem called handoff occurs. It is proved that handoff has critical influence to the quality of networks, such as reliability, security and real time flow's continuity. But the present IEEE 802.11 b/g is of inadequate support for it. It is rather valuable to research the WLAN handoff problems.In this paper, the 802.11b protocol and MAC layer handoff process were analyzed. Considering the factors what cause the latency and packet loss, the improved methods called preferential selective scanning and parallel algorithm using dual wireless cards were presented.A daemon C program in which implements scanning and MAC layer handoff was accomplished based on Linux with Wireless Tools' APIs. Under the seamless wireless networks and open authentication, some tests were made to analyze the algorithm by using this program.The total MAC handoff time using preferential selective scanning is around 95ms compared with 250-350ms of original full channels scanning. And the single channel scanning which can use in some special situations is about 35ms. Dual wireless cards algorithm is simulated by direct handoff using cache AP's information, and the time is around of 15ms.The results show that the improved algorithm can enhance the speed of MAC layer handoff.