The Study Of The WLAN Active Control Strategy Driven By The Wireless Access Point

Recently the wireless LAN has been widely deployed and studied, especially for the ones under the IEEE 802.11 standard. Through the experiments and the analysis of the previous studies, three major factors are thought to inhibit the WLAN to be more efficient, they are transmission collision, transmission error, and handover between different wireless access points. Accompanying them the shortcoming of congestion control in upper protocol also damages the performance of WLAN. In this paper, the active control strategy that is driven by the wireless access point is proposed to resolve these problems.The active control strategy introduces a mechanism to exchange information and instructions between the wireless access point and the wireless terminals, by this mechanism wireless access point can make an integral view of transmissions on the wireless link. After that, the wireless access point adjusts the content windows of the wireless terminals according to the competition intensity on the wireless link to resolve the collisions and avoid being idle for a long time. By this means, the utilization of the wireless bandwidth is preserved.The active control strategy constructs a mathematical model for the transmission error to get their characteristics. Based on the analysis of the burst of the transmission error the wireless access point can choose a suitable frame length for the wireless terminals taking account of the existence of transmission error. On the other hand, the wireless access point keeps detecting the interference around, if the existence of some interference is verified, the wireless access point will change the channel dynamically to keep a good working condition.The active control strategy put eyes on the motion trend of the wireless terminals, so it can deduce the moving direction of the wireless terminal and wireless access point that may accept the association with handovering wireless terminal. After knowing some wireless terminal will handover, the wireless access point can inform the upstream router to adjust the forwarding speed for that wireless terminal and inform that wireless terminal the available channels around to shorten the probe time. While the wireless terminal is searching the new wireless access point, the packet relay between the wireless access points and the parallel forwarding of the packets from the upstream router to more than one wireless access point can reduce the packet loss during the handover and protect the performance of upper protocol accordingly.The active control strategy uses two methods to improve the performance of upper protocol. The wireless access point provides the local information about the WLAN actively to reinforce theend-to-end congestion control on the wireless terminals. At the same time, the wireless access point detects the change of the disposal capability of the WLAN, and notifies the upstream router to adjust the forwarding speed for one stream, or one wireless terminal, or a whole wireless link when a counteraction for the potential performance degradation is needed.The communication protocols used by the above mechanisms are also described in this paper to complete the description of the active control strategy. After that, an elementary verification of the effect of the active control strategy is described next, which is carried out with the network simulator (NS-2) including three items. The items include the promotion of the bandwidth utilization under the MAC protocol, the alleviation of performance degradation during the handover, the enhancement of congestion control. The results of the simulations are positive as expected.