| Summary: | 碩士 === 國立中興大學 === 電機工程學系所 === 95 === In IEEE 802.11 Wireless Local Area Network (WLAN), an access point (AP) is used to connect wireless network to wired network. Before a station is allowed to send a data message via an AP, it should first become associated with the AP when the signal strength of the AP decreases, the channel quality of service would decrease as well. In the original 802.11 handoff decision, the station will select the best AP based on the measured SNR from the probe response packets received from each AP. After a station is associated with an AP, it will wait until the signal of the AP drops to a threshold and start to rescan for new AP’s. However, the measured SNR from the probe response packets received from each AP can not show whether the channel is busy or not. It means that if the station selects an AP that has the best SNR but high contention rate, the station may not receive the best service due to the high collision rate. In this thesis, we present a new 802.11 handoff scheme based on Contention Window and make the station roam with proper quality of service by considering the channel congestion status.
In a wireless environment, as the collision rate increases, the channel quality of service would decrease. However, it is more complicated to estimate the congestion status by directly computing the collision rate which is not a parameter in 802.11 MAC. So, we propose a new congestion estimation scheme by using Contention Window instead of using the collision rate directly.
In our proposed handoff scheme, each of the stations will estimate the channel congestion status periodically and piggyback its channel information with the transmission packets. After AP receives the information, it would have a complete picture of the channel utilization. Then, AP will use SyncScan mechanism to send the channel information to the stations with the Beacon frame. Finally, when a station wants to associate with a new AP, it could choose the AP with the best channel congestion status according to the information it received and then complete all the process of our proposed handoff decision based on contention window.
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