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標題: IEEE 802.11 無線網路以競爭視窗為基準之漫遊決策
IEEE 802.11 Wireless LAN Handoff Decision based on Contention Window
作者: 盧彥霖
Lu, Yen-Lin
關鍵字: Wireless LAN;無線網路;Handoff Decision;Contention Window;Congestion Status;漫遊決策;競爭視窗;壅塞狀況
出版社: 電機工程學系所
引用: [1] 賴彥呈, “IEEE 802.11 Wireless LAN Roaming Based on Channel Quality Estimation ,”國立中興大學,2006. . [2] C. Siva Ram Murthy and B. S. Manoj,“Ad Hoc Wireless Networks Architectures and Protocols,” Prentice Hall PTR, 2004. [3] Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach,” Morgan Kaufmann Publishers, 2003. [4] Fred Halsall ,“ Data Communications, Computer Networks and Open Systems,” Addison-Wesley Publishing Company 4th Ed,1996 . [5] Mattbew S. Gast ,“ 802.11 Wireless Networks,” O’Reilly , 2006 [6] Kevin Fall and Kannan Varadhan, “The ns Manual”, 2006. [7] IEEE 802.11 WG, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification, Standard, IEEE, 1999. [8] shwar Ramani, Stefan Savage , “ SyncScan:Practical Fast Handoff for 802.11 Infrastructure Networks,” Proceeding of the IEEE infocom Conference, Miami, FL, vol.1, pp.675-684, 2005. [9] Yang Xiao, “Concatenation and Piggyback Mechanisms for the IEEE 802.11 MAC,” Wireless Communications and Networking Conference, vol. 3, pp.1642-1647, 2004. [10] Yang Xiao,“ Performance Analysis of Priority Schemes for IEEE 802.11 and IEEE 802.11e Wireless LANs,” IEEE Transactions on Wireless Communications, vol. 4, pp.1506-1515, 2005. [11] Hongqiang Zhai, Xiang Chen and Yuguang Fang,“ How Well Can the IEEE 802.11 Wireless LAN Support Quality of Service,” IEEE Transactions on Wireless Communications, vol. 4, pp.3084 – 3094, 2005. [12] G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function,” IEEE Journal on Selected Areas in Communications, vol. 18, pp.535-547, 2000. [13] Ping-Jung Huang, Yu-Chee Tseng and Kun-Cheng Tsai, “A Fast Handoff Mechanism for IEEE 802.11 and IAPP Networks”, IEEE Vehicular Technology Conference, pp.966 – 970, 2006. [14] Seongkwan Kim, Sunghyun Choi, Se-kyu Park, Jaehwan Lee and Sungmann Kim, “An Empirical Measurements-based Analysis of Public WLAN Handoff Operations,” First International Conference on Communication System Software and Middleware, pp.1-6, 2006. [15] Jung-Ryun Lee, Sang-Wook Kwon, and Dong-Ho Cho, “Adaptive Beacon Listening Protocol for a TCP Connection in Slow-Start Phase in WLAN,” IEEE Communications Letters, vol. 9, pp.853-855, 2005. [16] “AWK Tutorial Guide, version 2.2”,,中央研究院計算中心. [17] “Tutorial for the Network Simulator ns”, [18] Ns version 1 -LBNL Network Simulator.
IEEE 802.11無線區域網路中, Access Point (AP)是用來連接無線網路跟有線網路,在AP訊號範圍內的工作站必須先跟AP註冊才能透過AP存取有線網路的資源。一般來說當連線訊號減弱時也意味著連線品質變差,而在原本802.11的漫遊決策機制中,工作站僅會根據AP訊號強度的大小,在目前連結的AP訊號衰弱到某一個臨界值時,才會開始掃描尋找其它AP的訊號,並且要超過一個標準值後才會確定漫遊的目標,但是AP信號的強弱,並不能顯示這個AP範圍內通道忙碌的情況,也就是說當工作站漫遊至訊號最強但範圍內通道情況為忙碌的AP,可能因碰撞率的增加而使訊號連線品質不佳,所以我們提出一種新的漫遊決策機制,稱為以競爭視窗為基準之漫遊決策機制,將壅塞狀況納入考量,來使工作站能在保持連線品質的狀態下進行漫遊。
在無線環境中,當碰撞率越高的時候,連線的品質越差,然而直接計算碰撞率來估測壅塞狀況,在現實上顯得比較複雜,因為要使用原本80.11 MAC中不具有的參數來做計算,所以我們提出一種壅塞狀況估測機制,使用802.11MAC原本具有的參數競爭視窗(Contention Window)來取代直接計算碰撞率的方式,即時性的估測出壅塞的情況。在此機制下,所有的工作站會週期性的偵測目前壅塞的情況,並且在傳輸的過程中將目前偵測到的壅塞狀況的資訊附加在封包中傳送出去,當附近的AP收到這些訊息後,就可以了解目前通道的使用狀況,再利用同步化掃描的方式,將它所收集到的通道的資訊附加在Beacon的封包中,於是當某個工作站進入無線傳輸的範圍內,希望跟附近的AP註冊的時候,就可以利用Beacon封包內的資訊,選擇一個壅塞狀況比較好的AP來漫遊,從而完成整個以競爭視窗為基準之漫遊決策。

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.
其他識別: U0005-0808200714112900
Appears in Collections:電機工程學系所

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