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標題: IEEE 802.11 無線網路漫遊中隨機掃描之改良
Improvement of Random Scan for IEEE 802.11 Wireless LAN Roaming
作者: 辛偉豪
Hsin, Wei-Hao
關鍵字: Wireless local area networks (WLANs);無線區域網路;Roaming;Random Scan;漫遊;隨機掃描
出版社: 電機工程學系所
引用: [1] 蔣大偉, “802.11無線網路技術通論第二版,” 歐萊禮, 2006. [2] 柯志亨, “計算機網路實驗:以NS2模擬工具實作,” 學貫行銷, 2007. [3] 張詠盛, “IEEE 802.11 無線網路漫遊之隨機掃描,” 國立中興大學, 2009 [4] 賴彥呈, “IEEE 802.11無線網路通道品質預測之漫遊決策,” 國立中興大學, 2006. [5] 盧彥霖, “IEEE 802.11 無線網路以競爭視窗為基準之漫遊決策,” 國立中興大學, 2007. [6] 黃昭憲, “IEEE 802.11 無線網路以工作站判決之漫遊決策,” 國立中興大學, 2008. [7] 梁世聰, “支援IEEE 802.11基礎架構式無線區域網路模擬器之設計與建置,” 台北市立師範學院, 2004. [8] Kevin Fall and Kannan Varadhan, “The ns Manual”, 2006. [9] “AWK Tutorial Guide, version 2.2”, [10] “The ns Manual”, [11] “Tutorial for the Network Simulator ns”, [12] Haitao Wu, Kun Tan, Yongguang Zhang, “Proactive Scan: Fast Handoff with Smart Triggers for 802.11 Wireless LAN,” 26th IEEE International Conference on Computer Communications, pp.749-757, 2007. [13] Ishwar Ramani, Stefan Savage , “SyncScan:Practical Fast Handoff for 802.11 Infrastructure Networks,” Annual Joint Conference of the IEEE Computer and Communications Societies, vol.1, pp.675-684, 2005. [14] Yuh-Shyan Chen, Ming-Chin Chuang, and Chung-Kai Chen, “DeuceScan: Deuce-Based Fast Handoff Schemein IEEE 802.11 Wireless Networks,” IEEE Transactions on Vehicular Technology, vol. 57, pp.1126-1141, 2008. [15] “IEEE 802.11 WG, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification,” Standard, IEEE, 1999. [16] 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. [17] “NS by Example”,
IEEE 802.11 標準的制定降低了無線網路裝置的製造成本,使得無線網路能提供有效率的服務。無線網路透過網路擷取點在空間上的建置而擴充為涵蓋範圍廣闊的網路,當移動式的工作站在網路中往返移動時,可以藉由這個擴充網路從一個擷取點漫遊至下一個擷取點以完成無間縫漫遊。整個漫遊所造成的時間延遲可以區分為:掃描延遲、身份認證延遲、重新聯結延遲,漫遊延遲中以掃描延遲佔據最大的時間比例,而漫遊延遲愈長,服務中斷的時間也愈長。
隨機掃描(Random Scan)是一個減少IEEE 802.11無線網路之漫遊所造成服務中斷的機制。此機制將標記間隔(Beacon Interval)切割成數個區段(Section),在每個標記間隔中,只對一個區段進行掃描,剩下的時間可繼續正常的傳輸,以降低服務被中斷的時間。隨機掃描啟動時,會隨機產生通道順序(Channel Sequence)與區段順序(Section Sequence)以掃描通道。根據通道順序與區段順序的排程,工作站切換至目標通道搜尋網路擷取點。

The IEEE 802.11 standard has enabled low-cost and effective wireless local area network (WLAN) services. Due to its limited communication range, we need to deploy multiple access points (APs) to cover a large geographical area. To roam from one AP to another seamlessly, roaming procedure is necessary and it results in roaming delay. The delay can be divided into three parts: channel scan, authentication, and re-association. In these three parts, the channel scan time is the longest.
Random scan is a new channel scan method to reduce the delay. The scheme divides the beacon interval into several sections. In each beacon interval, only one section will be scanned and it will take multiple beacon intervals to scan all the section to cover the entire beacon interval. The section sequence is randomly generated.
In the thesis, we improved on the original random scan. The first improvement is to utilize the results from the previous scan in generating section sequence. From the results of the previous scan, we put the section where the AP is found to the first place of the section sequence. The second improvement is the removal of random scan threshold. Through experiments, we found that the setting of the random scan threshold depends on the speed of the mobile stations. A better alternative is to remove the threshold and scan the channels continuously. In simulations, we found that the new scheme improved the performance of random scan.
其他識別: U0005-2607201012563900
Appears in Collections:電機工程學系所

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