臺灣博碩士論文加值系統

近年來隨著IEEE 802.11無線網路的發展，目前搭載無線上網功能的行動裝置已經非常普及。透過網路存取點（AP，Access Point）在空間上的建置所形成涵蓋範圍廣闊的網路，讓移動式的無線上網裝置在網路中往返移動時，可以藉由這個擴充網路從一個存取點漫遊至下一個存取點以完成無間縫的漫遊。而整個無線漫遊所造成的時間延遲可區分為三個部分：掃描延遲、身份認證延遲、和重新連結延遲。漫遊延遲中則以掃描延遲佔據最大的時間比例，所以漫遊延遲如果愈長，服務中斷的時間也會愈長。
隨機掃描（Random Scan）為一個減少IEEE 802.11無線網路漫遊所造成服務中斷的機制，此機制將指標信號間隔時間（Beacon Interval）切割成三個區段（Section），在每次的指標信號間隔時間中，只針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。隨機掃描開啟時，會隨機產生通道順序（Channel Sequence）與區段順序（Section Sequence）來掃描通道，工作站（Station）就根據產生的通道順序與區段順序，依排程切換至目標通道來搜尋存取點。
延續隨機掃描的漫遊機制，本論文將針對被動式掃描與隨機掃描進行換手模擬的研究，比較出兩者的差異，並且利用設定提前隨機掃描門檻的方式將通道掃描的時間提前到真正決定漫遊之前，稱之為提前執行的隨機掃描，以期解決隨機掃描在較高移動速率時所遇到通道掃描時間不足的問題，最後在不同移動速度之下探討三者漫遊的效能，證實隨機掃描機制能有效縮短通道掃描時間而且在較高的移動速率之下提前執行的隨機掃描更能快速有效的進行漫遊。

With the development of IEEE 802.11 wireless network in recent years, mobile devices equipped with wireless network capability have become very popular. The wide deployment of wireless network access points (AP) forms a wireless network with very large coverage. To enable a mobile device connecting to the wireless network uninterruptedly, we need fast and reliable handoff procedures between AP’s. The handoff delay can be divided into three parts: channel scan, authentication, and re-association. Among these, the channel scan time accounts for the largest proportion of handoff time.
Random scan is a channel scan method in IEEE 802.11 wireless networks design to reduce the channel scan time. It divides a beacon interval into three sections. In each beacon interval, one section in randomly selected and a randomly selected channel is scanned for available AP’s. Normal transmission can be conducted in the remaining sections. It takes three beacon intervals to scan all the sections. The sequences for channels and sections are randomly generated. Each station switches to the target channel according to the sequences.
To extend the studies of random scan, this thesis focuses on handoff simulation of random scan and passive scan and compares the differences between the two. To alleviate the problem that scanning time may not be enough in random scan when the moving speed is high, we add a threshold to activate channel scan before handoff is required. In simulations, we find that random scan can reliably find suitable AP to switch and significantly shorten the channel scan time at various speeds.

誌謝 i
摘要 ii
Abstruct iii
目次 iv
表目次 vi
圖目次 vii
第一章 緒論 1
1.1　簡介 1
1.2　動機與目的 1
1.3　論文架構 2
第二章　802.11無線區域網路 3
2.1　無線區域網路簡介 3
2.2　無線區域網路的基本元件 5
2.3　無線區域網路的基本架構 6
2.4　無線區域網路的系統服務 7
2.4.1　分散式系統服務 8
2.4.2　工作站服務 9
2.5　IEEE 802.11 MAC協定 10
2.5.1　集中式協調機制(PCF) 11
2.5.2　分散式協調機制(DCF) 12
第三章 802.11無線區域網路的漫遊程序 15
3.1　漫遊程序簡介 15
3.2　被動式掃描的機制 17
3.3　隨機掃描的機制 19
3.3.1　隨機掃描的工作原理 19
3.3.2　隨機掃描的實際例子 21
3.3.3　隨機掃描的流程圖 23
3.3.4　提前執行的隨機掃描 24
第四章 隨機掃描實驗模擬 26
4.1　Network Simulator ns-2介紹 26
4.2　模擬環境之設定 27
4.3　實驗模擬 29
第五章 結論與未來展望 40
參考文獻 41

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[16] “Tutorial for the Network Simulator ns”, http://www.isi.edu/nsnam/ns/tutorial/index.html.

[17] “NS by Example”, http://nile.wpi.edu/NS/.

[18] “The ns Manual”, http://www.isi.edu/nsnam/ns/doc/ns_doc.pdf.

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