臺灣博碩士論文加值系統

IEEE WLAN工作小組在1997年制訂了IEEE 802.11標準。它定義了兩種不同操作類型的無線區域網路基本架構：有基礎架構的無線區域網路(Infrastructure Wireless LAN)與無基礎架構的無線區域網路(Ad-Hoc Wireless LAN)。其中，DCF是IEEE 802.11 MAC所採用的基本媒介競爭機制。
DCF可同時用在無基礎無線區域網路架構與有基礎無線區域網路架構中，主要是利用載波偵測多重擷取及衝撞避免(Carrier-Sense Multiple Access/Collision Avoidance，簡稱CSMA/CA)與隨機後退時間機制(Random Backoff Scheme)來提供工作站傳送與接收資料。
雖然DCF利用了以上的技術，也確實可以避免一些傳輸上的碰撞，但是，有時候卻也造成工作站等待時間過長。每當大量的工作站同時要競爭通道的存取權時，容易產生相同的後退時間，而導致碰撞；碰撞後，又必須等待一段時間才可再進入競爭週期，所以常常會耗費許多等待的時間。所以，如何在面臨大量的工作站時，產生適合的競爭視窗值，來減少工作站發生碰撞的情形，實在是一個很重要的課題。
本論文提出一個新的競爭視窗的產生方法，來減少發生碰撞的機會。主要的想法是針對通道處於一個較高負載的情況之下，容易產生相同的後退時間，發生碰撞。而在碰撞之後，便採用我們所提出的方法，達到減少碰撞發生的機會，提升整體的傳輸效能。

IEEE WLAN work team established IEEE 802.11 standard in 1997. It defined two different operation types of Wireless LAN - Infrastructure Wireless LAN and Ad-Hoc Wireless LAN. Among these systems, what basic media competitive scheme IEEE 802.11 MAC uses is DCF scheme.
DCF can be used in Infrastructure Wireless LAN and Ad-Hoc Wireless LAN simultaneously, and it utilizes Carrier-Sense Multiple Access, Collision Avoidance (can be abbreviated as CSMA/CA), and Random Backoff Scheme to make station to transmit and receive information.
Although DCF uses the techniques mentioned above may avoid some collision during the transmission, it sometimes also causes stations waiting for a long time. Every time when a lot of workstations compete for accessing the channel, it is easy to create the same backoff time which may produce collision; however, after collision, repeatedly, it needs to take a long time to get into the competitive cycle. For this reason, how can we create a compatible contention window to reduce the collisions when facing the great amount of stations is a important study.
In this paper, we provide an approach of generating a new contention window to reduce the opportunities of collisions happened in the systems. The main idea is aiming at that, under the channel condition of heavy load, stations produces the same backoff time and collides easily. After the collision occurs, we may adopt approaches we assumed to lessen the opportunities of it and to promote the performance of whole transmission.

誌謝……………………………………………………………………ii
摘要……………………………………………………………………iii
ABSTRACT………………………………………………………………iv
目錄……………………………………………………………………v
圖目錄…………………………………………………………………vi
表目錄…………………………………………………………………vii
第一章 緒論…………………………………………………………1
1.1 研究動機………………………………………………………1
1.2 研究目的………………………………………………………2
1.3 論文架構………………………………………………………2
第二章IEEE 802.11無線區域網路架構……………………………3
2.1 無線網路與有線網路的環境差異性…………………………3
2.2 無線區域網路基本架構………………………………………4
2.3 碰撞偵測機制…………………………………………………7
2.4訊框間隔(Inter-Frame Space)………………………………8
2.5 IEEE 802.11 MAC層.…………………………………………9
2.5.1分散式協調機制…………………………………………10
2.5.1.1後退機制(Backoff Scheme)………………………11
2.5.1.2 DCF範例……………………………………………12
2.5.2 集中式協調機制…………………………………………14
2.5.3 隱藏節點(Hidden Nodes)的問題………………………15
2.6 總結……………………………………………………………16
第三章 競爭視窗之線性調整………………………………………17
3.1同步化掃描........……………………………………………17
3.2 同步化掃描的分析討論…………………………………………20
3.3 競爭視窗的滑動值與通到預估值………………………………21
第四章 模擬結果與討論……………………………………………25
4.1模擬平台介紹…………………………………………………25
4.2參數設定……… …………………………………………….26
4.3比較……………………………………………………………27
4.4總結……………………………………………………………36
第五章 結論與未來展望……………………………………………37
參考文獻……………………………………………………………38

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