我們目前使用在無線網路的IEEE 802.11 DCF機制，因為不能像有線網路一樣，完全了解網路上的情形，所以使用CSMA/CA方法，並且因為使用共享傳輸媒介的原因，造成節點在傳送時的競爭和碰撞，也影響的封包的傳送到達時間，增加了封包的延遲，延遲常常對於即時資料流是非常重要的，可能因為封包的過度延遲，造成即時應用程式品質的降低。而DCF也沒有對於這個問題有補償或者解決的方法，所以後來制定的IEEE 802.11e EDCF提供了多種優先權，利用差異式服務(DiffServ)的方式，讓即時資料流的品質可以獲得改善。但是對於具有變動性的隨意式網路，EDCF仍會顯得不足，所以本論文提出動態競爭控制機制(DCC)，利用無線網路上MAC層的ACK，和反應式(Reactive)路由協定的封包，來測量傳送路徑上每個跳躍的延遲，並且利用這些訊框和封包格式，來累積路由路徑上所有跳躍的延遲，接著我們就可以藉此評估端點對端點所花費的時間，也就可以知道哪些封包很有可能會遲到，需要調整優先權，或者應該在退後程序(Backoff Procedure)中，依照傳送所花費的時間，來給予合適的後退時間(Backoff Time)。我們藉由這些機制，就可以讓網路上傳送的即時封包降低延遲，增加單位時間內符合延遲要求的封包數量，即時應用程式的品質也可以獲得更大的改善。最後我們和制定中的IEEE 802.11e Draft的EDCF，和其他與本論文目的相似的研究AEDCF作比較，透過NS-2模擬的方式，驗證我們提出的機制確實可以改善即時資料流的品質。

IEEE 802.11 has become the standard in wireless LAN. Originally, 802.11 is designed for the best-effort services only. To support the increasing demand of delay-sensitive applications, IEEE 802.11 Task Group E is developing a QoS-aware MAC protocol, EDCF, for differentiated services. However, when the network becomes congested, there exists unexpected packet delay due to collisions and retransmissions. This thesis proposes a dynamic contention control (DCC) scheme to reduce packet delay and increase the percentage of packets arriving in time. DCC estimates per-hop delay, denoted as Mrtt, and end-to-end delay, denoted as Sigma_t, based on either the received MAC-layer ACK or the control packets of a reactive routing protocol. Then, Mrtt and Sigma_t are used to dynamically adjust the associated contention window for each priority. Besides, when a frame is retransmitted, the backoff time is determined according to the remaining end-to-end delay instead of a uniformly distributed random number. For the propose of evaluation, we perform simulations on the well-known network simulator, NS-2. DCC is compared with the EDCF and one previously proposed scheme, AEDCF. The simulation results demonstrate the effectiveness and superiority of DCC.

第一章 導論 1
1.1 研究動機 1
1.2 研究方向與模擬 2
1.3 章節介紹 3
第二章 無線網路服務品質的背景與相關研究 4
2.1 IEEE 802.11和IEEE 802.11e機制簡介 4
2.2 改善IEEE 802.11e MAC服務品質的機制 6
2.2.1 修改EDCF的傳送程序 6
2.2.2 頻寬保留和允許控制的機制 7
2.2.3 差異式服務的機制 8
2.3 差異式服務的機制的比較 10
2.4 本論文的動態競爭控制機制 12
第三章 動態競爭控制的更新式EDCF 13
3.1 動態競爭控制機制 13
3.1.1 名詞定義解釋 14
3.1.2 Mrtt和Sigma_t的計算方式 15
3.1.2.1 路徑完整時的計算方式 15
3.1.2.2 路徑發生斷裂時的計算方式 19
3.1.3 Non-Uniform Random Backoff Time的計算 22
3.2 運作流程 25
第四章 模擬分析及結果量測 30
4.1 與DCC相關的NS-2模組架構 30
4.2 模擬環境與網路拓樸 33
4.3 模擬結果與分析討論 35
4.3.1 變動性較小的模擬網路拓樸 35
4.3.2 變動性較大的模擬網路拓樸 39
4.4 保障高優先權資料流的架構 43
4.4.1 模擬結果與分析 44
第五章 結論與未來工作的方向 46
5.1 結論 46
5.2 未來工作 47
REFERENCES 48
INDEX 52

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