臺灣博碩士論文加值系統

無線通信在現代生活中扮演越來越重要的角色。電氣與電子工程師協會(IEEE) 所制定的無線區域網路標準 (IEEE 802.11 Standard)，已逐漸成為現行的準無線網路解決方案。此標準在媒介存取子層 (MAC) 層所使用的”載波偵測多重擷取與衝撞避免 (CSMA/CA)”技術中，解決訊框重送問題 (retransmission) 的方法─二元指數撤退機制 (binary exponential backoff) ─會造成很大的訊框延遲 (frame delay) 和傳輸延遲變異值(jitter value)，這將使對其敏感的即時資料 (real-time traffic) 傳輸造成困擾。本篇論文針對此問題提出一個以雜亂訊號干擾為基礎的重送機制 (jamming-based retransmission mechanism) 以解決IEEE 802.11無線網路上即時資料傳輸的問題，實作上本機制可相容於傳統IEEE 802.11裝置，並能有效減少訊框延遲和訊框傳送變異值，當網路的交通量 (traffic load) 變重時，本方法也能表現穩定不受影響。
本論文首先探討IEEE 802.11中負責媒介存取控制子層協定改進研究的工作群組 (task group E) 所提出的一些改良協定，接著詳述我們所提出的改進方式並討論其中參數的最佳化。透過模擬實驗我們比較我們的方法與task group E 提出的兩個方法 ─ VDCF及TCMA，結果顯示我們的方法能針對即時資料在訊框延遲 (delay)，傳輸延遲變異值 (jitter value)，訊框丟棄率 (dropping rate)，頻道使用率 (channel utilization) 等方面有良好的表現。

The Medium Access Control (MAC) protocol of the IEEE 802.11 standard is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The basic retransmission mechanism, binary exponential backoff, may cause large packet delay and packet delay jitter that are not suitable for real-time traffic. In this thesis, we will first investigate some MAC enhancement mechanisms discussed in the IEEE 802.11 task group E, which was formed for enhancing the current 802.11 MAC protocol to support for applications with Quality of Service (QoS) requirements. Then, we will propose a jamming-based retransmission mechanism that is compatible with the 802.11 standard and could reduce the packet delay for real-time traffic. Besides, this mechanism performs stably when the traffic load is heavy. The optimal setting of our proposed mechanism is discussed analytically. We perform simulated experiments by comparing our proposed retransmission mechanism with the other two mechanisms discussed in the 802.11 task group E. We show that the jamming-based retransmission mechanism can reduce the packet delay and the packet dropping rate.

1. Introduction 1
1.1 The Wireless Local Area Networks (WLANs) …………1
1.2 Collision Resolution Schemes in WLAN ………………2
1.3 MAC Enhancement for QoS equirement …………………3
1.4 Thesis Organization ……………………………………4
2. Backgrounds and Related Works 5
2.1 IEEE 802.11 MAC Protocol ……………………………5
2.2 MAC Enhancements in IEEE 802.11 Task Group E ……11
2.2.1 Virtualized Distributed
Coordination Function (VDCF) ………………………12
2.2.2 Probabilistic Distribution
Coordination Function (pDCF) ……………………14
2.2.3 Tiered Contention Multiple Access (TCMA) ……17
3. Proposed Mechanism 19
3.1 The Jamming-Based Retransmission Mechanism ……19
3.2 Parameters Optimization ……………………………23
3.3 The Frame Format and State Diagram …………………27
4. Simulation Results 30
4.1 Performance Evaluation …………………………………30
4.2 Simulation Result ………………………………………33
5. Conclusions and Future Works 36

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