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標題: 802.11無線網路適應性公平增強分散式協調機制之競爭視窗值研究
The Study of Contention Window in 802.11 Adaptive Fair Enhanced Distributed Coordination Function
作者: 鄭啟彬
Cheng, Chi-Pin
關鍵字: 802.11 Wireless LAN;802.11無線區域網路;distributed coordination function;medium access control;Quality of Service (QoS);分散式協調機制;媒介存取控制;服務品質保證
出版社: 電機工程學系所
引用: [1] Larry L. Peterson and Bruce S. Davie, “Computer Networks”, Morgan Kaufmann Publishers, 3nd Ed. [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] Jong-Deok Kim and Chong-Kwon Kim, “Performance analysis and evaluation of IEEE 802.11e EDCF”, Wireless Communications and Mobile Computing, vol.4, pp.55-74, 2004. [4] Lamia Romdhani, Qiang Ni, and Thierry Turletti, “AEDCF: Enhanced Service Differentiation for IEEE 802.11 Wireless Ad-Hoc Networks”, IEEE WCNC’03 (Wireless Communications and Networking), vol.2, pp.1373-1378, 2003. [5] Mohammad Malli, Qiang Ni, Thierry Turletti, Chadi Barakat, “Adaptive Fair Channel Allocation for QoS Enhancement in IEEE 802.11 Wireless LANs”, IEEE International Conference on Communications, vol.6, pp.3470-3475, 2004. [6] Giuseppe Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE Journal on Selected Areas in Communications, vol.18, pp.535-547, 2000. [7] Q. Ni, L. Romdhani, and T. Turletti, “A survey of QoS Enhancements for IEEE 802.11 Wireless LAN”, Journal of Wireless Communications and Mobile Computing , vol.4, pp.547-566, 2004. [8] Qiang Ni, Imad Aad, Chadi Barakat, and Thierry Turletti, “Modeling and Analysis of Slow CW Decrease for IEEE 802.11 WLAN”, IEEE 2003 International Symposium on Personal, Indoor and Mobile Radio Communication Proceedings, vol.2, pp.1717-1721, 2003. [9] Federico Cali, Marco Conti, and Enrico Gregori, “IEEE 802.11 Protocol: Design and Performance Evaluation of an Adaptive Backoff Mechanism”, IEEE Journal on Selected Areas in Communications, vol.18, pp.1774-1786, 2000. [10] “The ns Manual”, [11] “Network Simulator (ns), version 2”, [12] “AWK Tutorial Guide, version 2.2”,,中央研究院計算中心.
在802.11無線區域網路中,分散式協調機制(Distributed Coordination Function , DCF)是主要的媒體接取控制機制,它採用了一種名為載波偵測多重存取(Carrier-Sense Multiple Access/Collision Avoidance ,CSMA/CA)的技術來進行資料傳輸。由於DCF不能提供任何服務品質的保證,這對於日漸使用頻繁的即時性多媒體資料而言,是個很大的致命傷,因此802.11小組制訂了一個可以支援服務品質的標準,名為802.11e。其中包含了一個以競爭視窗為基底的增強分散式通道存取機制(EDCA)。儘管EDCA達到了在高優先權的資料流可以得到較好的服務品質,但是也發現當工作站數目增多時,將使得EDCA的效能嚴重下降。
因此學者提出了適應性EDCF(Adaptive EDCF,AEDCF)與適應性公平EDCF(Adaptive Fair EDCF,AFEDCF)的方法改良EDCA。AEDCF是對傳輸成功後的競爭視窗值進行較為緩慢的更新,稱為緩慢後退(Slow Decrease,SD)機制,以改善負載較大時的傳輸效率。AFEDCF則是利用適應性快速後退機制來減少空閒的時槽,以增加流量。

The distributed coordination function (DCF) scheme is the main medium access control (MAC) scheme in IEEE 802.11 wireless LANs. The DCF scheme uses the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to control the access to the media. DCF scheme is only appropriate for non-real time data service, so it can't provide any Quality of Service (QoS). Therefore, the IEEE 802.11 group has defined a new standard called IEEE 802.11e to support QoS. It includes a new contention-window based access control algorithm called enhanced distributed channel access (EDCA). Though EDCA could provide QoS for high priority flows, its performance is poor when the load is very heavy.
Researchers developed “Adaptive EDCF” (AEDCF) and “Adaptive Fair EDCF” (AFEDCF) to improve EDCA. AEDCF proposed the “Slow Decrease” (SD) scheme. It updated the contention window (CW) slower than EDCA to improve the efficiency at high load. AFEDCF used an adaptive fast backoff scheme to reduce the idle time slots and improve the throughput.
In this thesis, we added SD scheme into AFEDCF and tested several different functions to change the way CW updated. Using ns-2, we simulated and analyzed the influence on system performance for these different CW update functions. The results showed that CW update functions have little impact on system performance.
其他識別: U0005-2007200916194400
Appears in Collections:電機工程學系所

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