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National Chung Hsing University Institutional Repository - NCHUIR > 工學院 > 通訊工程研究所 > 依資料類型分類 > 碩博士論文 >  802.11無線網路適應性公平增強分散式協調機制之後退時間更新研究

Please use this identifier to cite or link to this item: http://nchuir.lib.nchu.edu.tw/handle/309270000/111754

標題: 802.11無線網路適應性公平增強分散式協調機制之後退時間更新研究
Backoff Time Update in IEEE 802.11 Adaptive Fair Enhanced Distributed Coordination Function
作者: 陳志偉
Chen, Chih-Wei
Contributors: 林宗男;歐陽彥杰
Tsung-Nan Lin;Yen-Chieh Ouyang
廖俊睿
Jan-Ray Liao
中興大學
關鍵字: 802.11 Wireless LAN;Distributed Coordination Function;Medium Access Control;Quality of Service (QoS)
802.11無線區域網路;分散式協調機制;媒介存取控制;服務品質保證
日期: 2011
Issue Date: 2012-09-04 16:19:24 (UTC+8)
Publisher: 通訊工程研究所
摘要: 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改善整體的資料流量,因應當下之通道環境,計算出快速後退之臨界值,將後退程序利用臨界值區分為線性後退與快速後退兩階段,在快速後退階段,每一次更新,後退時間便會遞減為原先後退時間的二分之一。
本論文中,我們在AFEDCF中提出兩個臨界決策函數FS (Fast Decay -> Slow Decay)與SF (Slow Decay ->Fast Decay),函數FS遞減特性開始為快速,後段將趨於緩慢,函數SF則反之,用以計算其快速後退的臨界值,利用ns-2模擬在不同網路情形時臨界值選取對效能的影響,並進一步調整快速後退速率。在既有的AFEDCF機制中,是將AFEDCF快速後退速率設定為2,我們將不同的臨界決策函數,分別調整速率為2,4,6,7,分析不同臨界決策函數在不同速率的影響,結果可以發現透過適當的選擇FS函數及速率,可有效改善AFEDCF效能。
我們提出一動態速率更新(Dynamic Rate Update)機制於FS函數,於快速後退區間內每更新一次後退時間,就遞增或遞減一次速率,來模擬分析動態速率對FS函數的影響,結果發現其效能的影響主要決定於第一次快速後退速率。
Distributed coordination function (DCF) is the main scheme for medium access control (MAC) in IEEE 802.11 wireless LANs. It uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to control access to the medium. DCF is designed for non-real-time data service but it cannot provide Quality of Service (QoS). Therefore, a new standard called IEEE 802.11e is developed to support QoS. 802.11e includes a new contention-window based access control algorithm called enhanced distributed channel access (EDCA). Although EDCA can 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 slower than EDCA after successful transmission. AFEDCF used an adaptive fast backoff scheme. Before a threshold was reached, the contention window decreased linearly. After the threshold, the contention window decreased exponentially and it is called “fast decrease stage”. This approach shortened the idle time caused by the backoff.
In this thesis, we experimented on three aspects of the AFEDCF. The first was to test two threshold decision functions called FS (Fast Decay -> Slow Decay) and SF (Slow Decay -> Fast Decay). The FS function has a larger slope at the beginning and a smaller slope at the end. On the contrary, the SF function has a smaller slope at the beginning and a larger slope at the end. The second was to test different rate for exponentially decreased contention window. In the original AFEDCF scheme, the contention window is divided by 2 at the fast decrease stage. We tested three other rates: 4, 6 and 7. The third aspect is what we called “dynamic rate update” mechanism. It increased or decreased the update rate of the fast decrease stage dynamically.
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