Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/6519
標題: 802.11無線網路適應性公平增強分散式協調機制之修改
Modification of Adaptive Fair Enhanced Distributed Coordination Function in 802.11 Wireless LAN
作者: 張立群
Zhang, Li-Qun
關鍵字: Wireless LAN
Array802.11無線區域網路 媒介存取控制 分散式協調機制 服務品質保證
medium access control
distributed coordination function
Quality of Service
出版社: 電機工程學系
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,亦即工作站須感測傳輸媒介閒置一段特定時間後才能傳送資料,每個工作站都有相同的機會競爭到傳送權。由於DCF並不能提供任何服務品質的保証,這對於日漸使用頻繁的即時性多媒體資料而言,是個絕大的致命傷,因此802.11工作小組制訂了一個可以支援服務品質的標準,名為802.11e。其中包含了一個新的媒介存取機制稱為EDCF(Enhanced DCF)。 儘管EDCF達到了高優先權資料流可以獲得較好服務品質的效果,但是實驗結果發現,由於工作站的增加,將使得EDCF的效能嚴重地下降,這又是另一個急需解決的問題。因此我們提出了一個叫做適應性公平EDCF(Adaptive Fair EDCF)的方法改良EDCF。這個方法主要是利用適應性快速後退機制來改善整體的資料流量,因應當時的通道環境,算出進入快速後退的臨界值,再搭配計算平均碰撞率的方式去調整快速後退的程度。 。 我們使用ns2針對不同的網路情形,模擬分析所提的適應性公平EDCF及其它增強機制的效能差異結果,並更一步地修改適應性公平EDCF內部的參數及演算法,以期達到良好的效能
The distributed coordination function (DCF) scheme is the primary medium access control (MAC) scheme for IEEE 802.11 wireless LANs. The DCF mechanism employs the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) strategy to provide multiplexed data transmission. In this mode, a station must sense the medium before sending a packet. If the medium is found idle for a specific time then it can transmit a packet. DCF mechanism is only appropriate for non-real time data service, so it can not provide any Quality of Service (QoS). Thus, the IEEE 802.11 working group has defined a new standard to support QoS, called IEEE 802.11e. In 802.11e, a new medium access mechanism called enhanced DCF(EDCF) is defined. Though EDCF could provide QoS for high priority flows, we find that it performs poorly when the medium is heavily loaded. So we use a mechanism called “Adaptive Fair EDCF” that extends EDCF. This new method uses an adaptive fast backoff mechanism to improve the total throughput. Our scheme computes an adaptive backoff threshold for each priority level by taking into account the channel load and uses the average collision rate to adjust fast backoff. We use Network Simulator Version 2 (NS2) to simulate adaptive fair EDCF and other enhanced schemes. We experiment it with different network topologies to evaluate the performance of multimedia flows with different channel loads. Moreover, we adjust parameters for adaptive fair EDCF to achieve better performance.
URI: http://hdl.handle.net/11455/6519
Appears in Collections:電機工程學系所

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