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標題: 802.11無線網路之適應性公平增強分散式協調機制
Adaptive Fair Enhanced Distributed Coordination Function in 802.11 Wireless LAN
作者: 謝明熹
Hsieh, Ming-Hsi
關鍵字: Wireless LAN
medium access control
distributed coordination function
Quality of Service
出版社: 電機工程學系
摘要: 隨著即時性多媒體應用在無網路上的廣泛使用,使得服務品質的支援是個關鍵性問題。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。DCF採用避免碰撞之載波偵測多重存取(Carrier-Sense Multiple Access/Collision Avoidance,CSMA/CA)的方式進行資料的傳輸,在一般的情況下,雖然可以讓大部份的訊框避免不必要的碰撞,不過當DCF在高負載時,將明顯的造成資料流量降低與較高的延遲。原因在於浪費了太多的頻寛用於決定通道的存取。此外,DCF只適合非即時性的服務,並不能提供任何服務品質。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 EDCF。 在此論文中,我們可以發現EDCF容易因為工作站變多而嚴重的降低其效能,因此我們使用名為適應性公平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。方法為考量通道當時環境並針對不同等級的優先權,分別計算進入快速後退的臨界點,再搭配計算平均碰撞率的方式去調整快速後退的程度。並針對為了使得相同優先權的不同資料流具有較高程度的公平性,特別採用了當通道媒介繁忙時就視為發生碰撞似的增加競爭視窗值。 最後我們使用NS2針對不同的網路情形,實際的模擬分析所提的適應性公平EDCF、EDCF與其它的增強機制的效能差異結果。
With the emerging use of real-time multimedia applications over wireless networks, support of Quality of Service (QoS) becomes a key problem. 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. If the network load, the majority of frames transmitted via CSMA/CA strategy will not cause collision. However, DCF suffers from significant throughput degradation and high delay under heavy load. The reason is that bandwidth is wasted in the increased time used for negotiating channel access. Moreover, DCF mechanism is only appropriate for non-real time data service and can not provide any Quality of Service (QoS). Thus, the IEEE 802.11 working group is currently working on the support of QoS in a new standard, called IEEE 802.11e EDCF. In this thesis, we found that EDCF performed poorly when the medium was heavily loaded. So we used a mechanism named Adaptive Fair EDCF that extended EDCF. This new method used an adaptive fast backoff mechanism to improve the total throughput. Our scheme computed an adaptive backoff threshold for each priority level by taking into account the channel load and used the average collision rate to adjust fast backoff. Besides, it provided a higher degree of fairness among different flows of the same priority. By increasing the contention window during deferring periods when channel was busy. Finally, we used Network Simulator Version 2 (NS2) to simulate adaptive fair EDCF, EDCF and other enhanced schemes. We experimented it with different ad-hoc network topologies to evaluate the performance of multimedia flows with different channel loads.
Appears in Collections:電機工程學系所



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