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標題: 802.11e無線網路之混合式協調機制之公平排程機制
Fair Scheduling Scheme for Hybrid Coordination Function in 802.11e Wireless LAN
作者: 錢昌盛
關鍵字: Wireless LAN;Array802.11無線區域網路 媒介存取控制 集中式協調機制 服務品質保證;IEEE 802.11e;Medium Access Control(MAC);Point Coordination Function(PCF);Quality of Service(QoS)
出版社: 電機工程學系
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。IEEE 802.11e Medium Access Control(MAC)是可以支援服務品質(Quality of Service,QoS)的新標準。802.11e的混合式栛調機制(Hybrid Coordination Function,HCF)明顯提高了802.11無線區域網路中服務品質的支援。802.11e提出一個比較簡單的標準 HCF排程演算法,此演算法利用資料流(traffic flow)的服務品質要求(QoS requirement)來計算分配給工作站的傳輸時間 (transmission opportunity,TXOP),此標準演算法是用平均傳輸速率來計算分配給工作站的傳輸時間。但此簡單的標準HCF排程演算法只適合用來傳送固定性傳輸速率(Constant Bit Rate,CBR)的資料,並不適合傳送非固定性傳輸速率的資料。因為有很多的即時性應用,像視訊會議等應用的傳輸速率與封包大小,會隨著時間而變動,因此有變動性傳輸速率(Variable Bit Rate,VBR)的特徵。此論文中,我們會提出一個新的HCF排程演算法,稱為混合式協調機制之公平排程機制 (Fair Scheduling Scheme for HCF,FHCF),這個新的演算法不但可以讓CBR與VBR資料流都有更好的公平性,也改善了傳送VBR資料訊框的延遲時間。FHCF排程演算法用估計佇列長度的方式,把時間分配給各工作站。我們會用一系列的拓樸來模擬FHCF排程演算法,然後與其它機制做比較。
最後,我們使用Network Simulator Version 2(NS2)針對兩種網路拓樸結構,來模擬我們所提的FHCF機制的基地台端排程者(QAP scheduler)與工作站端排程者(QSTA scheduler)。最後則比較FHCF、標準HCF以及EDCF等機制之間的效能差異。從模擬結果得知,FHCF排程演算法除了可以提供更佳的公平性之外,在高負載的情況之下,也能提供各資料流所需要的頻寛與延遲時間。

With the emerging use of real-time multimedia applications over wireless networks, support of Quality of Service (QoS) becomes a key problem. The IEEE 802.11e Medium Access Control (MAC) protocol is an emerging standard to support QoS.
Some recent works prove that 802.11e Hybrid Coordination Function (HCF) can improve the QoS support significantly. A simple HCF scheduler has been proposed in 802.11e which takes the QoS requirements of flows into account and allocates time to stations on the basis of the mean sending rate. As we show in this paper, this HCF scheduling algorithm is only efficient for flows with strict Constant Bit Rate (CBR) characteristics but not for non- Constant Bit Rate traffic flows. There are many real time applications, such as videoconferencing, may have bit rate variations and have Variable Bit Rate (VBR) characteristics. In this thesis, we propose a new HCF scheduling algorithm, FHCF, that aims to be fair for both CBR and VBR flows and also can reduce the latency of VBR traffic packets. The FHCF scheme uses queue length estimation to tune its time allocation to stations. We present a set of simulations and provide performance comparisons with other schemes.
Finally, we used Network Simulator Version 2 (NS2) to simulate the performance of FHCF scheme’s QAP scheduler and QSTA scheduler with two network topologies. Finally, we compare the performances between FHCF、standard HCF and EDCF schemes. Our performance study indicates that FHCF provides good fairness while supporting bandwidth and delay requirements for a large range of network loads.
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