無線區域網路下高效率媒體存取控制機制

Abstract

隨著筆記型電腦和其他行動裝置的快速發展，無線區域網路已成為近年來的一項新興技術。由於這些裝置的電源供應及在無線環境下可用的頻寬都是有限的，因此在設計無線區域網路時，必須多考慮電源的消耗和頻寬的利用率。在本論文中，我們首先探討IEEE 802.11媒體存取控制層的運作效能，其媒體存取控制層中有兩種運作模式：分散式協調功能(DCF)和集中式協調功能(PCF)。當系統負載量低和使用者個數較少時，DCF可以減少因集中控制所需的額外傳輸訊息；而在高負載量和使用者較多時，PCF可以避免掉競爭的情況。根據這樣的特性，我們提出了一個新的協定機制，稱為高效率協調功能(ECF)。ECF整合了兩種方法來作傳輸：一種是像DCF的競爭模式，另外一種是像PCF的無競爭模式。我們也提出了一個具有適應性的演算法來作為決定傳輸模式的根據，主要是依目前的網路狀態，我們的演算法會動態地調整在競爭模式下的使用者個數以適應目前的系統負載量。如此我們就能夠利用到每種模式的特性來避免競爭及額外的資源浪費。此外我們也加入了保留的觀念以提供較佳的服務品質及減少系統中使用者的競爭個數。模擬結果中顯示，根據我們所提的ECF方法，相較於其它的傳輸機制時，ECF整體的效能包括平均產量、頻寬的利用率及電源的消耗量等都有較佳的表現。

With the rapid growth of notebooks and mobile devices, the wireless local area network (WLAN) has become an emerging technology recently. Owing to the lack of power supply and bandwidth, the design of WLAN has to concern more about power consumption and utilization of bandwidth. This thesis first investigates the Medium Access Control (MAC) protocol performance of the IEEE 802.11 WLAN, which consists of two access modes: Distribution Coordination Function (DCF) and Point Coordination Function (PCF). It is useful to use the DCF mode for low traffic load and to reduce the overhead of central control in small networks, and the PCF mode for high traffic load and to avoid contentions in large networks. According to our performance evaluation, we develop a novel control mechanism, called Efficient Coordination Function (ECF). ECF integrates two basic strategies for medium acquisition: contention, as in DCF, and collision-free as in PCF. Each strategy can be evaluated with respect to two important performance metrics, delay and channel efficiency. By analysis results, we provide an adaptive algorithm to determine when to use DCF and when to use PCF. Based on the current network condition, our algorithm can dynamically adjust the permitted number of stations in the DCF mode to suit the current traffic load and network size. In this way, we can take advantage of each mode to avoid increased contentions and access overhead. We also use the idea of reservation to support better QoS and reduce the contending number of stations. Evaluation has shown that by using our ECF algorithm, the average throughput is 10% to 20% better than that of the IEEE 802.11. And the ECF performance in terms of average throughput, bandwidth utilization, and power consumption is improved and is also better than that of DCF, PCF and UMAV schemes.