有效率的動態選擇封包合併機制在無線網狀網路中

Abstract

無線區域網路在真實環境中所達到的實際效能比起理論值是相對地遜色許多。因其MAC協議：CSMA/CA在傳輸過程中，所產生高度額外的負載為主要的因素。再加上現今盛行的多媒體通訊應用和網路控制協議通常使用小型的封包來進行資料的傳輸，如此，若使用新興的技術像802.11n如此高的傳輸速率來傳送資料，那麼在整個傳輸過程中，所耗費在控制協調的比例就相對來的較高。若加上因多點跳躍的傳輸環境，為傳輸單一封包而得耗用更多額外的資源，會更顯著地大幅降低傳輸效能。因此一個解決傳輸效能低落的方法之一是在傳輸封包之前，將小封包聚集成大封包再進行傳送。 故本論文先陳述三種普遍認定的封包合併機制，其使用限制、傳輸特徵及其效益，而後提出一個針對802.11s無線網狀網路傳輸環境下，基於機率上的假設來有效率地動態選擇最適合的封包合併機制的排程演算法。此演算法依據佇列內封包數量的多寡、封包的分布情形和當下的傳輸品質，決定兩件事情:第一是採用何種封包合併機制，第二是何時把合併的封包傳送出去。藉由此排程來提升整體無線網狀網路的頻寬使用效率。透過模擬結果，驗證此演算法能有效地提升整體網路的傳輸吞吐量達將近95%。

WLAN achieves poor throughput performance compared to the underlying PHY data rate. This is mainly caused by the overhead of CSMA/CA. Besides, the data of multimedia traffic and control protocols is usually transmitted in small frames. When transmitting a large number of small-size frames with high data rate, such as 802.11n, the ratio occupied for CSMA/CA control overhead is relatively high so that it results in worse efficiency. The degree of throughput degradation is further severe under multi-hop transmissions. Thus, aggregating several small-size frames into one transmission is a way to improve this. This works first reveal the three common frame aggregation mechanisms about their transmission characters, benefits, and the restriction of usage, and then propose a novel algorithm, which could dynamically adopt the appropriate aggregation mechanism according to hypothesis of probability, to achieve a high-throughput and high-efficiency mesh network. Based on channel conditions, the quantity and the distribution of frames in the transmission queue, two things will be determined, one is what aggregation mechanism to be adopted; the other is when to send the aggregated frames. Through the policy described above, the bandwidth utilization will be maximized as high as possible. Simulation results demonstrated that the algorithm actually increases the channel efficiency of the 802.11 MAC and further improves the overall throughput of wireless mesh networks by approximately 95%.