IEEE 802.11中提供了多種傳輸速率，但卻未針對如何選擇速率以及何時應調整速率詳細定義。多重的傳輸速率主要是藉著實體層針對訊號的調變方式而達成，但是不同的調變方式，對於環境中雜訊的抵抗能力也有著不同的表現。本論文提出FaRM（Fragment and Rate Matching）協定，利用即時偵測所得到的噪訊比（signal-noise-ratio，SNR），得知當下網路上通訊的品質，並藉著使用Finite State Markov Chain（FSMC）預測網路中訊號品質的波動，如此將比傳統使用SNR臨界值之方法，更能貼近網路品質狀況的真正表現。此外本論文採用動態分割訊框大小（dynamic fragmentation），以期能藉由縮短資料長度，達到較高的資料傳送可靠度，並依此選出最有利之傳輸速率與訊框長度之配對，縮短傳輸所佔用之網路時間，以提升網路產能及改善傳輸延遲時間。透過FaRM動態地選擇速率以及訊框長度，可獲得更理想的傳輸效能。實驗結果呈現出，FaRM考量未來網路品質變化，並快速地針對網路品質調整傳輸方式，可得到較好的網路效能，以及更高的傳輸可靠度，並同時降低資料傳輸的延遲時間。 In the wireless environment, the channel condition varies due to several factors, especially in mobile ad hoc networks. Previous researches use SNR thresholds to determine the channel condition. However, SNR thresholds may not be an accurate decision, especially in a quickly varied environment. This paper proposes a fragment and rate matching (FaRM) MAC protocol for mobile ad hoc networks. In FaRM, stations dynamically detect the current signal-noise-ratio (SNR) to present the channel quality through the control frame exchange. Then FaRM uses Finite State Markov Chain to predict the variation of the channel quality. FaRM therefore can get closer results of the error occurrence in the channel than previous researches. According to the results generated from Finite State Markov Chain, FaRM dynamically selects the transmission rate with an acceptable fragment length. FaRM not only gains better throughput through the selection of high rate, but also increases the reliability by fragmentation. Comparing to other researches, the simulation results show FaRM has better performance, higher transmission reliability and lower transmission delay time by fragmentation according to the prediction of the network.
