FEC (Forward Error Correction)是一種在無線網路中常用的傳輸錯誤回復機制，一般適應性的FEC(Adaptive FEC)技術會根據接收端所回傳(feedback)的資訊調整FEC保護比率(FEC rate)。然而，經過計算與回傳的資訊因為時間上的延遲，可能會造成所得到的FEC rate無法準確反應出網路目前的狀態。本論文先後提出兩個做在無線網路接取點(wireless Access Point)上的錯誤更正機制，分別為(1)隨機早期偵測前向糾錯機制(Random Early Detection Forward Error Correction，RED-FEC); (2)增強式隨機早期偵測前向糾錯機制(Enhanced RED-FEC，ERED-FEC)，用以改善無線區域網路之影像傳輸品質。與之前的AFEC機制相比，我們所提出的RED-FEC和ERED-FEC機制是藉由無線接取點(Access Point)進行網路狀態判斷，以決定出最適合的FEC保護比率，並由無線接取點負責產生錯誤保護封包(FEC packets)。RED-FEC機制主要根據網路的負載情況，再利用RED佇列管理技術調整最後所傳送出去的FEC保護封包數量。而ERED-FEC機制是針對RED-FEC本身的功能進行設計上的改良，在考慮網路狀態時包含兩個部份，一個是藉由封包的重傳次數判斷無線通道狀態(wireless channel state)，並搭配影像品質模型進行分析，另外一個是觀察無線AP的佇列長度判斷目前網路流量負載。首先，ERED-FEC機制會量測目前的網路封包遺失率，並利用影像品質分析的模型決定出適合的FEC rate，接著再根據網路負載與利用RED佇列管理技術調整最後所傳送出去的FEC保護封包。經過這些步驟的計算，ERED-FEC機制可以傳送出最適量的FEC保護封包並有效改善影像傳輸品質。經由網路模擬數據分析與驗證，我們所提出的AP-based RED-FEC機制在高、低網路負載都可有效提升錯誤回復效能，並充分利用無線網路頻寬資源。

FEC (Forward Error Correction) is a popular technology used to perform packet error recovery. Typical Adaptive FEC schemes (AFEC) tune the FEC rate on the basis of feedback information supplied by the receiver. However, the processes of calculating and feeding back an appropriate FEC rate, inevitably have a finite duration, and hence the FEC rate implemented at the sender end may not accurately reflect the current network condition. This dissertation proposes two FEC schemes which are implemented at the wireless Access Point (AP): (1) Random Early Detection Forward Error Correction mechanism (RED-FEC) and (2) Enhanced RED-FEC mechanism (ERED-FEC) to improve the quality of video delivered over Wireless Local Area Networks (WLANs). In contrast to previous AFEC schemes, in our proposed methods, the FEC redundancy rate is calculated directly at the wireless AP in accordance with the network conditions. In the RED-FEC scheme, the final number of FEC redundant packets is tuned based on the current network traffic load by a RED mechanism. In the ERED-FEC scheme, the number of redundant FEC packets is determined in accordance with both wireless channel condition and network traffic load. The wireless channel condition is indicated by packet retransmission times; while the queue length of the wireless access point indicates the network traffic load. First, the FEC redundancy rate is determined from the estimation of packet loss rate and the analysis model of video quality. Then, the ERED-FEC tunes the final number of FEC redundant packets based on the current network traffic load by a RED mechanism. By adopting this approach, the proposed AP-based RED-FEC schemes significantly improves the video quality without feeding back information. The numerical results show that the proposed AP-based RED-FEC mechanisms achieve better performance than the related FEC mechanisms in both light and heavy load.

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