近年來無線網路技術及資訊軟硬體發展已經非常成熟，故而無線網路相關應用也非常之普及。行動裝置透過無線網路的應用也甚普遍，尤其是在多媒體影音傳送的應用。故而，針對在無線網路環境上傳送不同的服務，需要有QoS機制來確保即時性影音資料的傳送品質。 IEEE 802.11e的標準協定則是用於解決無線網路環境中的QoS問題，該協定可將不同服務等級的資料，配置到四個不同的傳送佇列中，來達到此目的。而跨階層(Cross-layer)設計則是改善無線網路傳送品質的方法之一。

然而，就算無線網路技術與產品再成熟，仍然有其不足之處，但是諸如訊號不佳、斷線、以及傳輸品質不穩定等問題仍然未能獲得有效解決。在無線網路環境中即時影音資料的傳送過程中，當資料傳送過程中因封包遺失或毀損，可能導致於接收端的影像畫面無法完整的解碼。以上這種情況，便是浪費了網路的資源傳送無效的資料，也消耗了載具或基地台電力。

本研究則提出了基於IEEE 802.11e無線網路的佇列管理演算法，可提升影像傳送的品質，該演算法可針對不同的影像的訊框(frame)，即 I-frame、P-frame、B-frame，訂立不同的傳送策略以改善影像傳送品質，即將不同重要性的影像訊框配置到不同優先權的佇列。此外，有效率的傳送資料也是該演算法用來改善影像傳送品質的策略之一。此機制除了不傳送沒有用的封包外(如：毀損的訊框封包)，當無線網路處於高負載的情況下，亦會優先傳送重要的影像訊框，第一優先為I-frame，其次為P-frame與B-frame。

另外，以MPEG-4為例，影像訊框之間是有相依性的，當最重要的I-frame傳送過程造成資料遺失，除了接收端無法正確解碼該訊框，相依的P-frame與B-frame亦會無法解碼，本演算法的分析機制亦會考慮此種情況，將有限的網路資源運用最大化。

本研究透過模擬實驗，針對不同研究提出之演算，比較傳送前與接收後的影像PSNR來分析影像傳送品質；再者，也分析訊框遺失比率做為參考指標之一。研究結果顯示，本研究相較於IEEE 802.11e EDCA及adaptive mapping演算法，都有較好的表現。

Recent developments in hardware, software, and communication technologies have resulted in increasing interest in the use of wireless local area networks (WLANs). Mobile devices with embedded WLAN functionality are becoming increasingly popular. Such devices must be designed to support applications that require high quality of service (QoS) and have favorable characteristics to maximize battery capacity. IEEE 802.11e has been introduced to support QoS in WLANs by allocating different traffics to four access categories. Cross-layer design is a method to improve the transmission quality of multimedia over wireless networks.

The resources of queues in IEEE 802.11e networks may be taken up by the transmission of information that is useless to the receiver. It is not easy to transmit packets in WLANs with a heavy traffic load, and if the network becomes congested, important packets may be dropped. Accordingly, efficient transmission of video packets is important for improving the quality of video transmission..

Further, video packets tend to be affected by congestion considerably more than data packets. Therefore, controlling the traffic of insignificant packets to avoid the transmission of useless data can improve the quality of video transmission. On the basis of this concept, in this study, we propose strategies that are different from the approaches proposed in earlier studies. The proposed strategies of QoS-Aware Delivery Approaches that exploit different methods to process intra-coded (I)/predictive-coded (P)/bidirectionally predictive-coded (B) video frame packets to improve video transmission quality in WLANs. First, identification of the dependence of video frames in a group of pictures (GOP) can prevent the transmission of useless data. Thus, the transmission channel resources are saved by avoiding the transmission of undecodable video frames. Second, in heavily loaded WLANs, it is not sufficient to utilize the queues to transmit important video frame packets. The proposed mechanisms adopt the method of an aggressive dropping of unimportant packets to reserve the transmission channel resources for important video packets.

In order to evaluate the quality of MPEG-4 video delivery over IEEE 802.11e networks by the proposed mechanism, this work adopted two evaluation metrics: peak signal-to-noise ratio (PSNR) and the ratio of frames loss. The simulation results demonstrate that, in terms of both PSNR and the ratio of frames loss, the proposed scheme outperforms exiting schemes such as enhanced distributed channel access (EDCA) and adaptive mapping.

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