摘要
隨著無線網路廣泛地使用以及多媒體應用需求的增加，例如:多媒體串流、視訊電話、高解析度電視(HDTV)，對於在無線環境中支援多媒體傳輸更加被重視。由於無線頻道之衰減、多重路徑、干擾等特性，使得所提供的頻寬會遭遇瞬時劇烈波動的現象，無法維持一致，導致無可避免的傳輸服務品質(Quality of Service, QoS)下降。傳統改善無線環境中頻寬波動之傳輸品質的現有方法為熟知的適應性傳輸調變(link adaptation)及適應性改變資料傳輸速率機制(rate adaptation)。然而，這些方法沒有考慮到資料本身的屬性和編碼技術的特性，並且端賴實體層提供緩慢的反饋資訊來調整適當的傳輸速率。因此，對於即時性的多媒體應用，特別是有嚴謹頻寬需求和延遲限制的影音串流服務，無法提供較好的傳輸服務品質也無法及時地因應瞬時頻寬波動的情形。
在這篇論文中，我們在IEEE 802.11e無線網路環境中，提出了一個cross-layer的機制來改善瞬時頻寬波動情形下之H.264影音串流傳輸的服務品質。我們的方法藉由在應用層(application layer)根據對H.264的slices的特性做分類，使用錯誤頻道傳輸模型來預測頻道情況並且結合多重存取控制層(MAC layer)的封包選擇性傳輸機制來達成。
透過NS2模擬工具評估我們所提出的方法，相較於原本的802.11e EDCA，最重要封包的丟棄率和平均封包延遲皆大幅改善，証明所提出之方法於無線頻寬瞬時波動的情形下可提供較佳的H.264影音串流之傳輸的服務品質並有效率地使用珍貴的無線網路資源。

Abstract
With the increasing usage of wireless networks and the demand for multimedia
applications, such as video streaming, videophone, high definition television (HDTV) etc.,
it is very important to provide quality of service (QoS) for real-time applications over
Wireless Local Network (WLANs). To support QoS in WLAN, the 802.11 Working
Group has developed a new protocol, IEEE 802.11e, to provide service differentiation at
the medium access control (MAC) layer. Because of wireless channel characteristics,
such as shadowing, multi-path, fading, and interferences, the allocated bandwidth is
usually not fixed. It results in ineluctable degradation in QoS. Solutions to deal with
wireless bandwidth fluctuations consist of link adaptation (LA) and rate adaptation (RA).
However, these approaches rely on the slow feedback of physical layer to adapt
transmission rate and do not consider characteristics of traffic and codec. Thus, they may
not satisfy requirements of strict bandwidth guarantee and latency limitation for video
streaming. In this thesis, we propose a cross layer mechanism to improve H.264 video
streaming transmission over IEEE 802.11e wireless networks, when encountering
short-term bandwidth fluctuations. The proposed cross layer mechanism consists of slice
classification in the application layer, a dynamic packet selective transmission (DPST)
scheme in the MAC layer, and channel condition prediction using two-state Markov
chain. Performance of the proposed mechanism is evaluated through extensive
simulations. Results show that the proposed cross layer mechanism provides better video
quality and lower packet delay.

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