無線網路多媒體傳播之行動管理 與服務品質保證

Abstract

無線網路技術的成長與建置的普及化，提供大眾獲取網路服務更豐富的選擇，原來有線網路上的許多應用，也逐漸移植到無線網路的環境，其中多媒體服務在無線網路上的應用更是不可避免的趨勢。 在無線網路的環境透過多點傳播(multicast)的技術，傳播單一資料流給特定群組中的接收端，對於頻寬需求較大的多媒體串流傳輸，能夠更有效率地利用頻寬資源，減少不必要的浪費。我們選擇結合可調性視訊編碼標準(Scalable Video Coding)的壓縮技術，支援multicast傳輸多媒體應用成為更富彈性的服務，讓MN能夠依據獲得頻寬與所想要的影像品質選擇服務。 由於無線網路特有的移動特性以及認證機制，必須確保mobile node(MN)在移動過程中不會影響其原有通訊連線，並能提供多媒體的服務品質保證(Quality of Service，QoS)，因此我們根據所針對多媒體傳輸機制的特性，提出「預先下載(pre-download)」的設計，在MN移動的過程中，保證其能夠不斷線地享受到基本觀賞品質的服務，並且，我們不強制保留無線的頻寬給這些預先下載的traffic，更能彈性且有效率地使用珍貴的無線資源；同時，兼顧對於handoff MN的保護，我們提出「搶佔 (preemption)」的設計，讓handoff MN在系統資源不足時，能夠擠壓下載高品質內容traffic的空間來取得頻寬傳輸；最後，我們在允入控制(Admission Control)的設計上提出Full quality Utilization(FU)的概念，用於限制下載高品質內容的比例，並能根據網路狀況動態地調度資源。 除外，我們觀察使用者觀賞電視節目的習慣，將切換節目的行為視為另一種層次的handoff，更進一步地提出節目叢集(program cluster)的概念。 最後，AP上實做QoS的機制會對於系統的performance有所影響，我們以當今state-of-the-art之IEEE 802.11e標準提供的兩種機制來做評比，分析比較EDCA與HCCA在網路電視廣播服務中所能支援QoS程度與對系統效能的影響。

With the widely deployed wireless networks, people have another choice to access the Internet. Therefore, many applications originally provided on wired networks will be used over wireless environment, and multimedia services may be the most popular one in the near future. Delivering multimedia content via multicast in our system saves unnecessary bandwidth wastage and uses limited wireless resource more efficiently. Besides, we compress video content through the adaptive mechanism─scalable video coding according to the video quality users need. However, nodes in wireless networks move anytime and multimedia services may disconnect during handoff, mobility management issues must be considered when providing seamless services. Moreover, real-time applications are sensitive to delay and jitter, Quality of Service (QoS) guarantee need to be provided. We propose “pre-download” scheme in order to pre-construct multicast tree, then MN will not suffer from delay and could always get basic quality content during handoff. Besides, our most important contribution is to design “preemption” scheme. Through the flexibility provided by scalable video coding, we let handoff MN preempt others’ high quality enhancement layer (EL) stream to continually download basic quality content when system resource insufficient. Therefore, we not only favor handoff MN than new arrival one to keep handoff dropping probability low but also utilize the channel. In the end, we propose concept of Full quality Utilization (FU) on admission control unit to manage the resource adaptively. We evaluate both our mobility management scheme and MAC mechanism used to provide QoS on AP (IEEE 802.11e MAC). In the simulation results, our scheme outperforms previous work favor handoff MN by bandwidth reservation on three important performance metrics : utilization、new blocking probability and handoff dropping probability.