支援無縫交接之可適應性視訊串流群播系統

透過無線區域網路取得視訊串流及其他多媒體服務已成為未來視訊服務最便利的方式之一，然而無線區域網路有其固有之特性與限制：如頻寬的不穩定性、信號強度常因障礙物或遠離接取點（Access Point, AP）而快速衰減與多路徑干擾造成（連續）封包漏失等。目前在無線區域網路播送UDP之群播視訊串流時，未能依據無線頻道狀態而適應性地調整視訊串流編碼位元率、實體層傳送速率與合適之封包大小。在移動性方面，現行IEEE 802.11標準尚無完備之機制可讓收看視訊串流的使用者在不同的接取點涵蓋範圍間移動，達到無縫交接　（seamless handover），使視訊節目不中斷。這些不理想的特性會大幅降低收視品質。　　針對上述問題，本論文利用智慧型接取點，設計並實作一個支援無縫交接之可適應性無線區域網路視訊串流群播系統，此系統中使用者將可在不同接取點之涵蓋範圍間移動，而其間視訊不致中斷；且依據使用者回授訊息之內容，智慧型接取點會動態轉送群播封包，並調整視訊串流位元率與實體層傳輸速率，以適應該區域之無線頻道狀態，有效適應頻道狀態。本系統另一重大概念之突破為讓使用者端不以client而是以接取點模式（使用Wireless Bridge模式）進行接收，提升主控權。實驗結果顯示本論文實作之系統能大幅改善現行系統之表現，並可成為進一步改良無線區域網路多媒體串流系統之原型架構。

關鍵字

無線區域網路；視訊串流；群播；交接；智慧型接取點； WDS與監控模式

並列摘要

Accessing multimedia services, like video streaming, via Wireless LAN is expected to become one of the most popular approaches for video services. Though WLANs provide simple and low cost methods of connectivity, some properties like bandwidth instability, bursty packet loss, fading and multipath interference make video streaming over WLANs a challenge. When transmitting multicast video stream packets over WLANs, the Access Points do not adaptively adjust the video bit rate, physical bit rate and proprietary packet size according to the changeful channel condition. In 802.11 WLANs, when a mobile station (STA) moves from the original AP toward the new AP, the video playback may be interrupted. In other words, the existing system cannot provide seamless handover for video streaming. In this thesis, we propose the adaptive multicast video streaming system with seamless handover support using smart APs. In such implementation, STA can move among the coverage of different APs and playback the video smoothly. Smart AP can adjust its forwarding policy of multicast packets, the transmission rate and packet size adaptively according to the feedback information of beacon signals generated by STA. In addition, we propose to employ AP-mode in the client’s terminal, to improve its controllability. The experiment results show that the proposed system outperforms the existing approach and achieves bandwidth efficiency.

並列關鍵字

Wireless LAN ； video stream ； IP multicast ； handover ； smart AP ； WDS ； monitor mode

參考文獻

[1] IEEE. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE Standard 802.11, 1999.

[5] D. Waitzman, C. Partridge, BBN STC, S. Deering, “Distance Vector Multicast Routing Protocol,” RFC1075, Stanford University, November 1988.

[6] J. Moy, “Multicast Extensions to OSPF,” RFC1584, Proteon, Inc., March 1994.

[8] A. Ballardie, “Core Based Trees (CBT version 2) Multicast Routing,” RFC2189, September 1997.

[9] J. Yuan, C. Liu, and X. Li, “SROAM: An End-to-End Service Model for Improved Performance in Roaming over Wireless LAN Networks,” Proceeding of the 2003 International Conference on Computer Networks and Mobile Computing (ICCNMC’03).

國際替代計量

支援無縫交接之可適應性視訊串流群播系統

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