在IEEE 802.11e下針對視訊串流之動態優先分配

Abstract

近年來，上網服務越來越多元化，不再是傳統的資料傳輸，有更多的多媒體傳輸服務如即時影音串流，視訊會議等，加上無線網路已成為行動裝置的必備功能，舊有的無線網路標準IEEE 802.11跟上述的多媒體傳輸無法達成最佳效能，大幅度降低使用者的使用意願，並浪費多餘的無線資源。為了解決目前潛在的問題並順應潮流的發展，我們以跨越時代的想法開發新架構，使得多媒體資料能在高品質的保證下傳輸。 我們以多媒體無線網路的跨層設計為出發點，提出跨層動態影像優先權分配(Cross-layer Adaptive Video Prioritization，CAVP)，不同於以往傳統分工的架構，主要以應用層、媒體存取控制層這二層的跨層設計來達到多媒體資料傳輸最佳化。跨層設計之優點在於能協調網路架構之間的各層，使得在多媒體傳輸中不會造成額外的負擔，且能夠有效地運用。所以我們預期透過無線網路跨層設計能夠開發出一套高效能的多媒體無線傳輸系統，使得多媒體無線傳輸能夠達到最好的效能。 IEEE 802.11標準提供了兩種不同的協調功能：

The wireless environment is built everywhere, especially IEEE 802.11 WLANs. More users use wireless network for searching information in Internets, TV program streaming, and video/voice conference. The packets include not only traditional data but also interactive multimedia data. There is an extension to IEEE 802.11 standards called IEEE 802.11e, improving the QoS capabilities on WLANs. This protocol enhances QoS data as above applications and features. The contribution, Cross-Layer Adaptive Video Prioritization (CAVP), in this thesis paper is divided into two parts. First, it provides MAC-Layer adaptive prioritization (MAP) to prioritize the packet which has the information of video frames at the Medium Access Control (MAC) Layer of IEEE 802.11e. In IEEE 802.11e, the priority of packets is very important, and it affects the order of received packets and the probability of successfully-received packet. The method to prioritize packets is estimating the access time of each access category (AC) for the arrived packet and putting the packet into the AC which has the minimal access time. Second, Video Frame Prioritization (VFP) prioritizes the video packet. Because the importance of video frames is different, we set the initial priority at Application Layer by choosing the maximal PSNR-difference frame as the highest priority frame. The maximal PSNR-difference frame critically influences the overall video sequences. These two parts of our contribution can greatly improve the quality of video streaming. The cross-layer design, CAVP, involves MAC Layer and Application Layer. The information of a packet arrived at MAC Layer is gotten from Application Layer. The benefit of cross-layer design is effectively using the information of different layers and combining their feature. In simulations, we use the popular simulator ns-2 as the experiment platform. We design several scenarios to show that our cross-layer design improves the quality of video streaming. The result shows that our cross-layer design proposal not only greatly increases PSNR of video but also improves the video in vision after the wireless transmission.