無線網路即時新聞視訊回傳及其加密防護機制之研究

A Study of Real-time Transmission and Content Protection for TV News Gathering via Wireless Internet Networks

劉星劭(Hsing-Shao Liu)

指導教授 ： 謝清佳

共同指導教授 ： 王家輝 ； 張瑞益

國立臺灣大學/管理學院/資訊管理學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.02518

摘要

面對行動電視、網路電視等新興服務所帶來龐大之競爭壓力，傳統新聞電視媒體被迫重整新聞採訪、編輯、播出(簡稱新聞採編播)等流程，以提供更低成本、更高效率之服務模式。本研究依新聞電視台透過新一代無線傳輸技術WiMAX，來提升新聞採編播效能之努力，提出兩項研究議題來促進此發展。
第一項研究議題，乃奠基於電視媒體「即時回傳新聞採訪影音內容」之需求，傳統以高成本之衛星傳輸技術來完成，現今則可考慮採用低成本之WiMAX傳輸技術。然而，已知單一座WiMAX基地台僅約能提供2∼3Mbps上鏈傳輸頻寬。在影音內容維持至少standard definition (SD)畫質的要求下，現有MPEG4壓縮技術仍需2.5Mbps以上之data rate，此使其難於保證(gurantee)即時新聞採訪影音內容之傳輸服務；另一方面，由於傳輸一路SD畫質之影音內容，已幾乎消耗所有WiMAX上鏈頻寬資源，而限制其它行動應用服務使用該傳輸資源的機會，此亦將難以符合WiMAX營運商(WiMAX operator)之商業考量。據此，本研究提出多重鏈路傳輸的方法，藉由結合多個無線基地台之傳輸資源，分擔傳輸SD影音內容之負荷，並能達到網路備援(backup)的效用，以進一步保障傳輸服務品質。經分析比較後，本研究採用建置於FreeBSD 7.1版之stream control transmission protocol (SCTP)模組(其已內建多重鏈路傳輸功能──Concurrent Multipath Transfer (CMT))，並在此平台上實作多重鏈路傳輸系統，然後驗證其合併鏈路頻寬資源的效能。在經過多項實驗測試後，本研究發現諸多有利於高畫質即時影音傳輸之系統參數，同時辨識出CMT在合併鏈路頻寬資源上的問題。依所發現的問題，本研究提出改良CMT的方法──VC-CMT，以改善合併鏈路後之傳輸效能。經NS2.31 (Network Simulator with Version 2.31)模擬驗證後，證實VC-CMT能有效改善CMT傳輸效能。
第二項研究議題，乃基於電視媒體提供行動影音播出服務(包括Internet視訊服務)之需求，針對影音內容傳輸之安全性，提出解決方案。由於行動影音服務多提供於公眾無線網路傳輸環境，而在無線網路環境下存在資料容易被竊取的風險，造成內容供應業者之營運損失。然而，過去即時影音內容傳輸加密方法，通常為了滿足即時傳輸的需求而降低安全等級，使得影音內容仍存在易被竊取的問題。另一方面，由於行動終端設備之性能愈益提升，而能承載更豐富之行動影音內容。然而，豐富之行動影音內容不但需要消耗較多之傳輸頻寬，同時，也增加即時影音內容保護的困難。此外，由於現今行動終端設備的種類繁多、性能差異大，目前既有之加密機制，是否能適用於各種類型之終端設備，還有待進一步驗證。而針對不同之行動終端設備，採用不同之加密技術來保護影音內容，對於Internet視訊服務業者來說，將會增加營運上的困難。因此，本研究提出具one-time pad特性之即時影音傳輸加密機制──SVOD (Secure Video on Demand)，其可適用於不同影音內容類型、不同終端用戶設備、以及適應於網路傳輸狀態的變化，並能在影音內容「即時性」與「安全性」之間取得平衡，以有效確保行動影音服務的品質。此機制藉由定期取得終端用戶播放緩衝區之使用狀況，來動態決定加密複雜度，進而調節影音內容之發送速度(sending rate)。本研究實作系統來驗證SVOD之效能，實驗結果亦顯示所實作的SVOD系統，能確實達到前述特性，而同時兼顧行動影音服務之QoS (quality of service)與QoP (quality of protection)。

關鍵字

即時新聞轉播 ； 合併頻寬 ； 多重鏈路傳輸(CMT) ； Stream Control Transmission Protocol (SCTP) ； 安全之隨選視訊 (SVOD) ； 播放緩衝區佔用狀況 (PBO) ； 服務品質 (QoS) ； 防護品質 (QoP)

並列摘要

Facing the intense competition resulted from the emerging multimedia services such as mobile TV and Internet TV, tranditional news TV stations are forced to rebuild the processes of news coverage, editing, and broadcasting to deliver their TV services more efficiently but in a low-cost manner. According to the context of traditional news TV media and the endeavor which news TV stations have made to use novel wireless communication technology WiMAX to facilitate above processes, we propose two research issues to assist such development.
The first issue is based on the requirement of live news transmission for TV stations. Traditionally, it is done by satellite network which is greatly expensive. Currently, it is possible to be completed by using cost-effective transmission technology WiMAX. However, to maintain the standard-definition (SD) quality, the video of live news content will normally have data rate of 2.5Mbps encoded by modern compression technique MPEG4. Given that one WiMAX base station usually has uplink bandwidth of 2 to 3Mbps, it is difficult for WiMAX to guarantee the service quality for delivering live news video program. Besides, transmiting one SD program almost exhausts the uplink bandwidth of one WiMAX base station and thus prevents other mobile applications from using this base station. It is also difficult for WiMAX operators to dedicate one WiMAX base station only to single service. Therefore, we propose the multi-link transmission approach, which aggregates bandwidth resources from multiple wireless base stations to share the load of transmitting one SD program and have the benefit of link backup for network dynamics. After surveying and literature review, we use concurrent multipath transfer (CMT) which has been embedded into the module of stream control transmission protocol (SCTP) in FreeBSD 7.1. We futher build up and fine-tune CMT system to fulfill the bandwidth aggregation and conduct experiments for evaluating this system’s performance. After several experiments, we have identified several critical parameters which can enable CMT system more suitable for real-time transmission applications. Meanwhile, we also recognize the problem of throughput degradation in CMT. Thus, we further propose a novel approach called VC-CMT (VC stands for virtual connection) to improve the throughput gains obtained from aggregated links in CMT. By using NS-2.31 (Network Simulator with Version 2.31), we can investigate VC-CMT performance. The result shows VC-CMT can more effectively aggregate the link’s bandwidth than CMT.
The second research issue is based on the requirement of delivering mobile video service and Internet video service in TV stations. We propose a solution to protect the video services from eavesdropping. Given that mobile video servces are always rendered in a pulic wireless network environment, the risk of content leakage and eavesdropping is raised and thus the content providers’ interests are easy to be infringed. However, tradidtional encryption approaches often sacrifice the security level to attain the demand of real-time transmission. Thus, the risk of content leakage and eavesdropping still exists. Besides, given that the capability of mobile devices used by clients is enhanced very soon and can play out with richer video content, to transmit richer video content will not only consumes more network bandwidth but also increase the protection difficulty. Furhtermore, due to the diversity of mobile TV devices nowadays, the performance difference between them is large. It requires further examination for current encryption approaches to realize if they can be applied for various kinds of mobile TV devices. To maintain multiple encryption approaches to protect video content due to the diversity of mobile TV devices will be a difficult task for Internet TV service providers. Therefore, we propose an encryption mechanism called SVOD (Secure Video on Demand) with one-time pad feature, which can not only be applied in different types of video content and mobile devices, but also adapt to the network dynamics. It can also leverage the requirements between real-time transmission and content security levels to effectively guarantee the service quality of mobile video applications. SVOD uses the information of playback buffer occupancy (PBO) which is fed back by client periodically to determine the encryption complexity to intervene the video server’s sending rates. We implement SVOD system and investigate its performance. The experiments show that this SVOD system can achieve the advantages mentioned above and can leverage the quality of service (QoS) and quality of protection (QoP) for mobile video services.

並列關鍵字

Live News Broadcasting ； Bandwidth Aggregation ； Concurrent Multipath Transfer (CMT) ； Stream Control Transmission Protocol (SCTP) ； Secure Video on Demand (SVOD) ； Playback Buffer Occupancy (PBO) ； Quality of Service (QoS) ； Quality of Protection (QoP)

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