基於封包遺失模型之錯誤保護機制應用於提升點對點視訊串流可靠度之研究

Packet Loss Model Based Error Protection Mechanisms for Reliable Peer-to-Peer Video Streaming

羅啟文(Chi-Wen Lo)

指導教授 ： 陳永昌 ； 林嘉文

國立清華大學/電機資訊學院/電機工程學系/博士(2012年)

摘要

由於近年來網路科技的發展，網路使用者的上傳頻寬逐漸增加，使得點對點(peer-to-peer)視訊可以被實現，進而在較低的伺服器端成本的情況下來達成大規模的視訊串流服務。然而因為點對點網路中，節點會不預期的離開、網路雍塞，造成點對點視訊有嚴重的封包遺失問題，而導致視訊品質的惡化。由於視訊資料封包是由節點之間互相傳輸，當節點移失封包時，後續的節點也無法接收到遺失的封包，導致封包遺失的情形會擴散到後續的節點上，我們稱之為封包遺失的擴散(packet loss propagation)，這使得封包遺失在點對點傳輸架構比傳統主從式架構下更為複雜。使用前項錯誤更正碼(forward error correction code)增加冗餘資訊(redundancy)是一項常用的封包保護機制。在點對點網路中，每個節點的上傳頻寬是有限的，無法滿足每個節點的冗餘資料的需求，而且節點是分散式的管理式各自的上傳頻寬，在分散式的架構中節點如何有效的分配各自的上傳頻寬來達到最佳化的分配也是一項挑戰。
在本論文中，我們針對點對點視訊串流提出一系列的封包保護技術。在現今的點對點視訊中大都採用網狀架構，我們針對網狀架構的不規則網路拓樸提出一個封包遺失機率模型，這個模型考慮到了節點加入/離開行為、傳輸時所造成的封包遺失、錯誤更正碼冗餘保護等特性，並根據此模型推出封包遺失的在點對點網路中是一個馬可夫隨機場(Markov random field)，封包遺失的擴散影響可以簡化到兩層的傳輸內，顯著地簡化了封包遺失擴散的分析。我們利用這樣的分析結果傳輸提出了一個貢獻度節點選擇法來有效的分配節點的上傳頻寬來達到點對點網路中封包遺失率的最小化，進而提升視訊串流的可靠度。針對可調式編碼(scalable video coding)在點對點網路中的封包保護，我們提出一個接收端/傳送端混合式的封包保護機制，每個節點在系統上傳頻寬限制之下利用JSCC(joint source-channel coding)方式來決定每個視訊層不同的冗餘封包接收量，以達到視訊品質的最佳化。然而點對點網路中，上傳頻寬來源因為節點的加入、離開行為而動態的變動，將會導致JSCC結果產生波動進而影響降低視訊品質。我們提出利用一致性擴散(consensus propagation)來預估一個穩定的系統上傳頻寬，節點在針對這個頻寬來要求可調式編碼中不同的視訊層封包。
實驗結果顯示，我們提出的封包遺失機率模型可有精準的估算出節點在各種網路環境下的封包遺失率。我們提出的節點選擇法因為運用了封包遺失機率模型，在分配上傳頻寬時可以推估節點得到冗餘封包後可以得到多少的封包遺失貢獻度，所以我們的方法與其他節點選擇法相較，可以得到最低的封包遺失率。而我們提出針對可調式編碼所提出的接收端/傳送端混合式的封包保護機制可以有效的分配各個節點接收不同視訊層中冗餘封包來達到最佳的視訊品質。

關鍵字

點對點視訊串流 ； 可調式編碼 ； 錯誤更正馬 ； 封包遺失

並列摘要

Peer-to-peer (P2P) video streaming is an emerging streaming service, which can support large-scale services with a lower infrastructure cost compared to traditional client-server structures. The key of a successful video streaming system lies in the video quality perceived by users. However, one of the major challenges to P2P video streaming services is the visual quality degradation due to packet loss. Because peer dynamics lead to more packet loss opportunities for peers, and the packet loss of a peer will propagate to its neighboring peers through inter-peer transmissions. Such packet loss can seriously damage the reconstructed video quality.
To address the packet loss problem, we propose efficient error protection mechanisms for reliable P2P streaming systems. First, we propose a sender-driven peer selection scheme involving estimation of packet loss propagation, evaluation of peers’ contributions, and peer selection based on child-peers’ contributions, for non-scalable streaming over a mesh-based P2P video streaming systems. The proposed packet loss propagation model takes into account the link packet drop rate, peer dynamics, and forward error correction (FEC) protection to characterize the heterogeneous packet loss behavior of individual substreams transmitted over a mesh network. The evaluation of candidate peers’ contributions is modeled through Markov random fields (MRFs) to significantly reduce complexity.
We then propose a hybrid sender/receiver-driven error protection scheme to transmit scalable video packets over packet-lossy peer-to-peer networks. A joint source-channel coding (JSCC) scheme based on receiver-driven subscription is proposed to minimize the distortion of the videos received by child-peers under an estimated system uplink capacity by selecting the appropriate amount of source and channel coding packets. Because the bandwidth for inter-peer transmissions may fluctuate largely due to peer dynamics, in our method peers estimate an average system uplink capacity through consensus propagation to avoid the oscillatory allocations of the JSCC scheme. To efficiently utilize the uplink bandwidth of peers, parent-peers utilize sender-driven contribution-guided peer selection to reject the low-distortion contribution subscriptions from candidate child-peers. Simulation results demonstrate that our methods can significantly mitigate packet loss and therefore increase the visual quality, compared to other state-of-the-art schemes.

並列關鍵字

P2P video streaming ； FEC ； SVC ； packet loss

參考文獻

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