隨著網路科技進步與智慧型行動裝置普及，視訊串流相關應用之需求日增，如何提供適用使用者頻寬與運算能力之視訊串流技術為主要關鍵技術。視訊串流技術從早期需要中央伺服器控管推播串流方式轉換為高通用性的HTTP自適性串流(HTTP adaptive streaming)，不僅克服防火牆與NAT穿透的問題，更能確保每位用戶都能透過線上即時影音的服務取用相應多媒體資料。目前自適性串流技術的發展如Apple的HTTP Live Streaming、Microsoft的Smooth Streaming、Adobe的HTTP Dynamic Streaming與唯一之國際標準Dynamic Adaptive Streaming over HTTP (MPEG-DASH)，自適性串流技術可以因應網路頻寬發生變化適當調整視訊內容編碼參數，以提供使用者在收視期間畫面不會中斷或停頓。本論文研究與實作一基於MPEG-DASH架構之雲端自適性串流系統，使用開源軟體自行建置一雲端轉碼與串流系統平台，包含雲端伺服器、網路通道及Android用戶端的系統環境，此平台整合視訊轉碼技術與MPEG-DASH檔案封裝方式重新封裝多媒體資料，經由HTTP協定傳輸至用戶端。本論文提出一個在使用者所在網路環境下能夠提供更佳視訊品質與播放順暢度的自適性串流調整機制，其中包含基於網路流量的頻寬估測方法與根據預測播放緩衝器儲存量之視訊品質選擇策略，並且考慮在多用戶情況下如何讓每個用戶都可以穩定接收視訊串流服務與公平使用同一網路頻寬。實驗結果顯示本論文提出之基於網路吞吐量與未來播放緩衝器狀態(Network Throughput and Future Buffer，NTFB)的自適性串流演算法，可以不受播放緩衝器大小與多用戶情況下影響，且在系統效能與PSNR上相較於參考方法都有更好的效果。

With the prevalence of video streaming services (e.g. YouTube, Netflix), cloud video streaming technologies become important. HTTP-based adaptive streaming (HAS) has emerged as the prominent technology for video delivery over the Internet. The HAS allows video streaming services to go through firewalls and NAT friendly and has the potential to improve the quality of service (QoS), as comparison with the traditional centralized streaming service. In the HAS approach, video contents are segmented into small packets with fixed time duration and then encoded at multiple resolutions and bitrates to meet the requirements of user network conditions and heterogeneous devices. In this thesis, we design and implement a cloud HTTP adaptive streaming system based on MPEG-DASH standard, which includes a server and multiple DASH clients. We also propose a client-side adaptive algorithm, based on network throughput and future buffer usage (NTFB). As multiple clients share the same network resources and compete for available bandwidth, the NTFB will select the required video segment from a best client, which compromises stability, fairness, and efficiency for robust video adaption. Experimental results show that, as compared with conventional algorithms, the NTFB can yield smoother bitrate transmission and higher video PSNRs with a smaller buffer.

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