Abstract: Aiming at the problem of low quality of video viewing experience in dynamic network environment with limited cache capacity, a Dynamic Adaptive Step-wise Bitrate Switching (DASBS) algorithm for HTTP streaming considering network bandwidth and cache capacity was proposed. Firstly, a sliding window was used to analyze the recent downloaded fragments, obtaining the initial bandwidth estimation. Then, according to the real-time bandwidth fluctuation degree and cache state, two correction factors were set to further smooth the bandwidth estimation. Finally, a cache threshold was set to establish a correlation with the current bitrate, and the bandwidth estimation and the cache dynamic threshold were used to jointly control the bitrate switching. Experimental results on platform libdash show that DASBS is better than Video Quality Control for QoE (VQCQ) algorithm in switching smoothness and its average bitrate of video playback is higher, which effectively improves the bandwidth utilization. Although the average bitrate is slightly lower than that of Evolution of Adaptive Bitrate Switching (EABS) algorithm, the number of switching times is greatly reduced, improving the switching stability. The experimental results show that the proposed algorithm has high bandwidth utilization, switching smoothness and switching stability in dynamic network environment, which can effectively improve user experience.

Key words: streaming media, bitrate adaptation, network bandwidth, cache capacity, step-wise switching

摘要： 针对在动态网络环境和有限缓存容量限制下用户观看视频体验质量低的问题，提出一种综合考虑带宽和缓存因素的动态自适应的HTTP流码率渐进切换（DASBS）算法。首先，利用滑动窗口分析最近下载分片得到带宽初步估值；然后，依据实时带宽波动程度和缓存状态设置双矫正因子进一步平滑带宽估值；最后，设置缓存阈值使其与当前码率建立相关关系，并利用带宽估值和缓存动态阈值共同控制码率切换。使用libdash平台验证算法性能，DASBS在切换平滑性上优于VQCQ（Video Quality Control for QoE）算法，视频播放平均码率更高，有效提高了带宽利用率；平均码率稍低于EABS（Evolution of Adaptive Bitrate Switching）算法时，切换次数得到大幅下降，整体切换稳定性表现更优。实验结果表明，所提算法在动态网络环境中具有带宽高利用、切换平滑和切换稳定的良好表现，能够有效改善用户体验。

关键词: 流媒体, 码率自适应, 网络带宽, 缓存容量, 渐进切换