HESP:亚秒延迟，快速通道变化和改进的ABR优于标准cdn

High efficiency streaming

Online viewers are more demanding than ever. 他们想要低延迟的视频流体验，用于交互式用例，如赌博, fan engagement and polls. They also expect a leanback TV experience over OTT, with a fast channel change, 与他们从广播环境中逐渐爱上的电击体验相当或更快.

However, 媒体公司无法通过像HLS这样的协议来满足观众日益增长的期望, DASH和WebRTC通常会在实时延迟之间进行权衡, fast channel change and/or cost of scaling.

这就是高效流协议(HESP)的由来, 一种基于http的流媒体协议，它提供高效率的流媒体，以满足不断增长的观众期望. In addition to sub-second latency at scale, it brings benefits such as improved adaptive bitrate (ABR), up to 20% bandwidth savings, and fast channel change, as low as 100ms, which makes it on par with analogue zapping. On top of this, HESP规范与CMAF标准可互操作，并包括诸如DRM和字幕的内容保护等功能，这些功能是其他超低延迟协议(如WebRTC)所缺乏的. 

图1:提高观众QoE期望需要高效的流媒体解决方案

HESP explained

基于http的流协议通常使用基于段的方法. This means a video is cut up into segments of a few seconds each, 哪个要求视频播放器等到新片段开始时才开始播放. 这种方法增加了通道更改时间并引入了额外的延迟.

HESP leverages a frame-based streaming approach, 哪一种不需要在实时延迟和通道切换时间之间进行权衡. More specifically, HESP uses two streams:

1. An Initialization Stream, which contains only key frames. This stream is not regularly used. It is only used when a new stream is started.
2. A Continuation Stream, which is a regularly encoded stream for low latency purposes, which can continue playback after any initialization stream image.

利用字节范围请求的块传输编码(CTE), 可以非常快速地启动流或根据网络条件的变化改变质量. As a result, 较低的玩家缓冲需要带来相同的观看质量体验, and hence lower latencies can be achieved, and this in combination with a fast channel change. Moreover, 由于HESP是一种基于http的流媒体协议，它可以轻松且经济高效地扩展到标准cdn上的任何受众规模.

Figure 2: HESP uses two complementary streams. Whenever a user wants to start a new video, firstly an image or frame is fetched from the initialization stream. Images can be requested at any moment to start playback. Subsequently, images are fetched from the continuation stream. 连续流可以在任何初始化流图像之后以实时延迟播放.

HTTP-based streaming

HESP的一大优点是它是基于http的流协议. This means that it can scale over standard CDNs, just like HLS and DASH, and hence it can reach any audience size. HESP brings a number of benefits over HLS and DASH though, such as lower latencies, fast channel change and improved ABR.

When all components of the workflow are optimized for low latency, including the encoder, HESP can also provide for sub-second latency, just like WebRTC. 与WebRTC相比，一个重要的区别是每个WebRTC客户端都需要与后端直接连接, 扩展是通过旋转额外的服务器基础设施实现的. 这使得WebRTC扩展到更大的受众变得复杂和昂贵, and it’s also more difficult to deal with flash crowds, 这些通常是通过CDN处理的基于http的协议，如HESP.

就像大多数其他基于http的流协议一样，HESP也完全兼容CMAF. 它使用CMAF容器进行媒体传递，并遵循CMAF媒体模型. 这使得它与流协议(如HLS和DASH)兼容，并允许它带来所有与CMAF兼容的功能. 这是一个关键的优势，因为它允许HESP流与工作室兼容的DRM(如Widevine)进行保护, 在CENC和CBCS模式下使用通用加密的PlayReady和Fairplay. 与WebRTC相比，另一个这样的优势是与字幕的兼容性:HESP支持今天在HLS和DASH中使用的TTML或WebVTT的标准字幕. Thanks to these capabilities, HESP can replace existing pipelines today, 不需要专有的内容保护和字幕解决方案.

Increasing HESP adoption

越来越多的供应商正在将HESP实现到他们的解决方案中, or ensuring compatibility, through the HESP Alliance. 这包括视频工作流程中的供应商，从编码/包装到CDN、DRM和播放器. HESP联盟有一个HESP就绪解决方案的验证程序，以确保整个生态系统的兼容性，这样媒体公司就可以轻松地将自己的工作流程与HESP联盟成员的点解决方案结合起来.

HESP is available as an IETF specification. The HESP standard includes details about, for example, the HESP manifest, the continuation stream and the initialization stream. 其优点是HESP延续流是CMAF兼容的. This means that it’s very easy to handle captions/subtitles, timed metadata and Digital Rights Management (DRM), items which are also included in the HESP IETF specification.

Secure high efficiency streaming

HESP的一大优势在于，它允许工作室批准的DRM系统通过通用加密标准来实现. 许多直播流媒体服务通常有使用DRM的合同义务，但不能达到亚秒级延迟，因为它们目前仅限于LL-DASH或LL-HLS实现, with 2-7 seconds of latency. When you want to create interactivity, however, or want to allow for betting, it’s important to have latencies as low as possible, to maximize your betting window. This is exactly what HESP can provide for.

Figure 3: Secure high efficiency streaming demo setup

To demonstrate DRM capabilities, EZDRM, Synamedia和THEO建立了一个端到端的HESP工作流，并测量了DRM对延迟和启动时间的影响. 测量表明，DRM对基于hesp的流的端到端延迟没有影响, and only a 250-400ms impact on startup time, related to the time needed to retrieve the DRM certificate & license, and CDM initialization time.

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