什么是自适应流?

[本文是流媒体的一部分。 "What Is" series, providing a high-level overview and definitions of key concepts in online video.]

Executive Summary

自适应流媒体技术使最佳 streaming video viewing experience for a diverse range of devices over a broad set of connection speeds. If streaming video is mission critical to your enterprise, 你今天没有使用自适应流媒体, 或者很快实现它, 你已经落后了. 本文档描述了什么是自适应流, 确定主要的技术竞争者, and discusses the factors that you should consider when choosing a technology.

自适应流定义

Adaptive streaming technologies share several critical aspects. First, they produce multiple files from the same source file to distribute to viewers watching on different powered devices via different connection speeds. Second, 它们自适应地分发文件, changing the stream that’s delivered to adapt to changes in effective throughput and available CPU cycles on the playback station.

Third, 它们对用户来说都是透明的, so that the viewer clicks one button (rather than multiple buttons as with the movie trailer experience where users select the bitrate and video quality beforehand) and all stream switching occurs behind the scenes. The viewer may notice a slight change in quality as the streams switch, 但他不需要采取任何行动.

All technologies share similar operating characteristics as well, 尽管有一些关键的区别. For example, all adaptive streaming technologies monitor factors like video buffer status to assess effective throughput and CPU utilization and dropped frames to assess the available computing power on the playback station. This information is used to determine when to switch streams.

For example, 如果视频缓冲区已满，CPU利用率低, the adaptive streaming technology may switch to a higher quality stream to enhance the viewing experience. 如果缓冲降到一定水平以下, 或者CPU利用率超过某些阈值, 该技术可能会切换到较低质量的流.

The key implementation difference between the technologies is the involvement of a streaming server. Specifically, 有些技术需要流媒体服务器, and constant communication between the server and player. If a stream switch is required, the server implements it by sending a different stream to the viewer.

其他技术无需流媒体服务器即可运行. The different quality streams are posted to different addresses on a web server or multiple web servers. The player monitors operating heuristics like CPU utilization and buffer status, decides when a stream switch is necessary and starts retrieving data from a different stream when appropriate.

Either way, adaptive streaming technologies enable producers to deliver outstanding quality streams at the high end of the bandwidth/power spectrum because they also serve the low end. 没有自适应流, most producers would either distribute a single mid-quality file that looks below average in the optimum viewing configuration, or create multiple files and force the viewer select the desired configuration.

自适应流媒体供应商和服务提供商

The players fall into three general categories: technology developers, 服务提供商和基于标准的技术. 主要的技术供应商包括 Adobe与基于flash的动态流, Apple with HTTP直播(HLS), and Microsoft with Silverlight平滑流. Move Networks, which pioneered this market, has largely dropped out as a general purpose technology provider. Several WebM-based HTML5 options are also under development or available, 包括来自 Anevia and Quavlive.

服务提供商主要包括Akamai Akamai HD Network, which is a platform that can deliver to iOS devices, Flash and Silverlight clients. Several companies, most notably Netflix, have developed their own adaptive streaming technologies for internal use.

Standard-based technologies include Scalable Video Coding (SVC), which is an extension of the H.264 specification. In addition, Apple has submitted the HTTP Live Streaming protocol to the Internet Engineering Task Force (IETF) where it is working its way through the standardization process.

选择自适应流媒体技术

Here are the primary factors that you should consider when choosing an adaptive streaming technology.

支持的播放平台

Intuitively, the most important consideration is whether a technology can reach your target viewer. 从基于计算机的播放开始, with HTML5-compatible browser penetration still well below the 60% range (according to netmarketshare.com), this means either a Flash or Silverlight-based solution. In this regard, while Silverlight’s penetration over the last few years has been growing, 约71%的全球渗透率(根据WWW).riastats.com) it’s still far behind the 97% Flash penetration reported by the same site.

If you’re distributing premium content, like the Olympics or Sunday Night Football, you may be able to assume that viewers will download and install the Silverlight client to watch your video. For more prosaic content, this assumption may be harder to make.

如果手机是一个重要的目标市场, you’ll have to implement at least two adaptive streaming technologies, since neither Flash nor Silverlight can play on iOS devices or most other mobile platforms. Android developer Google has helped things along by implementing HLS on the Android 3.0 platform, and hopefully other mobile platforms will fall into line.

However, HTTP Live Streaming is a non-starter on traditional computers because it requires the QuickTimeX player, 只在雪豹上可用, 而在Windows上则完全没有. Though there are some third party solutions in the works that enable HLS playback on Windows, the penetration of these players would have to become very significant before most streaming producers would consider abandoning Flash or Silverlight for HLS on general computers.

Fortunately, supporting multiple adaptive streaming technologies is not as formidable a task as it once was, as multi-platform capabilities like those offered by Akamai are becoming more mainstream, whether through a service provider like Akamai or via third party streaming servers. For example, Wowza Media Server 2 可以输入一个H.264编码的视频在多种格式, and dynamically transmux the container format and protocol to deliver to Flash, Silverlight, iOS设备和其他平台.

你可以自己购买Wowza媒体服务器, or choose an online video platform (OVP) or content delivery network (CDN) that uses the Wowza technology, 或其他类似产品, 向第三方提供这些功能. In addition, at NAB 2011, Adobe previewed the ability for the Flash Media Server to deliver to iOS devices, while Microsoft’s IIS Media Services can transmux an incoming Smooth Streaming video streaming for iOS delivery.

它使用哪种协议?

最初发布时, Adobe’s Dynamic Streaming exclusively used the Real Time Messaging Protocol (RTMP) to distribute video to viewers, which has multiple disadvantages as compared to technologies that use the more general Hypertext Transfer Protocol (HTTP), 包括HLS和平滑流. First, 作为一种基于服务器的技术, the initial implementation of Dynamic Streaming required a persistent connection between server and player, which potentially can increase implementation cost and limit deployment scalability.

Second, RTMP packets may have difficulties getting through certain firewalls, though the Flash Media Server has workarounds if these problems are experienced. Third, video packets delivered via HTTP can more easily leverage standard HTTP caching available within the networks of ISPs, corporations, 以及其他组织, which can improve distribution efficiency and quality of service. Finally, there’s a general perception among technical cognoscenti that stream switching works more effectively using the chunk-based delivery used by HTTP-based adaptive streaming technologies.

While these arguments create a strong case that RTMP is a tragically deficient protocol for delivering adaptive streaming, to a degree that’s akin to arguing that air is bad for mammals or water bad for fish and crustaceans. That is, 如果你不考虑大规模的事件, the overwhelming bulk of actual streaming video (as opposed to progressive download) is delivered via RTMP.

For example, the 华尔街日报，彭博社 and the Financial Times all distribute Flash video via RTMP, which presumably they wouldn’t use if the videos were rejected en masse 通过许多托尼观众使用的防火墙. MTV和CBS也通过RTMP流媒体, which should put issues about scalability and cost to bed, 以及缓存HTTP数据包的好处.

然而，总的来说，最终将RTMP与. HTTP debate to bed was Adobe announcing and shipping an HTTP version of Dynamic Streaming. So now if you want to deliver to the Flash Player via HTTP, you have an Adobe option.

DRM

最后的考虑, 至少对一些生产商来说是这样, is the availability of digital rights management (DRM) features to protect their content. 在这方面，Adobe提供 Flash Access 以及微软提供的其他技术 PlayReady 内容保护，而HLS不支持DRM, 该规范确实支持加密, and other HTTP technologies can be used to limit access to content, 比如HTTPS认证.

There are the high-level concepts, while the devil is in the details when it comes to deploying DRM. Just be sure to check the availability of DRM early in the process if it’s critical to your monetization strategy.

Conclusion

Adaptive streaming may be the single most important capability for delivering the optimum quality of service to a diverse range of viewers. As usual, however, not all technologies are created equal and there is no one-size-fits-all solution. 如果流媒体是企业的关键任务, you need to be considering a solution or combination of solutions that best meets your needs.

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