What Is a Content Delivery Network (CDN)?
This is an installment in our ongoing series of "What Is...?" articles, designed to offer definitions, history, and context around significant terms and issues in the online video industry.
Executive Summary
One of the basic terms in streaming and online video is the CDN, which stands for content delivery network. But what is a CDN, exactly? CDNs are used to rapidly and cost-effectively deliver a variety of content to numerous end points, whether the end points are web browsers, mobile devices, set-top boxes, or even gaming consoles. This article will define CDNs, look at their history, assess the current market landscape, and finally discuss what CDNs mean to you.
CDN: Definitions & History
There's not really a generic CDN, but for our purposes, they fall into three basic categories: those that deliver general content including, but not constrained to, video delivery; those that deliver on-demand or pre-recorded/pre-encoded video; and those that deliver live video.
Let's take a look at the differences and similarities between each of these. All the technologies described below, from full download to progressive download to streaming to HTTP delivery, are in use today—in a variety of business models—although some have proven more popular than others.
General Purpose CDNs
The growth of CDNs started before video, and chances are you've used their services if you've downloaded a software update, bought a song on the iTunes Store, or even just visited a popular website.
These general-purpose CDNs perform what's most frequently referred to as web acceleration. This is generally accomplished best by a CDN that has a number of servers in many locations, 理想情况下，靠近互联网服务提供商(isp)之间的大型连接点，甚至在与流行网站或游戏/应用程序提供商相同的数据中心内. The CDN caches content, storing a copy of content that will frequently be requested by a high number of internet users.
To understand web acceleration, 想想那些曾经伴随我们在邮件中获得的特定拨号服务的cd的营销宣传吧. The marketing pitch offered potential users the ability to "turbo boost" of their web browsing experience, at much faster speeds than their 56Kpbs modem would allow.
Accomplishing this was fairly simple, 因为拨号服务提供商的每个上网用户都需要拨号到它的一个接入点(pop)。. When a particular piece of content was request multiple times, the service provider would cache that content at one, several, or all of its POPs, 这意味着对该内容的任何请求都不会通过昂贵的互联网网络线路，而是从服务提供商自己的数据缓存中提供服务.
Today, however, web acceleration is a bit more complex, 因为即使是美国最受欢迎的互联网服务提供商，也只能为美国总人口的22%提供互联网连接.S. online population. The CDN delivering content that can be web accelerated needs to cache content at many locations, 导致在美国大多数数据中心和ISP头端/ pop都需要在全国范围内部署大量服务器.
The general-purpose CDN market is highly fragmented outside of the United States, falling along language lines and country boundaries, except in a few parts of Europe and Asia. Even in those markets, 单一国家的语言集群可以使多个cdn能够进入市场的各个部分. India is a good example of this, with more than thirteen different distinct official languages, depending on the given state or municipality.
On-Demand Video CDNs
Some general-purpose CDNs also provide CDN services for on-demand video content. The thinking is that video content is just a large file, like a game or large application download, so the serving of video content should not be much different than other content.
A few years ago, the disconnect between general-purpose and on-demand-video CDNs was quite distinct, as video delivery required the use of a streaming server.
Streaming servers deliver the content at the time of a request, but only deliver the bits requested rather than the entire length of the video clip. This was helpful for the content owner who was paying the CDN for delivery by the bit, 由于观看者选择在剪辑持续时间的中途放弃观看，因此不会下载整个剪辑, regardless of the viewer's internet connection speed.
CDNs didn't necessarily want to spend the extra dollars for hardware and streaming server software, just to limit the number of bits being delivered to the viewer, so a few other ideas were tried.
One was the direct download, where the entire video clip needed to be downloaded to a viewer's computer before it could be viewed. While this is acceptable for downloading a game or computer application, the tolerance level of viewers waiting for content to download—especially something such as a movie, which could be over 1GB in size—was low. No one wanted to wait, upon selecting a movie to view, twenty minutes or more to view the content.
Another option was the progressive download, which is used by a number of online video sites, such as YouTube. In this version, the CDN begins delivering the download, but a viewer can begin watching content within the first 3-5 seconds, 假设观看者的快速互联网连接将继续以足够快的速度下载视频片段，以避免耗尽可观看的内容. As the speed of internet connections outpaced the bitrates used for standard-definition content, though, many viewers would have the entire video clip downloaded before they were even halfway through viewing the content.
Abandoning the video clip partway through still meant that the content owner was paying for full delivery, since the entire clip had been downloaded. This issue, of course, is exactly what a streaming server was designed to avoid, 因此，人们开始寻找一种渐进式的下载解决方案，这种解决方案可以抑制内容交付(将内容下载的速度限制在比观看内容快一点的水平)，同时还可以避免使用专门的流媒体服务器来提供点播内容.
In recent years, the best solution to emerge is called HTTP streaming, and it is coupled very tightly with adaptive bitrate (ABR) encoding and delivery.
Just like it sounds, HTTP流使用通用的HTTP服务器(通常基于Apache或Windows Server)以与其他HTTP网站内容(如图像和文本文件)相同的方式交付按需视频文件. HTTP delivery has had the capability of throttling for a number of years, even before the advent of online video, and these same techniques can be applied to throttle on-demand video clip delivery.
The most interesting addition to the CDN arsenal is adaptive bitrate (ABR), which converts a video stream into fragments or chunks, often 2-10 seconds in length. ABR以各种比特率创建离散流，然后使用来自互联网用户的视频播放器的反馈来动态检测视频剪辑交付的最佳网络速度.
随着网络条件的变化——无论是好是坏——在给定的时间段内提供最合适比特率的流. As this is a streaming solution, it keeps the content owner happy by limiting the bitrate and delivery of content to just what the viewer will consume, and it benefits the CDN by limiting the number of specialized streaming servers needed for on-demand video delivery.
Not surprisingly, given the benefits of ABR, there are multiple ABR solutions in the marketplace today, including one each from the Big Three – Adobe, Apple, Microsoft – as well as several proprietary solutions, such as Move Networks. All act in a fairly similar manner, and some even allow delivery via HTTP, either HTTP streaming or progressive download.
Live Video CDNs
Despite the advent of ABR and HTTP streaming, there still is a need for live video delivery, since live video can't be cached like content that's previously recorded.
This area is probably the least mature of all the CDN models, for three reasons.
First, cdn提供的绝大多数视频内容都是点播视频:据估计，高达95%的在线视频都是点播视频内容.
Second, since live video can't be cached, 有必要修改基本的CDN基础设施，以便在中心位置和查看内容的最终用户之间拥有非常高带宽的管道，或者拥有稍微低带宽的管道，将直播流发送到离最终用户更近的中继器或反射器.
Third, given the two points above, 为非常受欢迎的直播活动构建和维护一个直播解决方案的成本是令人望而生畏的:构建和维护一个超过百万观众的直播解决方案是相当昂贵的.
考虑到必须使用实际的流媒体服务器，并且每增加一个同时观看的观众，带宽交付成本就会急剧上升(点播内容交付通常看到同时观看的带宽是直播观众数量的十分之一)，直播视频交付解决方案将需要在其峰值观看水平上几乎持续使用，以达到空中电视和传统有线直播视频传输的规模经济.
这并不是说实时视频CDN不可行:无线广播公司使用其部分数字频谱向移动电视或手机传输视频的举动只是CDN的另一个版本, 尽管它在设置输电塔和覆盖广泛的地理区域方面领先60年.
The problem with the blockbuster online live event, however, 是否经常需要几个月的计划来增加一个视频直播网络的规模，以达到预期的观众收视率. If successful, and more viewers attempt to watch than the system is built for, there is a risk of technical failure; if fewer viewers choose to watch the live event, there's the risk of financial failure from overbuilding the CDN for a specific event. The challenge is great, but the industry has shown it can overcome large challenges, so expect to see significant movement in this part of the CDN space over the next two years.
Current CDN Market Landscape
Dan Rayburn, who tracks the CDN market for StreamingMedia.com as well as his own blog, has seen the market go from 27 vendors in 2007 to almost twice that many at the end of 2010. 其中一个原因是CDN的定义——从流媒体的角度来看——也正在扩展到更“传统”的CDN服务，如小文件传输和网络加速. These aren’t new services, as the Tier 1 service providers has offered webpage acceleration for years, but their importance to streaming is growing with the continued inroads of Adaptive Bitrate and HTTP streaming.
Vendors in the CDN space often span the three areas mentioned above, so the list below breaks out CDNs by Tier 1 service providers, data service providers, and a few key stand-alone CDNs. It is not intended to be comprehensive; a complete list of CDNs is provided on Rayburn's blog.
Tier 1 Service Providers
AT&T, Bell Canada, Deutsche Telekom, Global Crossing, Level 3, Tata Communications, and Verizon
Data Service Providers That Also Provide Some CDN Services
Amazon and Rackspace
Stand-alone CDNs
Akamai, EdgeCast Networks, Highwinds, Limelight, Mirror Image, iStreamPlanet, Octoshape, PowerStream, StreamGuys, Streamzilla
Technology Providers to Many CDNs
Adobe, Cisco, Jet-Stream, Juniper, Microsoft (Microsoft also has Windows Azure, a cloud computing infrastructure)
Why CDNs Matter to You
So why should you care about the history and current state of the content delivery market? The answer depends on whether you are a consumer, a content publisher, or a technology service provider.
For technology service providers, 紧跟数字媒体交付领域的最新趋势是向内容所有者和cdn提供服务的关键. The trend toward HTTP-based video delivery may appear to be a “dumbing down” of delivery options, but as great amounts of premium content begin to move online, the owners of this content will be looking for ways to both monetize and protect content, while maintaining a lower overall delivery cost. All three of those needs converge at the CDN.
For consumers, an interest in CDNs may appear to be about as fun as watching paint dry, or understanding the inner workings of a radio or television transmission system. 然而，消费者可以随时在任何设备上轻松地观看内容，这是在线视频消费的潜在好处之一，这与传输和内容版权管理的进步直接相关. 了解CDN提供的内容类型，可以让您在分发自己的视频剪辑时更好地了解.
For content owners, understanding the various types of CDNs, as well as the ancillary services such as web acceleration and application delivery, is beneficial when it comes to presenting your compelling premium content in innovative ways. Whether it’s reduced delivery cost for content not destined for the television screen, or repurposing content that can reach an audience much bigger than just a national market, content owners need to be knowledgeable about the trends in content delivery to guarantee protected, efficient forms of media delivery for the most cost-effective monetization strategies.
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