General Purposes of CDN

What is a Content Delivery Network?
For what industry and content categories
are commonly used CDN?
Executive Summary
If you ever had to deal with online video or streaming, probably, you have heard about such a phenomenon as CDN, standing for content delivery network. But what does it mean, precisely? CDN is a network used to deliver site content to the end-users more quickly and more reliably, no matter how the site is accessed – from smart phones, game consoles, personal computers, tablets, web browsers or set-top boxes. This text covers various aspects of CDN, including the history of development, market landscape, and its significance for web site creators.
CDN essentials: definition and short history
CDN is a series of servers placed in different geographical locations and storing site content (static files, scripts, CSS files and other information). It delivers the content to end-users by connecting them to the physically nearest servers, which reduces data travelling distance and promotes faster and more reliable loading.
Basically, CDNs can be divided into three categories: networks for general content delivery, including video; networks serving on-demand or pre-recorded/pre-encoded videos; and CDNs specializing on live video only.
Let's observe how these types differ and what they have in common. As has already been described, all the three technologies are used equally often today, being applied in a number of business models. They provide a lot of possibilities, starting from streaming and progressive download, and finishing by HTTP delivery.
General Purpose CDNs
The need of CDNs arose even before video portals appeared, and you have benefited from then when downloading software updates, buying a track on the iTunes Store, or just visiting a popular web site.
What general purpose CDNs do is accelerate web sites. That is done by applying a CDN that has several servers in different locations, ideally, placed close to major connection points between several ISPs, or in data centers of one popular application/game provider. What CDN does is caches the most frequently used content, stores the copies of files and delivers them to the site visitors.
A CDN allows boosting site performance and speed, making content travel much faster than a conventional 56Kpbs modem allows. All that contributes to users' experience and helps to avoid traffic spike problems.
The principle of work is quite simple. An Internet user dials to one of PoPs (Points of presence), and the closest server is selected automatically. Besides, if a certain piece of content is requested several times in a row, it gets cached at one, several or all PoPs, and the request for it never goes out through the Internet lines again, instead being served from provider's data cache.
On-Demand Video CDNs
There are also general-purpose CDNs providing separate services for on-demand video file loading. But, in fact, video content is just a large file that is similar to game or large application, so video serving is no different from other content delivery.
Several years ago the difference between usual and on-demand video was more distinct, and a streaming server was required to serve the second type of content.
Streaming servers are used to bring the content to a user at the point of request, delivering only the requested pieces of video rather than the entire clip. It was beneficial for the site owner who had to pay for delivery bits, using a CDN, because a person ceasing the view halfway wouldn't have to download the whole file, no matter what Internet connection speed was.
The main idea behind all that was just to limit the number of bits delivered to end-users, not spending extra dollars on streaming server software and hardware. There were other attempts to do that.
Another method was direct download, when a viewer has to load the whole video on his computer to view it. While it is okay, if an application or a game is downloaded, waiting for loading of a film bigger than 1 Gb is something that users cannot tolerate. No one wants to wait too long.
An alternative option was progressive download, applied by some video sites, for instance, YouTube. In this case, while a CDN starts delivering a download, a user may start watching a video in 2-5 seconds, hoping that fast Internet connection would provide the later fast loading of the complete file. Thus, if standard content is viewed, usually, internet connection outruns its bitrates, and a user manages to watch the whole clip without interruptions.
But even if a visitor abandons the clip, the content owner would have to pay for the full delivery, because it had been downloaded entirely. Streaming server was created to avoid this issue.
Recently, a more advanced solution appeared, being called HTTP streaming. It is closely connected with adaptive bitrates (ABR) encoding and delivery.
As you might have guessed, HTTP streaming applies generic HTTP servers to load on-demand video clips the same way as other website content like texts and images. The same techniques can be used to throttle the delivery of on-demand clips.
One of the most interesting CDN features is adaptive bitrate (ABR) that transfers a video stream into several pieces 3-10 sec. long. It generates discrete streams in various bitrates, then applies feedback from a user's video player to define the optimal network speed to deliver the clip.
It adjusts to network conditions alterations, selecting bitrate to serve these fragments. As far as this is a a streaming solution, it limits delivery to only what is consumes, so CDN owner benefits by limiting the servers required for on-demand video delivery.
Taking into account endless benefits of ABR, there are various ABR solutions in the net, with the most popular provided by Adobe, Apple and Microsoft. All of them have similar principle and peculiarities of working.
Live Video CDNs
Although ABR and HTTP streaming have proven their efficiency, live video delivery stil requires separate specific solutions, because live video cannot be cached.
This CDN model is the least elaborated due to several reasons:
Most video content delivered by CDN is on-demand video. It comprises up to 95% of all online videos delivered in the Internet.
It is impossible to cache live video, so CDN structure must be somehow modified to provide the possibility of delivery. For instance, bandwidth pipe can be lowered to send the live stream to a repeater or reflector located near to the end users.
Taking into account the two above mentioned points, the cost of creation and
maintenance of live streaming is dreadful. A viewer for more than a million users is very expensive.
To be implemented into over-the-air television and traditional cable transmission, this
technology must somehow be developed and simplified. The cost of live video streaming is too high, because it is conditioned by bandwidth consumed and the number of users accessing the file at once.
Besides, to increase the size of a live video CDN so that it would be able to serve the growing audience, the site owner needs to plan and do it for several months. If a lot of people are watching live videos simultaneously, there is a risk of technical failures. And if there are few spectators, the financial risks appear, connected with overbuilding a CDN for a special event. There are many challenges, but the industry overcomes them one by one, gradually evolving and improving. So in the next couple of years we may observe significant leap in its development.
Why CDNs may be useful for you?
May a CDN come in handy for you? Yes. But what are the benefits in your exact case? The answer depends on who you are.
If you are a service provider, CDN is not just a current trend, it's a necessity for your site to work properly. Video delivery is a real challenge, both technical and financial. And as far as online video watching stays one of the most demanded things in the Net, content owner need to find means to both monetize and secure the files, keeping delivery cost moderate. All that is combined in CDN technologies.
If you are a consumer, the technical side of the issue is not of particular interest for you. Yet, online video consumption is much quicker and more convenient. Besides, if one day you decide to go online and distribute your own content, you know what advantages CDN has.
If you are a content creator or owner, the knowledge about CDN types and its application would be highly benefitial to accelerate and improve video delivery. Now you are aware of the innovative way of content distribution, which is going to turn from a trend to the norm of media delivery soon.
There are many CDN resellers and direct providers. How can a user know which company is better? In fact, there is no certain leader in this sphere – every provider has his own competitive strengths and drawbacks, and you should judge by your own needs. CDN comparison and profound analysis will definitely help to find an optimal variant, and you won't have to switch providers and lose money.
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See also