1. Bandwidth needs
What are your bandwidth needs? Are you going to use 50GB/month or 50TB/month? CDNs charge by GB transferred (in most cases). If you're only delivering a small amount of traffic, it may not be necessary to purchase CDN service. You might be able to get away with upping your current web host provider from a shared environment to a dedicated environment.
It doesn't make sense to pay a Tier 1 CDN thousands of dollars a year to deliver 4 videos. If you're having so many problems with your video or software downloads, then look at the root cause and fix it!
When you deliver about 500GB/month, it makes sense to offload that heavy lifting to a CDN. By now, you are getting several thousand requests per month or even per second, and your single web server in 1 data center won't be able to keep up with the traffic.
Certainly, when you are doing over 1TB/month of static content delivery, you should use a CDN. This will ensure that your videos, podcasts, music, images, documents, and software downloads get to your customers quickly and efficiently.
2. Network Performance
All CDNs, big and small, say they have the best network! There are basically 3 kinds of CDNs: Internet-based, Peering/Private based, and Peer to Peer (P2P).
The only internet-based CDN is Akamai. Akamai has thousands of servers all over the place. Then using some fancy algorithms, they route traffic from 1 PoP to the next, getting your content onto the backbone of whatever ISP your end-user is on. They then cache the content in that closest PoP, so the next person in that region/ISP has the content already close to them. Obviously, this method works as Akamai is the biggest CDN globally and boasts the most customers.
A peering/private CDNis one that puts servers in regionalized PoPs around the world. Then in those pops, they peer with or directly connect with as many ISPs and backbones as they can. Then when someone requests a piece of content, the file is delivered directly from the CDN to the end-user network and can bypass theInternet altogether, in most cases. Most other CDNs use this model. LimelightNetworks is the most successful in this configuration. They have a private fiber backbone as well to move content from Origin Server to PoP. Another CDNswho follows this model are Panther, EdgeCast, Level3, CDNetworks, and others.
Finally, the idea ofP2P is intriguing. Have all content viewers act as a PoP and replicate the content around the globe. There's little or no infrastructure cost, and theoretically, you can get your content onto any ISP in the world. P2P has its place, but this method should be avoided as a means to deliver mission-critical and revenue-generating content.
As a side note, there are Hybrid CDNs who employ P2P and Peering/Private methods. These are intriguing, however for secure delivery, using a P2P is less desirable. Your content will end up on hundreds to thousands of individual computers with little or no control over who gets access to it.
3. Technology
Does your CDN support the technology you require? For example, all CDNs will deliver content via HTTP Progressive download. But does your CDN support true Flash Streaming (RTMP), true WindowsMedia Streaming (MMS, RTSP), Quicktime, or Real Media streaming? What about flash Live or Windows Media Live? Can they do MP3 Live? Do they have a TokenBased Authentication secure URL product? Can they do pseudo-Flash streaming? Do they have any special services for HD delivery? What about a mobile CDN platform? Is it easy to get content to the CDN?
Finally, what about their analytics? Do they offer quality analytics? Is it easy to use? Does it show the number of requests per object? Is there a content management piece? Do they offer Geo-Reporting? Can you get raw logs?
4. Other products and services
What else can your CDN of choice do for you? Do they have a professional services department? Can they help with monetization? Do they offer encoding/live transcoding? What about digital rights management (DRM)? Do they offer a live event monitoring service? Is there a content management system or digital asset management system available? Does your service include embeddable media players? Can they cache whole websites? Do they support e-Commerce or shopping carts?
5. Support What kind of support can your CDN offer?
Ask for the number of the helpdesk and call it. How quickly did they answer? Did you get a person or just voice mail? Is there email support available? Do you have access to technical personnel during the integration phase? Who do you call if you have a question about your bill? Does your CDN even offer support? What happens if you call in the off-hours? What does their Service Level Agreement look like? MostCDNs offer a 100% SLA, but what does that really mean, and how do you get credit if they don't meet their SLA?
6. Contracts Does your CDN require an annual contract?
Do they offer a month-to-month contract? Are they asking you to commit to a minimum amount of money per month whether or not you use that much? What happens if you go over your commit, how much is that going to cost you? Can you pay with a credit card? Do you have to pay with a credit card?
Conclusion
Consider all these factors when deciding which CDN to go with. The biggest factor is how much traffic you are going to pass. You may have fun driving that Lexus, but you can still get to work in your Toyota. Choose a CDN that meets your needs and fits your budget.
