What to look for when choosing a video CDN

When you are managing a large video distribution platform, you probably know about the advantages of using a CDN, so I am not going to elaborate on that. But what factors do you need to consider when choosing a CDN? This blog post will give you some insights on this.

Bandwidth pricing

One of the most important aspects that comes to mind is bandwidth pricing. The second one is the CDN provider’s policy regarding bursting. Is bursting allowed and if so, to what level can you burst? Reasonable amounts for video on demand (VoD) platforms are two times the agreed 95th percentile or four times the average data traffic.

Multi-vendor strategy

Many customers nowadays choose to use a CDN balancing service. This can help you migrate from one CDN to another without the risks involved when doing a “big bang” change. It also helps avoid vendor lock-in and gives you the best position to negotiate prices and qualities. But you must make sure that the CDNs you want to balance offer the exact same features, or the balancing will not work.

Network quality

Network quality on CDN requests is measured in latency (milliseconds to receive an answer) and bandwidth (amount of bytes per second in the answer). Latency is influenced by various elements, including the geographic location of the nodes answering the requests. Bandwidth on the other hand, is dependent on the volume and amount of peering and transit connections, as well as their current usage. The network can be as fast as a Ferrari, but even the fastest car comes to a standstill when there’s a traffic jam.

Latency

As straight lines rarely occur in global communications, the speed of the data transit will never reach its optimum speed. Optical fibers provide the best method for transmitting data globally, however, even these fibers reduce the transit speed by up to 35%, depending on their refractive index.

Therefore, the nearer the CDN nodes are to the location of the website visitor, the lower your latency will be (in addition to a lower risk of packet loss, which in turn influences bandwidth). The calculation behind this atency is not just the round-trip time (two times the distance at 65% the speed of light), because a TCP connection has to be build up. A TCP connection does a “three way handshake” to establish a connection after which it can start sending data. It is best visualized like this:

If you are in Rome and the CDN you use has a PoP in Amsterdam, the theoretical minimal time-to-first-byte can be calculated by multiplying the latency by 5, for example:

• 807 mi (1298.74 km) = distance RA
• 186000 mi/sec = speed of light
• 65% = speed of light in fiber due to refractive index
• 5 = packets exchanged when first byte is received (see picture)
• Expected latency RA = 807 / 186000 * 65% ≈ 7 ms
• Time-to-first-byte RA = 5 * 7 = 35 ms

Bandwidth

Latency is a delay noticed only when starting the video, but a lack of bandwidth can ruin the entire video experience. Having not enough bandwidth causes “rebuffering”. Rebuffering is the most important measure for quality of video delivery. Bandwidth follows the “weakest link” principle, so if there is a network segment (even the local Wi-Fi) that cannot provide bandwidth at the bitrate of the video in the path from the visitor to a CDN node, the video will stop and the dreadful spinner will appear.

This is why bandwidth is actually much more important than latency for a video CDN customer. A good video CDN provider monitors connections continuously for capacity and upgrades them when needed.

As mentioned earlier, bandwidth in a network depends on the peering and transit connections. Here’s how the Leaseweb network, which at 3,5 Tbps capacity has the largest capacity in the world, is set up:

• Transit Providers: Level3, Telia Sonera, Global Crossing, Tata Telecommunications, Deutsche Telekom, Cogent
• Internet exchanges: AMS-IX, LINX, DE-CIX, and 27 others throughout Europe and US
• Multiple private peering agreements

As an example of how this all works out, you can easily are some test files on some of our nodes so you can measure the bandwidth from your location to some of our locations:

• Download test file The United States (Washington D.C.)
  10MB, 100MB, 1.000MB, 10.000MB
• Download test file The Netherlands (Amsterdam)
  10MB, 100MB, 1.000MB, 10.000MB
• Download test file Germany (Frankfurt)
  10MB, 100MB, 1.000MB, 10.000MB

Conclusion

To summarize: If you are running a video distribution platform, your main focus should be on network quality. Latency should not be too high, but having enough bandwidth capacity takes precedence, in order to avoid the rebuffering of videos and provide an optimal quality of experience (QoE). Combine this with a cost-effective pricing model and a good vendor strategy and you’ll have all the ingredients for a strong yet flexible CDN solution.