You may have heard of burstable billing, burstable rate or 95th percentile monitoring when looking at purchasing bandwidth. This is especially common for colocation and dedicated server providers, but also for Internet transit providers. But what is being measured and what does burstable 95th percentile billing mean to your bottom line? How much will this type of billing cost you?
First let’s look at three different but common types of bandwidth graphs.
When using bandwidth, most users have a fairly stable traffic graph. This is usually a rolling graph over time, where there is a peak usage time and then a low usage time and the graph is nice and steady.
Others might have a traffic graph that generally stays consistent at all times.
The thing with both of the above bandwidth graphs is that they are consistent. The user has an expected top amount of traffic and does not go beyond that. That type of bandwidth usage is easy to calculate and rather fair for everyone involved.
But what about users who generally have constant bandwidth usages, but have sharp but very brief spikes from time to time? Something like this:
The above graph has a fairly consistent bandwidth usage, with a few very large and very brief spikes in bandwidth. What is the best way to charge someone like this? It does not seem fair to charge this user for the peak traffic of around 960Mbps when they are usually only using less than 100Mbps.
So to provide a more fair form of bandwidth billing, many providers use 95th percentile billing. Basically, what this means is that the top five percent of bandwidth used is not charged. This would not be very significant for the first two graphs, as their bandwidth is distributed fairly evenly. But for the third graph this might be a huge difference. If the bandwidth provider is ignoring the last highest five percent used, then those peaks where the usage got up to 960Mbps might be ignored. The customer would actually only be charged for their more ‘normal’ usage of less than 100Mbps.
Of course, this depends on how many of the large blue spikes there are. It may be that they cover more than five percent of the billing cycle, in which case the customer will be charged for all the bursts that go beyond five percent of their usage.
Note that typical bandwidth billing does not combine inbound and outbound traffic. Only the higher of the two. On the above graphs, one the blue line is traffic going out of that port and the green is traffic going into the port, but for billing calculations, at any point on the graph the higher of the two colors is what is considered the usage amount for that time period. The two are not added together.
A typical billing cycle is 30 days. five percent of 30 days is 36 hours. So if you are being charged at a burstable 95th percentile rate, the highest bandwidth usage of 36 hours will be free. Some providers may have a 90th percentile bandwidth rate. If this is the case, then in a 30 day billing cycle, the top 72 hours will not be billed.
When is Burstable Billing a Bad Idea?
The 95th percentile billing idea works great for a lot of users, but not for all. If you have a traffic pattern that has a lot of down time and then a fair amount of very high usage, then the 9th percentile monitoring and billing system might be more expensive than flat-rate billing. Take a look at this traffic graph:
Notice that generally there is very little traffic being used. Almost none. But there are periods of high traffic usage. These periods last long enough that they will not be contained within the top five percent of the port’s bandwidth use. If this were a burstable port then the fees could end up being more than if it were a flat-rate port.
A flat-rate port is simply an agreed upon price for all the available bandwidth on a port. The price is the same no matter how much bandwidth is used.
So depending on what your traffic patterns will look like, the 95th percentile billing system may save you money, however, if your traffic consists of numerous high bandwidth periods followed by periods of almost no usage, then a flat rate port may be best for you. Likewise, if you expect to use close to all the bandwidth of a port on a consistent basis, then there is little reason to go with burstable billing. Go with flat-rate.
