What’s the 95th percentile? That’s how we measure bandwidth usage

95th percentile, metering method for bandwidth usage

Have you ever heard about the 95th percentile metering method? It has nothing to do with child growth charts, but with bandwidth usage. The 95th percentile or Burstable billing is the most efficient method to meter and bill a server’s bandwidth.

This system is used by companies that offer some kind of web hosting (dedicated servers and cloud hosting, among others) or colocation services. All of these machines use an amount of bandwidth that is billed to the customer, depending on the base commit rate he has purchased to his ISP.

What happens when a customer exceeds the purchased bandwidth?

As you might suspect, Internet Service Providers bill the excess of bandwidth usage to customers. However, they don’t bill all the data the customer bursts beyond his base commit rate. Instead of that, the ISP normally uses a method that favors the customer: the 95th percentile model.

How does the 95th percentile method work?

As we have mentioned before, the 95th percentile billing method is a way to meter the bandwidth used by the machines hosted by an ISP. This is done by sampling data transferred over the network every 5 minutes and then discarding the top 5% of the traffic. The 5 minutes intervals are metered in Mbps (Megabit per second).

Aggregated traffic Espanix
Bandwidth chart example. Source: Espanix

The 95% percentile metering and billing method works in this way:

  • The system meters data transferred every 5 minutes and stores the values of each 5 minutes interval in a database (in Mbps). In order to meter the average rate of an interval, the bits transferred during 5 minutes are divided by its duration (300 seconds).
  • With the samples stored during a month (understood as a 30-days period) a chart is created to verify the evolution of bandwidth usage. In this chart, data is sorted from lowest to highest.
Measuring bandwidth usage with the 95th percentile
  • Throughout a month, approximately 8.640 samples are taken from each machine. From these, the top 5% are discarded (around 432 samples or 35 hours of a 30-days billing period).
  • After discarding that percentage, the highest value is taken as the month’s bandwidth usage. If this value doesn’t burst beyond the base commit rate purchased by the customer, the ISP doesn’t bill any extra bandwidth. However, if after discarding that 5% the customer has exceeded his committed data rate, that excess is billed.

This metering method is designed for favoring the customer and for not penalizing him for data transfer peaks. For instance, peaks when posting a news on a media or blog, or when launching a new product on an eCommerce. The 95th percentile allows the customer to exceed the limit of purchased bandwidth during short periods of time.

The goal of this metering method is to guarantee the purchased bandwidth fits the project’s needs; without overestimating or underestimating resources.

If you want to know more about the 95th percentile, you can read the Burstable billing article on Wikipedia. There you can get further information about this bandwidth metering method, which is a standard among most of the ISPs worldwide.

Another metering alternative is to meter a server’s or website’s traffic by GB transferred. But, as a rule, you will end up paying much more this way if your traffic volumes/bandwidth are very high.

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