95th Percentile Methodology: What It Is, How It Works

A term pertaining to colocation bandwidth that you may have heard is the 95th Percentile Monitoring – also known as the 95% Method. So what does the term mean? To illustrate, we will look at a monthly graph of an Internet uplink port:

95thPercentileGraph

As you can see, this graph, has the data on the vertical axis, and the pass of time on the horizontal axis. About five weeks’ worth of data is shown here. Please notice that the sample rate has been consolidated to two-hour intervals in this graph – using smaller intervals does not make a statistically significant difference when graphing data over a larger time span.

So, how does a colocation service provider like us compute bandwidth usage for the month using this data? You might assume that it would be fair to add the Average In and Average Out values (Internet traffic travels in both directions) – a total of 36.3 Mbps in this case – and declare that to be the official usage for the month shown. In fact, many other data centers will do just that. However, there is another way: the 95th Percentile Methodology. Here is how it works:

  • A log of 30-days-worth of traffic samples at default settings (typically every 5 minutes) are accumulated;
  • The log is sorted in descending order, which places the highest traffic peaks at the top;
  • The top five percent of the log is discarded;
  • The value of the largest remaining peak becomes the effective bandwidth usage value for the month.

The advantage of the 95th Percentile billing method is that, over the course of a 30-day ‘month’ you can have up to 36 hours of peak traffic and that number is calculated only for the direction of greater flow, making the other direction free (as opposed to the average in + the average out method). In the example of the monthly graph above, the actual 95th Percentile traffic number for that data is 25.7 Mbps – some 10.6 Mbps lower than using the ‘sum of the averages’ method.

As you can see, the 95th Percentile Monitoring can be quite advantageous when acquiring bandwidth.

However, there is one exception to this. That would be in situations where there is normally very little bandwidth activity, but periodically large bursts of high traffic activity that occurs. The 95th Percentile could potentially work against you in the event that the cumulative bursts added up to more than 36 hours in the billing period.

In practice this situation is uncommon; the best way to address this is to try to know (or predict) your usage profile and order service accordingly.

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