95th percentile refers to a method of bandwidth tallying, most commonly associated with colocation. In 95th percentile billing, the customer is not charged for the top 5% of bandwidth used in any given month.
Advantages of 95th percentile billing
- Can help avoid large fees incurred due to DDoS attacks
- Allows users to "burst" (use larger than normal amounts of bandwidth) without having to pay for the highest amount used
Bandwidth is measured (or sampled) from the switch or router and recorded in a log file. In most cases this is done every 5 minutes. At the end of the month, the samples are sorted from highest to lowest, and the top 5% (which equal to approximately 36 hours of a 30-day billing cycle) of data is thrown away. The next highest measurement becomes the 'billable utilization' for the month.
With the 95th percentile billing method there is potential for paying for bandwidth that is not used. However, this method will also allow brief access to peak bandwidth when required. For example, if a website is Slashdotted for an entire weekend (48 hours) then billing would be based on a higher rate just because of that single weekend. Whereas if the peak is shorter (less than 36 hours), the peak would not appear on a usage bill for the month.
In the 95th percentile billing method inbound and outbound traffic is usually calculated separately. The highest value is then used for billing rather than the sum.
--Section information from Wikipedia.
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