The term "95th percentile billing" is often heard when an Internet connection is provided as "burstable" (variable rate) bandwidth. This is the case with the Internet connection used with colo and dedicated server web hosting plans. For these solutions, the Internet connection is provided as a Ethernet port on the data center's LAN, and billed according to your usage.

said by »www.seanadams.com/95/:

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The 95th percentile is the smallest number that is greater that 95% of the numbers in a given set. The reason this statistic is so useful in measuring data throughput is that is gives a very accurate picture of the cost of the bandwidth.

Here's an example. Suppose an ISP sells you a T1 line, but you're only using it to access the web. Even though you might frequently download very large files (filling the pipe) your cost to the ISP is negligible, because your usage is intermittent. A single T3 connection to the backbone could easily support hundreds of such downstream customers, and never become saturated. As another example, suppose you are hosting a very busy web site that half-way fills your T1 for several hours every day. This type of bandwidth is more expensive, because your ISP can't oversell their connection to the backbone as effectively.

The important thing to realize is that it doesn't cost your ISP anything to sell you a pipe of any particular size - it is the sustained rate of data transfer that costs them money. The sum of the 95th percentile usage of all of an ISP's customers predicts the peak amount of backbone traffic that the ISP will incur (in a given direction).

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How is the "95th percentile billing" calculated?

Counters on network devices are read every few minutes. The total number of bytes transferred (in and out) is determined each time the counters are read. The higher of these two numbers (inbound or outbound traffic), divided by the time period (in seconds), results in a single bps (bits per second) transfer measurement.

Each measurement is stored in a database. At the end of each billing cycle, the measurements are sorted in decreasing order. The top 5% of these measurements are thrown out. The next highest bps measurement is the "the 95th percentile", and that's the rate for that billing cycle.

More graphics and software for finding the 95th percentile can be found on »www.seanadams.com/95/

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by removed edited by big greg
last modified: 2004-06-26 17:19:43