Best of the ISP-Lists

Pay for 95 Percent

Members of the ISP-Colo list discuss how companies measure bandwidth use on burstable or flexible pipes. The measurement is more complex than advertised. Buyers and sellers, take note.

[March 12, 2001]

On the ISP-Colo list in February, MH queried,

"When a company says they charge by the 95th percentile, what does this mean exactly?"

A number of respondents offered their definitions of the terminology:

[JA explained] "Most burstable or flexible pipes are billed on the 95th percentile. This means that you pay as you use the bandwidth. The company usually will take two readings every ten minutes, one reading the traffic going up and the other reading the traffic going down. The higher of these two is recorded.

At the end of the month the numbers are lined up, highest to lowest. If there are 100 readings, the top five will be discarded, and the 94th reading will be the measure of the level of bandwidth used. Of course, there are many more than 100 samples, so they just chop off the top five percent—hence the 95th percentile."

[EJ noted] "This is how we put it on our web site: 'We sample your actual usage every five minutes. We then average the total and post the result as a five-minute usage point on your usage graph. Over the month, we will continue to plot the five-minute averages, which total about 8,640 points plotted on the graph.

We then take the top five percent of your usage (432 points, or 36 hours) and throw it out!

Your usage is determined based upon the highest remaining usage plotted. This method of billing provides you with a number of advantages. Any usage bursts that are untypical of your bandwidth requirements are not charged to you, and this equates to receiving approximately your highest 36 hours of bandwidth usage free each month.'"

[JM added] "The ISP takes what amounts to a five-minute 'snapshot' of your incoming and outgoing bandwidth usage, usually a two-second sampling. The ISP throws out the lowest sample and keeps the highest—it's put into a graph as a spike, like what you would see on a heart-monitor. At the end of the billing period, ISP throws out the top 5% and bills at the 95th percentile. Whatever the spike reads at that percentile, the customer is billed within the specified tier."

JT had a more cynical view of these explanations:

"I always love the spin on this! I don't think I've ever seen it where the 95th percentile wasn't explained as '... throw out the top 5% and bill you at...'. It could just as easily be stated as 'We throw out the lowest 94% and bill you at…'. Is the glass half full or half empty? It's a simple statistical calculation. Nothing is being 'thrown out,' though I'm sure this sounds more palatable to most people."

— End