What does 95th %tile mean?

Greg A. Woods woods at weird.com
Fri Apr 20 00:56:43 UTC 2001

[ On Friday, April 20, 2001 at 09:54:25 (+1000), Geoff Huston wrote: ]
> Subject: Re: What does 95th %tile mean?
> Now if you sample every five minutes, and the sample point is synchronized 
> to the peak and trough of the five minute rate you will get successive 
> readings of 'line rate', zero, 'line rate', zero, etc. The 95% sample value 
> will be 'line rate'.

Yeah, but synchronizing your usage that exactly with the sample taker is
almost impossible, at least with the average Internet pipe.

Not to mention but even if you managed to synchronise your usage profile
to the sample taker in that way then you really did use "line rate"
bandwidth for a significant period of time and that's what you should
fairly be billed at.

> If you change _nothing_ except shift the sample point two and one half 
> minutes forward in time the sample points will consistently produce 
> outcomes of 'half line rate', 'half line rate', ..., and the 95% point is 
> 'one half of line rate'.

(in this case the bill would be more in line with what the customer
might actually perceive as his usage, but not in line with the ISPs
perception of that usage)

What does it matter though when it's almost impossible for the customer
to adjust their usage profile in such an accurate way?
"Statistically" the sample period will measure the peak usage and if one
treats the 95th percentile rate as the actual bandwidth used over a
longer billing period then the bill will be fair for both parties.

Of course if your billing period is insanely short (eg. 1 hour) and
doesn't cover the peaks and valleys in actual use over at least several
days then you'll have screwy bills too, but they should still be fair.

The point is that in the real world of *Internet* usage Nth percentile
measurements, if properly done, are indeed fair, especially for
aggregated pipes where significant bursts that would raise the rate
can't be affected by one or two users doing a couple of big downloads.

							Greg A. Woods

+1 416 218-0098      VE3TCP      <gwoods at acm.org>     <woods at robohack.ca>
Planix, Inc. <woods at planix.com>;   Secrets of the Weird <woods at weird.com>

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