[ On , April 19, 2001 at 16:42:38 (-0700), Sean Donelan wrote: ]
Subject: Re: What does 95th %tile mean?
The power industry has a version of the 95% billing. Like ISPs, originally
it was a simple measurement every 5 (10, 15, etc) minutes. Figure out
the peak measurement, and you are done. Users figured out how to better
control their usage, i.e. turning on motors for 9 minutes and then turning
them off for six minutes.
Hmmm... I've never seen that type of power measurement in household,
small business, or farm usage; at least not anywhere across Canada.
Here the meters are the type that spin at a rate determined by the
current flow. I could check my texts on the subject but IIRC the meter
actually counts up the total current flow through the meter regardless
of "rate" and then the monthly reading is divided by the number of hours
in the month to get watt-hours and that's what we're billed for.
I.e. it's done almost exactly the way bulk throughput Internet usage is
billed, but since a power meter can't count watts as directly as a
router counts octets they have to measure the rate and derive the count.
The meter has to be a fairly precise and carefully designed instrument
though because it must react quickly enough to current flow changes so
as not to be fooled by a quickly pulsing load.
The only tricks to help reduce peak-usage that I've seen around here are
the likes of electric water heaters that are diabled during peak load
times by a little relay out on the street. Instead of using a separate
meter they simply bypassed the meter and charged a flat monthly fee
people figured out how to hook things up to their water heaters
illegally, etc., etc., etc. There's not much of that around here any
more because of that and partly because the pricing structure they
originally used for this scheme was flawed: it was based on some
statistical average usage for the customer base. Trouble is the
customer base changed and changed their usage patterns more dramatically
than they accounted for and their contracts locked them down in what
turned out to be inappropriate ways. In the long run it would have been
better to fork out the capital costs and use two meters with different
rates for the different types of usage (i.e. have a lower rate on peak
limited controlled usage). I wonder too if the usage patterns didn't
change enough that there was no more reduction of peak loads when the
water heaters were cut off, what with fewer housewives doing laundry,
more use of cold water for laundry, etc.
In any case the analogy with power meters is entirely flawed because
Internet usage on a fixed rate pipe is an entirely different critter.
In effect the custoemr is buying voltage, not current, and that's why a
95'th percentile metering of the peak "voltage" is more appropriate for
Internet billing in many cases.
As a customer I can download the same movie at full rate over a short
period of time (eg. the duration of the movie) or at a very low average
rate spread over a whole month. Obviously if I do it the first way then
my ISP want's to bill me for using the full rate (even if I only did
this once in the month), but if I do the latter then the ISP only needs
to bill me at the very low average rate because that's all I used.
Either way my total byte count is identical. However unless I'm
extremely patient I'm not going to wait a full month to view the movie,
no matter how little my bill might be at the end of the month.
Nth percentile billing works best when ISP hands the customer a rather
fast pipe, eg. a full-duplex Ethernet interface, but when the customer's
usage will not keep that pipe full. The customer wants the fast pipe,
but does not want to pay for it as if it were full all the time. The
important factor from the ISP's perpective is not the total throughput
(though that's definitely not to be ignored), but rather the peak usage.
So since the ISP can often save money by handing low-cost high-speed
ports (eg. etherent) to the customer, he still wants to get paid for
peak usage. The value of `N' in an Nth percential billing agreement is
set to balance between the customer's perceptions of his requirements
and the ISP's very real need to buy upstream bandwidth that might just
possibly be able to fill the customer's pipe on demand.
Oh, and obviously this is another reason why the ISP will also charge a
"port charge" and why they might include a certain base rate (eg. 1mbit)
in with the port charge.