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What is the 95th percentile, and why is it useful in measuring
bandwidth?
The 95th percentile is the smallest number that is greater
than 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).
Here are some examples. ISPs must charge for bandwidth
by one of three means:
- Sell a flat rate, possibly bandwidth limited connection,
and try to sell to customers whose usage patterns are
not so intense. Nearly all DSL providers do this. The
customers like it because they don’t have to worry about
how much bandwidth they use, and ISPs like it because
it simplifies billing, and they make more money as long
as they have plenty of low-usage customers. The problem,
particularly if the ISP is selling very fast connections,
is that the ISP can become overwhelmed by even a small
number of high-usage customers. Even residential customers
can be such high-usage clients, thanks to recently popular
services such as peer-to-peer file sharing.
- Sell a fast connection (eg 100Mbit Ethernet, which is
inexpensive) and charge for the volume of data transfer
– eg number of Gigabytes per month. This model works great
for web sites, which almost always generate traffic in
a predictable bell curve. However, it severely penalizes
customers who use bandwidth intermittently. For example,
suppose a customer runs an automated off-site backup every
night. This brief usage spurt costs the ISP almost nothing.
Although the recurring sustained data rate is low, the
customer gets charged for a huge amount of bandwidth.
- Sell a fast connection and bill by 95th percentile.
By now this should make sense – it’s a fair system where
everybody pays for what they get. The advantage to the
customer is that they get the performance of a high-speed
connection, while paying only for their actual usage.
ISPs like it because they don’t have to worry about high-usage
customers upsetting their overselling ratios.
Irrespective of billing concerns, the 95th percentile is
a very interesting and useful figure. Bottom line is it
tells you how much of your connection you’re really using
(and really need).
At Spydernet, we use the 95th percentile method of billing
for all dedicated and colo servers. This is the most accurate
and by far the fairest way to calculate bandwidth for our
customers.
The formula used to calculate is as follows:
First take the 95th percentile figure from the graph for
your server. For this example we will use 300kbps.
300kbps /8 = 37.5 KBps (Kilobytes per second)
37.5 KBps * 60 seconds/minute = 2.250 MB/minute
2.250 MBps * 60 minutes/hour = 135 MB/hour
135 MB/hour * 24 hours/day = 3.24 GB/day
3.24 GB/day * 30 days/month =97.20 GB/month
Your account would be billed for the difference between
what the plan you chose allowed and the 97 Gig of transfer.
Example:
If your plan allows you 50gig of transfer your account
would be billed for the additional 47 gig of transfer.
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