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The above calculations would only be valid if you used exactly 50 Mbps all
month, with no busy times and no slow times, or if you AVERAGED
50 Mbps. But Mbps isn't normally measured as an average, so you can't
just calculate it by multiplying by the number of seconds. Rather, bandwidth
is measured by either peak or 95th percentile.
Normally, bandwidth is billed under a formula called 95th percentile. Every
five minutes or so the average Mbps for that 5 minute block is recorded.
At the end of the month, all of the numbers are sorted from highest to lowest.
The highest five percent are thrown out, and you are billed for the highest
number remaining. That's generally going to be about double your average
Mbps, so figure 8 TB rather than 16 TB if you're asking how many TB you
can transfer and be billed for 50Mbps. That figure depends on how your
traffic varies throughout the day and throughout the month, though. For
typical web traffic, divide the TB in half or double the Mbps. If you're talking
about something other than web traffic the factor could be very different.
Bandwidth can also be measured peak, such as when talking about a
10Mbps or 100 Mbps ethernet connection. A 100 Mbps line can carry no
more than 100 Mb in it's busiest second. Figure your average is around
1/3rd of your peak, so if you're doing 16TB you'll peak out at about
150Mbps. If you decide it's OK for the site to be just a little slow during it's
busiest times, you could run a site pushing 16TB on a 100Mbps line.
The site would just be a bit slow during it's busiest times, when it would
use 150 Mbps if it could.
In the paragraph mentioning the 100 Mbps, I actually ignored one important
factor. 100Mbps or other line speeds are signaling rates, not data
transfer rates. The electrical signal can switch on and off 100 million times
per second, but due to protocol overhead it can only transfer about 86Mb of
data per second. So a 100 Mbps line is really a 86 Mbps line in terms of the
amount of data it can move.
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