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There are a couple ways of getting charged for bandwidth. You could by an “unmetered” connection rated at a given speed, say 10 or 100mbps. Sometimes those lines are shared so you won’t really be able to use it all, but that’s another discussion. But if you’re buying an “unmetered” connection, be absolutely sure to ask if it is shared with others. If it is, you’re at their mercy.

Also available are x GB or x TB of “bandwidth”. Technically it’s not really bandwidth, but traffic. Again that’s another discussion… But you’ll see it either way. The question to ask there is it incoming, outgoing or combined. You want to know ahead of time depending on what you’re doing.

Lastly is probably the fairest of them all, but also the scariest for smaller providers when buying… it’s called “95th percentile” billing. In this case you will typically have a port speed (10,100,1000mbps for example) as well as a “committed rate”. That committed rate will be how much you’re paying for every month regardless of usage. So if you commit to 5mbps, and only use 1, you still pay for 5. But the more you commit to, the less you pay generally per mbps. Even though you commit to say 5mbps, your connection is burstable all the way up to the capabilities of the line – 10,100,1000mbps (minus ethernet overhead, bottlenecks, etc.). 

That great in that if you get slashdotted, or digg’ed, your might be able to keep your site up… assuming you server, etc. can handle it. But you bandwidth is going to spike. The 95th percentile method computes the bandwidth usage as how much line is required to handle the load 95% of the time.  That is to say if chop off the top 5% or the usage TIME wise, that’s how much you need. There’s about 720 hours in a month (30 days times 24 hours a day),  5% of that is 36 hours. So if we chop off the peak 36 hours, how big of a line would be needed to handle the rest… THAT is what you’ll pay.

Using the normal logs, you can usually tell how much traffic you sent in and out. Or at least get a good estimate. That works for the “x GB / x TB” scenarios. With the “unmetered” scenario, you don’t really have to care provided you’re getting all the bandwidth you need, as there’s no overage charges… thus why people like them, even if they are shared as they often are. But (and it’s a big hairy one), your website logs are almost useless for telling what your 95th percentile usage is… Almost. It can give you an idea if you should get worried, but that’s about it. Think about it, anything less than 36hours at the full 100mbps and < 5mbps the restof the time, and there’s no overage charge with just a 5mbps commit. But 36 hours and 5 minutes (a common sampling rate) at 100mbps, and you suddenly have a 95mbps overage charge. Even if it’s $10/mbps (a pretty decent rate for mixed, excellent for premium) and you have a new $950 bill to pay. Scary huh?  The key is that any provider selling to you at 95th percentile should (and I’ve not seen one that doesn’t) provide you with real-time stats of usage.

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