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It's basically like this:
Per gigabyte billing related to megabit per second MBPS in the following way. If your server were to push out 1 megabit constantly for a 30 day month with no up or downswing you would push approximately 320 gigabytes. The reality is that a server's inbound and outbound traffic looks more like the stock market with highs and lows. So, with billing on per mbps on 95th percentile to actually push 320 gigabytes you would most definitely be at more than one megabit with average traffic patterns - probably around two but potentially as much as 3 megabit if you traffic was very spikey.
Both measurements can be taken on your server's 10/100/1000 port where a sample of the amount of traffic being pushed out every 5 minutes is taken. Over the course of a month all of the samples are stored and then at the end of the month the top 5% highest samples are dropped on 95th percentile per megabit billing (some 400 or so samples) that represent about your 30 hours of peak usage.
All hosts pay their upstream providers on either 95th percentile billing or by buying full ports, regardless of how much is used. For example, a host might purchase 100 megabit on a gigabit port and will pay for at least the first hundred megabit as he is committed to that and then will have to pay overage if his overall 95th percentile measurement comes in over 100 megabit. Conversely, a host might purchase 100 megabit on a 100 megabit port and if he doesn't use all of the bandwidth all of the time (which a host couldn't unless he was oversubscribing and then choosing to drop packets when he is otherwise peaking over 100 megabit) just like a host could flat out choose to purchase a gigabit of bandwidth regardless of whether or not he uses it.
Most hosts charge on 95th for clients that use more than a few hundred gigabytes because that is the only metric that ties back to their cost structure. Selling by the gigabyte unless it is at a high rate puts a host at risk for someone who might say peak at 30 megabit during the day but have lows of a few megabit and only end up pushing out a thousand gigs. I have a client who is very "peaky" like that because he does live web cams so his traffic is through the roof in the evenings. To charge him on any other measurement would put me at risk of losing money on the traffic.
Hope that helps!
Cheers,
Brad
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President at MojoHost | brad at mojohost dot com | Skype MojoHostBrad
71 industry awards for hosting and professional excellence since 1999
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