Every 5 minutes, your switch port is polled for it's current bandwidth counters, which is compared to the previous collected counters to calculate the rate for that 5 minute period.
At the end of the month, all the 5 minute periods are sorted, and the top 5% of samples are discarded. The highest remaining one is your 95th percentile rate.
95th percentile makes sense, because it measures how much capacity your provider needs to allocate for your use, with a reasonable (although somewhat arbitrary) measure of probability.
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If someone is selling you 2000GB of data transfer, and then trying to fit it into a 95th percentile billing model, something is very, very wrong. There are approximate ways to estimate how many GB per Mbps in 95th percentile, but usage patterns vary from server to server, customer to customer, making it impossible for there to be an exact way to convert GB usage into Mbps on 95th percentile ...
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Traditionally if your provider allows you 2000GB, it is of data transfer. If they offer 95% percentile billing, it should be billed in Mbps with overages. The only way I could see them billing you on 95% percentile for 2000GB of transfer would be by calculating your monthly 95% usage, and multiplying that number by 330GB / Mbps, which is a model very prone to error. Were I you, I would contact my provider and ask them to clarify their billing methods as well as provide an example for the previous month (if you haven't been with them a month, then the projected usage when the month is complete).
That sounds dodgy at best, deceitful at worst to be honest. If you're told you're getting 2000GB then it should be just that, 2000GB of data in/out, not some random approximation that may not come anywhere near 2000GB of data actually transferred.
I second scapeish comments. The definition of the 95th percentile billing method as acknowledged by the provider should be part of your contract. Do not accept a link to a web page, which states that you implicitly accept changes as determined by the provider.
I've seen a number of creative variances on the definition on the 95th percentile that if not caught early would cost the client significant bandwidth overage charges.