Every 5 or 6 minutes (or whatever increment your datacenter has set), two readings are taken of the total bandwidth used by your network. One outgoing and one incoming reading are taken on synchronous connections. The higher of these two readings is recorded as the bps transferred during this time. At the end of the billing cycle, these readings are sorted and the top 5% are discarded. The remaining readings are what you are billed on.
For these services, the data quota is based off a 95th percentile threshold. Billable quota is based on an entire month of readings at the 95th percentile. Many datacenters will report hourly data, but is only useful to get a general idea of how much bandwidth is being used. Billing is usually determined using an entire month of recorded measurements.
For example, if your datacenter takes 10 readings in an hour, then 240 readings will be taken in one 24-hour period. This adds up to 7200 readings in a 30-day month. In a one hour period the 95th percentile cannot be calculated into a round number, since removing 1 entry will leave 90 percent.
In order to be able to display the hourly data, your datacenter may round down to the 90th percentile. There are many more data points in a day or a month than in an hour, so 95th percentile can be calculated more accurately.
The graphs are examples of bandwith useage for a partial month, and consist of 4000 readings. The first graph shows an example of bandwidth with lots of sporadic peaks and spikes.
The second graph shows a much smoother curve once the 95th percentile billing model filters out the top 5% of infrequent peaks. You can easily see the average network trend and bill the customer based on this average. In this instance, the customer is billed for the highest remaining entry, which is 48.3Kbps. This places the customer in a 32Kbps – 64Kbps tier.
Some datacenters have a particular bandwidth commit, which means you pay for a minimum tier whether you use it or not. If you have a 1Mbit commit and only use 64Kbps for the month, then you are billed for 1Mbit even though you haven’t used the rest of your available bandwidth.
