Determining what you will actually get when you buy bandwidth for your server at a co-location facility or server farm can be tricky. You’ll see a host of terms like “95th percentile”, “fully burstable”, “CIR”, “EIR”, “PIR” and the like. Understanding the concepts and terminology can ensure you get the bandwidth you need without overpaying. “95th percentile” refers to a way of computing billing for your usage; “fully burstable” is pretty meaningless; and CIR EIR and PIR refer to different measurements of information rate.
Most Network Operations Centers oversubscribe their bandwidth, meaning that the total bandwidth they offer their customers mathematically exceeds the amount they buy from their providers. They rely on the fact that not all of their customers will need all the bandwidth they are buying all the time.
If your server needs to always have at least a minimum bandwidth available to it, but could occasionally use more, then you should carefully ask your co-location service if they can provide this. For example, say your server needs 5Mb/s to be available whenever it needs it, but could use 10Mb/s occasionally, should it be available. Then you should buy your bandwidth with a CIR of 5Mb/s and PIR of 5Mb/s. “CIR” stands for Committed Information Rate and you are guaranteed to always have this available. The “PIR” is the Peak Information Rate, and determines the maximum bandwidth you can get, assuming it is available when your server requests it. (If not, it will get something less, but always 5Mb/s or more.) In this example the CIR is at 50% of the PIR. Your bandwidth price will go up as this percentage goes up.
A reputable Network Operations Center, will never allow the sum of all the CIR values it provides it customers to exceed the maximum bandwidth its supplier provides.
For more information on co-location issues and bandwidth considerations, please see Choosing A Server Co-Location Provider.