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When you order a virtual circuit for a service such as frame relay or ATM, you can specify a guaranteed data rate that you want the carrier to provide. The data rate is negotiated with the carrier as the CIR (committed information rate).

When the data rate exceeds the CIR, the network starts dropping packets, so CIR should be a balance between the minimum and maximum bandwidth requirements. You can also negotiate a burst rate that lets you exceed the CIR rate to accommodate spikes in traffic. The ability to burst depends on whether bandwidth is available. CIR may also be negotiated as variable over time, so that during busy business hours more bandwidth is available.

Basically, CIR is the throughput rate that you negotiate with a service provider, and they will usually attempt to guarantee that rate. One way the carrier guarantees CIR is by dropping non-CIR traffic.

Carriers will often "overbook" the capacity of their networks, hoping that customers will not make excessive demands on the network at the same time. But if the carrier miscalculates, traffic will be dropped, even if it is guaranteed, although non-CIR traffic will be dropped first.

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