Frame has class of service defined so that the throughput of any given DLCI can be controlled.
There are two reasons for this. This occurs especially where a network is over-subscribed. At a lower level again, classes of service are required where there are more than 1 PVC's coming into a customer port. It is important to ensure that one PVC does not consume all the available bandwidth of the physical pipe and deprive others the use of it. The CIR is also generally referred to as throughput.