This doesn't make sense from the viewpoint of the workstation, physical access facility, or the network, because we don't want to waste this bandwidth. Allowing a user to transmit at a rate above the CIR, then, might have its merits in some circumstances. It is important to understand that CIR is not an instantaneous measurement of transmission, but an average rate over time. This distinction is not just a subtlety and nuance, but goes to the very heart of how CIRs work. The CIR is derived from two parameters.
Most data transmissions are, in fact, bursty in nature.
If network traffic is bursty, then the network ought to accommodate one device's excess traffic if there is available bandwidth. This is desirable because if the network were to discard a given user's burst of traffic, the user would just re-submit it later. To address this concern, most frame relay networks try to handle a momentary excess burst if there is bandwidth available. There is a maximum limit to how much information the user can submit in a given time interval. Tc, the excess frames are discarded immediately by the network.