The obstacle in running IP telephony over Frame Relay involves the treatment of traffic within and outside the CIR, commonly termed the burst range.

As Committed information rate (burst range) shows, traffic up to the CIR is guaranteed, whereas traffic beyond the CIR usually is not. This is how Frame Relay is intended to work. CIR is a committed and reliable rate, whereas burst is a bonus when network conditions permit it without infringing upon the CIR of any user. For this reason, burst frames are marked as discard eligible (DE) and are queued or discarded when network congestion exists. Although customers can achieve significant burst throughput, burst throughput is unreliable, unpredictable, and not suitable for real-time applications like IP telephony.

Therefore, the objective is to prevent voice traffic from entering the burst range and being marked DE. One way to accomplish this is to prohibit bursting by shaping the traffic to the CIR and setting the excess burst size (B_e – determines the burst range) to zero. However, this also prevents data traffic from using the burst range.