Most IXCs convert the long-haul delivery of Frame Relay into ATM, that is, the Frame Relay PVC is converted to an ATM PVC at the first Frame Relay switch after leaving the customer premises. It is not converted back to Frame Relay until the last Frame Relay switch before entering the customer premise. This is significant because ATM has built-in Class of Service (CoS). A customer can enter a contract with a carrier to convert the Frame Relay PVC into a constant bit rate (CBR) ATM PVC. ATM CBR cells are delivered with lower latency and higher reliability.

Finally, under the best circumstances, Frame Relay is still inherently more susceptible to delay than ATM or TDM. Therefore, after applying the best possible queuing mechanism, you can still expect a longer delay over Frame Relay than over ATM or TDM.