In frame relay, inference by user equipment that congestion has occurred in the network.The inference is triggered by realization of the receiving frame relay access device (FRAD) of transmission delays. Based on block, frame or packet sequence numbers, another protocol may recognize that one or more frames have been lost in transit. Control mechanisms at the upper protocol layers of the end devices then deal with frame loss by requesting retransmissions. See also block, Explicit Congestion Notification (ECN), FRAD, frame, packet, and protocol.