A flow control method used in Frame Relay to advise the sender that upstream network resources are busy.

Within a data packet, an overhead bit traveling in a frame-relay network. Bit is set to one if the packet travels through an area of network congestion that is opposite of its flow, signaling to higher level protocols to take flow control action.

A bit in the Frame Relay header that is set when the network router detects congestion from the source direction (or "backward," from the packet's point of view). See also Forward Explicit Congestion Notification (FECN).