A simple explanation FECN / BECN

These are technices used for congestion control within frame-relay.

A Bit marking 0 or 1 in the BECN oder FECN field, telling there is congestion.

Frame-relay is a layer 2 protocol and does no error checking. Another layer 3 protocol has to do this. But Frame-relay can send congestion notifications on the links, informing there is congestion and to slow down the sending device. It depends on the implementation of the remote device, if these notifications are noted.
Here is what can happen and when one of these are used. If congestion occurs, there are two possible ways of notifying this.

With FECN it’s possbile to send a notifcation to the receiving router, that congestion occured. If in case the receiving router does not have the bandwidth and would be overwhelmed.
With BECN it’s possible to send a notification to the sending local router, that congestion occured.

Source:

http://www.groupstudy.com/archives/ccielab/200209/msg01962.html

http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Frame-Relay.html