At this point, NCP sets the FECN and BECN indicators in transmitted frames. 900 FRFH does not set BECN during mild congestion.
Severe congestion is experienced when all the buffers are used.
At this point, NCP can no longer transmit through the subject outbound queue. The DE bit will be set on frames transmitted on the DLCI experiencing congestion. If all buffers are in use for a certain DLCI and frames are still coming than frames will have to be discarded. If this is the case, NCP first discards frames with the DE bit on. If there are no frames in the queue marked with the DE bit on, NCP flags all frames in the queue discard eligible.