Again, if there is severe congestion, all frames may be dropped. However, the management used by telcos will determines how this happens. Explicit Congestion Notification is one technique that has already been discussed earlier. It uses OSI Layer 2 data link control messages from network nodes to the user device. If traffic load exceeds the frame relay device's capacity or bandwidth, then buffering will occur.
Congestion is measured in degrees of buffer utilisation.
Mild Congestion Threshold - FECN and BECN bits are set, but no discarding of frames occurs. Severe Congestion threshold - FECN and BECN bits are set, frames marked with DE bit set are discarded. Absolute Congestion Threshold - All frames arriving to the device are automatically discarded. Many newer switches are able to prioritise PVC's so that high priority frames arrive and less important ones don't. It is also fairer on lighter use applications that in some instances fall over due to the load imposed by high demand applications.