Frame Relay

Frame Relay notes on FECN, BECN, DE etc.

Frame Relay Forum

PVC's and CIR

CIR = Committed Information Rate
PVC = Permanent Virtual Circuit

The difference between PVC and CIR is that PVC is a connection and CIR is a property that is two nodes connect to a Frame Relay service provider and to transfer data between them they use a PVC.  The speed of this PVC is defined as a CIR which the Service Provider commits to provide at least that bandwidth between the two nodes.


During periods of congestion a frame relay network will change the 3bits in the 2 byte DLCI to show indicate FECN (Forward Error Congestion Notification) or BECN (Backward Error Congestion Notification) state. A FECN is issued to the destination device along the path from the source to the destination to indicate congestion in the Frame Relay network along that path. A BECN is issued to frames returning along the path to indicate to the originating device that congestion occured along the path.

A FECN tells the receiving device that the path is congested so that the upper layer protocols should expect some delay. The BECN tells the transmitting device that the Frame Relay network is congested and that it should "back off" to allow better throughput.

If device A is sending data to device B accross a Frame Relay intrastructure and one of the intermediate Frame Relay switches encounters congestion, congestion being full buffers, over subscribed port, overloaded resources, etc, it will set the BECN bit on packets being returned to the sending device and the FECN bit on the packets being sent to the receiving device.  This has the effect of telling the sending router to Back off and apply flow control like traffic Shaping and informs the receiving device that the flow is congested and that it should inform upper layer protocols, if possible, that it should close down windowing etc to inform the sending application to slow down.

DE - Discard Eligibility

This bit is set to control which packets are thrown away.  The carrier network will generally set the DE bit on any packets which exceed the CIR during a give time period.  The sending device can manipulate traffic and select traffic that it would like to be marked as DE thus "protecting" other traffic.