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Frame Relay is generous with its bandwidth. If there is no congestion on your link, you are allowed to burst above the CIR rate. Any traffic sent above your CIR is marked as being Discard Eligible (DE) and, in the event of congestion, will be dropped.

When congestion does occur, congestion notification messages are sent out to notify both the sending and the receiving routers that congestion has occurred and that they should slow down their transmission rates. A Backward Explicit Congestion Notification (BECN) is sent back to the sender and a Forward Explicit Congestion Notification (FECN) is sent forward to the destination to notify them of congestion.

A BECN message is only sent back to the source when the destination sends a frame back. Because the provider must wait for a message to return in order to set the BECN bit in the frame header, the FECN bit is sent to the destination to request some traffic to be sent back in the reverse direction. Without this, the source might never know that congestion has occurred.

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Posted by jame_kristopher

Forward explicit congestion notification is a header bit transmitted by the source (sending) terminal requesting that the destination (receiving) terminal slow down its requests for data. BECN (backward explicit congestion notification) is a header bit transmitted by the destination terminal requesting that the source terminal send data more slowly. FECN and BECN are intended to minimize the possibility that packets will be discarded (and thus have to be resent) when more packets arrive than can be handled and

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Posted by crouse

Forward explicit congestion notification is a header bit transmitted by the source (sending) terminal requesting that the destination (receiving) terminal slow down its requests for data. BECN (backward explicit congestion notification) is a header bit transmitted by the destination terminal requesting that the source terminal send data more slowly. FECN and BECN are intended to minimize the possibility that packets will be discarded (and thus have to be resent) when more packets arrive than can be handled and
If the source terminal in a communications circuit generates frequent FECN bits, it indicates that the available network bandwidth (at that time) is not as great as can be supported by the destination terminal. Likewise, if the destination generates frequent BECN bits, it means the available network bandwidth (at that time) is not as great as can be supported by the source. In either case, the root cause is lack of available bandwidth at the times during which FECN or BECN bits are generated. This can occur because of outdated or inadequate network infrastructure, heavy network traffic, high levels of line noise, or portions of the system going down. Identifying and resolving these issues can improve overall network performance, especially when the system is called upon to carry a large volume of traffic

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Posted by HamidAliKhan

FECN (forward explicit congestion notification) is a header bit transmitted by the source (sending) terminal requesting that the destination (receiving) terminal slow down its requests for data. BECN (backward explicit congestion notification) is a header bit transmitted by the destination terminal requesting that the source terminal send data more slowly. FECN and BECN are intended to minimize the possibility that packets will be discarded (and thus have to be resent) when more packets arrive than can be handled.

If the source terminal in a communications circuit generates frequent FECN bits, it indicates that the available network bandwidth (at that time) is not as great as can be supported by the destination terminal. Likewise, if the destination generates frequent BECN bits, it means the available network bandwidth (at that time) is not as great as can be supported by the source. In either case, the root cause is lack of available bandwidth at the times during which FECN or BECN bits are generated. This can occur because of outdated or inadequate network infrastructure, heavy network traffic, high levels of line noise, or portions of the system going down. Identifying and resolving these issues can improve overall network performance, especially when the system is called upon to carry a large volume of traffic.

From:http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci787381,00.html#

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Posted by sagitraz