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A frame relay network supports congestion control procedures to prevent performance degradation and to maintain an acceptable quality of service. Congestion control includes congestion avoidance and recovery procedures. To determine vendor-specific congestion control features, refer to your frame relay carrier or switch vendor.
Congestion avoidance procedures are initiated before the network approaches a moderate congestion state. These procedures prevent severe congestion from occurring so that initiation of congestion recovery procedures is not necessary.
To avoid congestion, explicit congestion notifications are sent to source and destination machines.
To throttle down a destination-controlled transmitter, an FECN indication can be set in frames sent in the forward direction (congested node to destination node).
To throttle down a source-controlled transmitter, a BECN indication can be set in frames sent in the backward direction (congested node to source node).
Alternatively, a Consolidated Link Layer Management (CLLM) message can be sent in the backward direction.
Congestion recovery procedures are usually initiated when the network begins to drop frames because of severe congestion.
To recover from severe congestion, networks can discard DE-set frames and frames in excess of the CIR agreed upon at subscription time. Networks can also discard non-DE-set frames, clear existing calls, reject new calls, and negotiate CIRs downward, as necessary.
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