In practical terms, this will occur during working hours for Frame Relay due to the fact that it is a WAN technology and business-orientated. 10am and 2pm tend to be busiest because everyone tends to be on the WAN links then. Lunchtime sees a drastic depletion in traffic volumes. Government departments typically have a 9 - 4 pattern whilst corporates tend to be a 8 - 6.
Technically, a corporate requires longer hours of support and tend to be more demanding for service. However, their businesses are often not as disrupted should a link go down because they tend to be more server orientated with their processing being distributed to key nodes. On the other hand, government departments tend to suffer a complete melt-down when a link drops because they tend to still be using legacy architecture where the processing is based around mainframes and AS400 technology. Thus whole offices in the more remote places twiddle their thumbs until the telco brings the circuit back up.
Before the transmission of data, the node will determine how the transmission is to occur. If more bandwidth than the CIR is required, less important frames will de marked DE. Devices ahead of the node will drop those frames should the need arise. Some retransmission will be required. Again, if there is severe congestion, all frames may be dropped. However, the management used by telcos will determines how this happens. "Foresight" on Stratacom/Cisco BPX's is one system that governs transmission of frames.
Explicit Congestion Notification is one technique that has already been discussed earlier. (FECN / BECN)
Consolidated Link Layer Management (CLLM) is another signalling method. It uses OSI Layer 2 data link control messages from network nodes to the user device. Within the CLLM code is the reason for congestion;
- excessive traffic
- line failure
... etc
Both CLLM and LMI are carried on DLCI 1023.
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. - buy a new switch!
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.
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