Appendix B. OSI Model and Frame Relay Technology Overview
the frame relay network, the largest CIR available for purchase would be 56k. Although data could burst from site A to the frame relay network at the full T1 speed of 1.536 Mbps, it would queue up in the frame relay network until it could be sent across the
Managing Network Congestion
If congestion becomes a problem within the network due to excessive data being delivered from one of the sites, the frame relay switch attempts to flow control the data by sending bits that notify network devices that transmis- sions in the opposite direction are congested. These bits are called Backward Explicit Congestion Notification (BECN) and Forward Explicit Congestion Notification (FECN).
For example, if a frame relay switch begins to experience congestion, it sends the upstream site a FECN and the downstream site a BECN. This notification indicates to the frame relay equipment that the frame relay switch is experi- encing difficulty and that the frame relay device should begin to flow control its traffic.
Figure B-4 shows an example of FECN and BECN messages being transmit- ted to the frame relay equipment when congestion occurs. Both ends are no- tified that congestion is occurring within the switch. You might wonder why the receiving end should receive notification of congestion and then flow control its data when the other end is causing the problem by sending large amounts of data. Flow control is used by the receiving end so that upper lay- er acknowledgments from the destination slow down, thereby reducing the amount of data being transmitted from the source.
Data Destination
Frame Relay/Router
| Switch Congestion | |
Data Source | BECN |
|
Frame Relay/ |
|
|
Router | Frame | Frame |
| ||
| Switch | Switch |
Frame Relay/Router
Figure B-4. Network Congestion and Flow Control
ATLAS 550 User Manual |
