Appendix B. OSI Model and Frame Relay Technology Overview
61200305L1-1 ATLAS 550 User Manual B-7
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 56-kbps DDS circuit. This queue could cause
network congestion.
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 toflow
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 aframe relay switch begins to experience congestion, it sends
the upstream site a FECNand the downstream site aBECN. This notification
indicates to the frame relay equipment that the frame relay switch is experi-
encing difficulty and thatthe 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 isoccurring 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 sothat upper lay-
er acknowledgments from the destination slow down, thereby reducing the
amount of data being transmitted from the source.
Figure B-4. Network Congestion and Flow Control
BECN
Data Source
Frame Relay/
Router
Frame Relay/Router
Switch Congestion
FECN
Frame Relay/Route
Frame
Switch
Frame
Switch
Data Destination
