Frame Relay Congestion: DE, BECN, and FECN
FR networks, like any other multiaccess network, create the possibility for congestion caused by
speed mismatches. For instance, imagine an FR network with 20 remote sites with 256-kbps links,
and one main site with a T1 link. If all 20 remote sites were to send continuous frames to the main
site at the same time, about 5 Mbps of data would need to exit the FR switch over the 1.5-Mbps
T1 connected to the main router, causing the output queue on the FR switch to grow. Similarly,
when the main site sends data to any one remote site, it sends at T1 speed, potentially causing the
egress queue connected to the remote 256-kbps access link to back up as well. Beyond those two
cases, which are typically called egress blocking, queues can grow inside the core of the FR
network as well.
Frame Relay provides two methods of reacting to the inevitable congestion, as covered in the next
two sections.