A Frame Relay switch has a simple job: It forwards all the data that it can. If there is more data
than bandwidth, the switch will first drop the data with the DE bit set and will then drop committed
data if needed. In addition, the Frame Relay switch will also send out messages that congestion
is occurring.
Backward explicit congestion notification (BECN) and forward explicit congestion notification
(FECN) are the primary notification mechanisms used for handling congestion on the
Frame Relay switching internetwork. BECNs and FECNs both send notices that congestion is
occurring. A BECN is transmitted in the direction from which the traffic came, and an FECN
is transmitted in the direction in which the traffic is going.
Under normal circumstances, only Frame Relay switches send BECN and
FECN messages.
The end devices receive these notifications indicating that they should reduce the amount of
traffic that they are sending. A Frame Relay switch does not enforce the reduction; it simply
notifies the end devices. It is the responsibility of the end devices to reduce the traffic.
The congestion mechanisms are important to understand because congestion occurs frequently
on Frame Relay networks. As providers attempt to maximize the use of their lines, they sell more
bandwidth than they can actually provide. This is called oversubscription. Oversubscription is a
growing trend, so you must be aware of the implications and effects of the resulting congestion.
Some providers will attempt to sell you a zero CIR. Although inexpensive, you
have no guarantee that mission-critical data (or any data, for that matter!) will
get through.