User manual

Congestion Control
Page 29-12
Notification By FECN
Frame Relay headers also contain a congestion control bit called FECN (Forwards Explicit
Congestion Notification), which is usually pronounced “Feckon.” Like BECN, the FECN bit also
notifies a WSX or Bridge/Router of congestions problems. However, it is set by the Frame
Relay network in frames that are actually experiencing congestion. When the WSX receives
frames with their FECN bit set, it knows that congestion is already occurring on the virtual
circuit in the direction that these FECN frames are travelling. The WSX reacts by reducing the
data flow down to the CIR for data in the opposite direction. If the FECNs persist, then data
flow is stepped down even further. Data flow will gradually increase back up to the normal
rate as soon as FECNs or BECNs are no longer received.
Congestion Notification Using a FECN
Congestion. FECN flag
set in frames experi-
encing congestion.
Frame Relay Virtual Circuit
WSX or Bridge/Rout-
er at this end
receives FECN and
can adjust traffic flow
down to the CIR, or
below, for outgoing
traffic in the opposite
direction.