Alcatel Carrier Internetworking Solutions Omni Switch/Router - Manual (Page 883)

Congestion Control
Page 29-11
Notification By BECN
Each data link header contains a congestion control flag called BECN (Backwards Explicit
Congestion Notification), which is usually pronounced “beckon.” Normally this flag is turned
off. As with other
WAN packet-based networks, frames in Frame Relay may build up in
queues at certain points. When a queue is full, due to congestion, frames will be dropped.
The senders of this data (Bridge/Router or
WSX ) may not be aware of the congestion. Frame
Relay uses a congestion notification technique to notify the Bridge/Router that traffic is
jammed further down the circuit.
When a frame on one side of the bi-directional virtual circuit sees data congested on the other
side, the Frame Relay network sets the frame’s
BECN flag On. Any subsequent frames that see
the congestion also have their BECN flag set On. These BECN frames continue down the
virtual circuit until they reach the Bridge/Router or WSX on the other end. The receiving WSX
sees the BECN flags and adjusts data flow in the opposite direction. Normally the WSX will
slow the speed of data down to the CIR. If the BECNs persist, then data flow is stepped down
even further. Data flow will gradually increase back up to the normal rate as soon as BECNs
or FECNs (see below) are no longer received.
Congestion Notification Using a BECN
BECN notification only works if traffic flows in both directions. If traffic in the uncongested
direction did not exist then there would be no frames for the Frame Relay network to set
BECN flags on.
Frame sees traffic con-
gestion in other direc-
tion. Sets its
BECN flag.
Congestion.
Frame Relay Virtual Circuit
WSX or Bridge/
Router at this end
r e c e i v e s B E C N
a n d c a n a d j u s t
traffic flow down
t o t h e C I R , o r
below.