Alcatel Omni Switch/Router, User Manual

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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.