Committed Information Rate; Forward And Backward Explicit Congestion Notifications; Local Management Interface; Supporting Frame Relay - 3Com 3C421600A Management Manual

Committed
Information Rate
Forward and
Backward Explicit
Congestion
Notifications
Local Management
Interface
Supporting Frame
Relay

Congestion Control

Frame Relay controls the data throughput rate with the Committed
Information Rate (CIR) parameter. CIR is the data rate the carrier
guarantees without data loss. CIR is determined at the time the Frame
Relay circuit is ordered and typically determines the cost of the Frame
Relay service. Bursting above the CIR is allowed by most carriers however
data in excess of the CIR may be discarded by the carrier in case of
congestion on the Frame Relay network.
Frame Relay employs a method of flow control using Forward Explicit
Congestion Notification (FECN) and Backward Explicit Congestion
Notification (BECN) to control data loss when a network is congested. By
sending a FECN the receiving station is notified of network congestion. A
BECN notifies the sending station when there is network congestion. The
routers and switches can then reduce the data rate until congestion is no
longer reported. Thus reducing discarded data due to congestion.
Frame Relay devices such as routers and switches manage the Frame
Relay interface the using Local Management Interface (LMI). LMI provides
a polling mechanism that allow switches and routers to request the status
of all PVCs on a given interface.
The RAS 1500 provides a synchronous serial interface capable of
supporting data rates between 9.6 kbps and 2.048 mbps. The RAS 1500
routes IP and IPX over Frame Relay. The RAS 1500 supports RFC 1490,
which is a standard that provides for routing of multiple protocols over a
single Frame Relay link. RFC 1490 also provides for interoperability with
Frame Relay equipment from other manufacturers.
The RAS 1500 automatically reduces the Committed Burst Size (Bc) to
control congestion when it receives too many BECN frames. The RAS
1500 uses BECN Monitoring to determine if more BECN frames than
non-BECN are received during a BECN Congestion Monitoring Period
(BECN_CMP). When the number of BECN frames is greater than the
number of non-BECN frame, the RAS 1500 adjusts the Committed burst
size downward. Each downward adjustment is equal to 1/8 of the
difference between Bc_MAX and Bc_MIN until the Bc_MIN value is
reached. When congestion is no longer occurring and the number of
non-BECN frames is greater than the number of BECN frames the unit
will begin to adjust Bc upward in steps of 1/16 of the difference between
Bc_MAX and Bc_MIN until Bc_MAX is reached. For more information on
Frame Relay Terminology see Table 53.
Overview 203