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Overview of Frame Relay
In general, the only DLCI numbers you see are those numbers assigned to the physical
circuits on the perimeter of the Frame Relay cloud.
Line Speed
The physical circuit between point A and the network must be ordered with a certain
line speed. This speed is the maximum physical bandwidth for your connection to the
Frame Relay network. Expansion beyond this limit is not possible without a hardware
change and a new circuit installation.
Port Speed
The connection into the telecommunications provider's Frame Relay network must be
ordered at a particular port speed, which is the maximum bandwidth rate that the
telecommunications provider accepts from your connection. This number must be less
than or equal to the line speed. This speed is the maximum rate at which you can
transmit data to any of your PVCs under any circumstances. The port speed differs from
line speed only in that it can be upgraded through software without a circuit installation
or hardware change.
CIR and Burst Speed
Each PVC has a property known as committed information rate (CIR), which represents
the guaranteed minimum bandwidth available to the particular PVC under all
conditions. In some implementations, an additional property can be assigned to a PVC,
known as burst speed or maximum burst. This speed represents the highest rate at
which data is allowed to flow over a given PVC, regardless of bandwidth availability.
Discarding Frames
The PortMaster 4 transmits as much data on the serial port as it can for any PVC that
has traffic, regardless of CIR. The Frame Relay switch passes on as much of the data as
possible to the next link. However, once a particular PVC has transmitted its CIR-worth
of bits each second, the switch marks any additional frames as "discard eligible." If the
switch receives more frames than it can pass along, the frames are automatically
discarded in the following order:
•
Frames that would be marked discard eligible even if they are forwarded
•
Frames received that were marked as discard eligible
If the switch must discard other frames, the behavior is undefined. In this case, the
Frame Relay network is improperly configured because the CIR total exceeds the line
speed or port speed.
Ordering Frame Relay Service
In general, when ordering Frame Relay service for a private network, order
large-bandwidth physical circuits (for example, T1) with a port speed appropriate to
your application, and a CIR that is high enough to provide minimally acceptable
12-2
PortMaster 4 Configuration Guide
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Table of Contents
Troubleshooting
