Congestion Control
Page 29-9
Discard Eligibility (DE) Flag
The Frame Relay network keeps track of data that is eligible for discard by using a single bit
within each frame. When the data rate exceeds the CIR, frames are tagged (i.e., the DE bit is
set to 1). If congestion in the network nears saturation, those frames tagged with the DE bit
will be dropped before untagged frames. Unless totally congested, data below the CIR level
on all virtual circuits is usually guaranteed delivery. Normally, frames are not dropped on an
entire Frame Relay connection, but only those frames that exceed the pre-defined CIR level.
Interaction Among Congestion Parameters
The following example helps illustrate the interaction among congestion regulation parame-
ters. A Frame Relay line has an access rate of 64 Kbps. The guaranteed Committed Informa-
tion Rate (CIR) is 16 Kbps. The Committed Burst Size is 16 Kilobits and the Excess Burst Size
is also 16 Kilobits. These parameters mean that any data exceeding 16 Kilobits (within a Tc
sample period) normally will be tagged with a Discard Eligibility flag and could be discarded
if congestion occurs on the virtual circuit. In addition, since the Excess Burst Size is 16 kilo-
bits, any frames sent exceeding 32 Kbps will have a higher probability of being discarded.
The following graph illustrates this example.
Effect of Congestion Control Parameters on Data
Time (Seconds)
Data (Kilobits)
0 .25 .50 .75 1.00
16
32
48
64
Data Not Discarded
Data Discarded
Committed
Burst Size
Excess
Burst Size
CIR
Data Discarded if Congested
Access Rate
Frames
Time Between
Frames
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