3Com 3C421600A, Management Manual

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204 C
HAPTER
13: U
SING
F
RAME
R
ELAY
Table 53 Frame Relay Terminology
How Congestion Control Works
The chart below illustrates the congestion control process. This example
assumes that all Frames are 4 k bits in size. Figure 13 shows what
happens as the number of transmitted bits increases during the
Committed Rate Measurement Interval (Tc). When 16 k bits have been
transmitted, CIR is reached.
After CIR is reached, the unit begins to count bits until Bc is reached at 24
k bits. Once Bc is reached between 24k bits and 40k bits depending on
congestion levels all data is marked with the Discard Eligible (DE) bit set.
This allows the network to discard these frames if congestion is
encountered. At 48 k bits the Excess Burst Size (Be) is reached. Any data
in excess of Be is discarded. All data is discarded until a new Tc begins.
Abbreviation Term Description
Bc Committed
Burst Size The number of bits above CIR that are
transmitted during a time interval Tc without
setting the DE bit.
Be Excess Burst Size The number of Bits that are transmitted during a
time interval Tc in excess of Bc that will have the
DE bit set.
Tc Committed Rate
Measurement
Interval The time interval in which Bc and Be are
measured. See formulas below.
DE Discard Eligible A bit in the Frame Relay frame informing the
network that the frame can be discarded in case
of congestion.
CIR Committed
Information
Rate The data rate the carrier guarantees without
data loss. Bursting above the CIR is allowed by
most carriers, but data in excess of the CIR may
be discarded by the carrier in case of congestion
on the Frame Relay network.