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OC3port Plus
Users Manual
VBR / POISSON Parameter Setup
Many parameters affect the way VBR / Poisson works:
•
(Sustained Cell Rate)
SCR
Determines the "center" transmission rate, around which the cell inter-arrival
times are distributed.
•
(Peak Cell Rate)
PCR
Determines the upper bound on the cell transmission rate. The instantaneous
cell rate varies randomly between the PCR and SCR - (PCR-SCR) as long as
the CDV limit is not exceeded. This is only enforced if the Shaper is enabled.
•
(Maximum Burst Size)
MBS
Limits the number of cells that can be generated sequentially at the PCR rate.
This is only enforced if the Shaper is enabled.
•
CDV LIM
Constrains early arrival times to an integral number of cells before the
expected SCR interval. This is only enforced if the Shaper is enabled.
Results
If you view a VBR / Poisson source with an instrument that presents average
results (any sampling interval greater than 100 cells or so), you will see an average
arrival rate and bandwidth very close to the
capable of detecting instantaneous minimum and maximum transmission rates,
you will see a stream that varies around the
If you capture a group of VBR / Poisson cells that have been transmitted with the
Shaper off, you will see cell inter-arrival times that are random but centered
around the
SCR
effect this is a "rogue" cell stream that may exceed contract parameters over short
periods.
If you capture a group of VBR / Poisson cells that have been transmitted with the
Shaper on, you will see cell inter-arrival times that are random but are constrained
to the PCR and CDV LIM values. In effect, they adhere to the ATM contract
parameters.
Because these are RANDOM sources, every time that you load the traffic settings
on the OC3port Plus, even with the same parameters, a source with new
characteristics is transmitted.
B-10
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(Cell Delay Variation Limit)
setting. No limits are placed upon the inter-arrival variations. In
setting. If the instrument is
SCR
setting.
SCR
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