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OC3port Plus
Users Manual
B-2
CBR Parameter Setup
Two transmission stream parameters affect the way a CBR transmission source is
transmitted:
•
PCR
(Peak Cell Rate)
Determines the amount of bandwidth consumed by the CBR source and the
inter-cell arrival times for the periodic transmission. The OC3port Plus places
cells in transmission queue that are separated by a minimum interval equal to
the inverse of the PCR. If a cell slot is not available at that interval, the next
open slot is used. For example, a PCR set to 45 Mbits results in a source that
consumes 45 Mbits of OC3 bandwidth (almost 30 %). The cells, on average,
are periodically spaced approximately 9.42 microseconds apart.
•
CDV LIM
(Cell Delay Variation Limit)
When the shaper is left enabled for a stream, the
CDV LIM
parameter
guarantees that the cells, which are adjusted for slotting and contention, are
never spaced more closely than (1/PCR) -
CDV LIM
. For example, a CDV LIM
set to 2.0 cells and a
PCR
set to 45 Mbits guarantees cells that, on average, are
spaced 9.42 microseconds apart but that may be separated by as little as 3.76
microseconds (9.42 - (2*2.83).
Results
If you set up a small number of CBR streams and if these streams are in the
highest priority positions (that is, Stream 1, Stream 2, etc.), the resulting ATM
transmission stream will match (to within 1 %) the bandwidth and spacing
assigned by the PCR parameter.
If you configure a low-priority CBR transmission stream and if other higher
priority transmission streams are using the majority of the available OC3
bandwidth, the low-priority CBR stream will be transmitted at a rate lower than
the specified PCR.
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