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PM7329 S/UNI-APEX-1K800
DATASHEET
PMC-2010141
ISSUE 2
ATM TRAFFIC MANAGER AND SWITCH
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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proportional to 1/ShpIncr. The CDV introduced is within 1 fundamental time unit.
When there is neither contention (brief period where multiple VCs are scheduled
to transmit at the same time) nor congestion (over-subscription of the port/class),
the shaper will always transmit at SCR. See ideal emission of Figure 14.
The PCR and MBS only come into effect when the VC experiences one or more
periods of contention or congestion (and hence the term passive dual rate
shaper). An internal “l(fā)ate counter” is maintained that represents how late the
current cell’s scheduled emission time slot is relative to the ideal emission time
slot. A non-zero late counter will cause the shaper to attempt to recover the lost
opportunities by scheduling the cell with an increment value no smaller than
ShpIncr – ShpCdvt. The ShpCdvt parameter, user defined on a per-VC basis, is
in terms of the shaper’s fundamental time unit. The difference (ShpIncr –
ShpCdvt) is minimum number of fundamental time units inserted between cells,
and is proportional to the VC’s 1/PCR. Given the opportunity, the shaper will
burst at PCR rates until the late counter returns to zero
1
. The size of the counter,
programmable on a per-VC basis, therefore defines the MBS. See case #1 of
Figure 14.
If congestion persists for an extended period, the late counter will continue to
accumulate and eventually wrap around once MBS is reached. The resulting
emission pattern is one where the duration of bursting is the remainder of the
rolled counter. Every time the counter wraps, a CDV, equal to the MBS, is
introduced into the emission stream. Recovery of the cumulative CDVs can only
occur if the ingress stream pauses long enough for the VC queue to empty
entirely. MaxCDV can be imposed by limiting the length of the VC queue via the
per-VC max congestion threshold. See case #2 of Figure 14.
1
Note that the inter-cell transmission times may actually exceed 1/PCR. Factors include the number of
active WAN ports, the number of active loop ports, and back pressure created by the external WAN port.