ATM Pace Control
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
40-21
Figure 40-9. Example of Three APC Priority Levels Combining
APC Scheduling Tables and APC PTP queues
40.11 APC Scheduling Flow
The following flow describes the schedule mechanism which is done in the APC per APC level, scheduling
one cell in ESAR mode. The flow demonstrates how APC PTP queues combine with the APC scheduling
table in an APC priority level. It also shows the use of the different service types (TCT[TSERVICE]), the
use of buffer-not-ready (BNR which is set automatically by the transmitter if the buffer is not ready) for
the different services, the use of the OUT bit which is set by the CP when issuing host deactivate command,
the scheduling when VBR2 bit is set, and the use of the leaky bucket (credit) mechanism in VBR.
Note that the flow describes the scheduling mechanism done for one cell in one APC level. If NCITS>1,
the flow may be repeated several times per APC request. Note that the leaky bucket mechanism is always
receiving one credit at the SCR rate and checked against BT (maximum burst tolerance, not shown in the
flow). Note also that when over subscribing at the lowest APC level (typically for UBR), a channel’s PCR
is used by the APC as the limiter (max pace), but the channel’s actual pace is based on the available
bandwidth shared among all active channels at this level.
Level1
Level2
Level3
Share the same priority
(EAPCST EQ=1)
PTP
Queue 1
APC Scheduling Table 1
Used for CBR
Used by UBR channels
PTP
Queue 2
PTP
Queue 3
APC Scheduling Table 2
APC Scheduling
Order
PTP is at a higher
priority (EAPCST EQ=0)
Summary of Contents for PowerQUICC MPC870
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