Motorola PowerQUICC II MPC8280 Series Reference Manual page 1028

ATM Pace Control (APC) Unit
credit for bursting at the peak cell rate (PCR). (Gaining credit implies that the buffer at the
switch is not full and can tolerate a burst transmission.) If a TxBD becomes ready, the APC
schedules the VC to burst at the PCR as long as credit remains. When the burst credit ends
(the network's UPC leaky bucket reaches its limit), the APC schedules the VC according
to SCR.
31.3.5.3.1 Example for Using VBR Traffic Parameters
Suppose the traffic parameters of a VBR channel are PCR = 6 Mbps, SCR = 2 Mbps, MBS
(maximum burst size) = 1000 cells, and CPS = 8.
Equation C (see Section 31.3.4, "Determining the Time-Slot Scheduling Rate of a
Channel") yields
SCR_FRACTION, which the user writes to the channel's TCT.
9.72 = 9 + (0.72 × 256/256) = 9 + 184.32/256 ~ 9 + 185/256
SCR = 9
Equation D yields the number of slots the user writes to the channel's TCT[BT].
BT [slots] = (MBS[cells] - 2) × (SCR[slots] - PCR[slots]) + SCR[slots]
(D)
31.3.5.3.2 Handling the Cell Loss Priority (CLP)—VBR Type 1 and 2
The MPC8280 supports two ways to schedule VBR traffic based on the cell loss priority
(CLP). When TCTE[VBR2] is cleared, CLP
according to the GCRA state. When TCTE[VBR2] is set, CLP
PCR or SCR according to the GCRA state, but CLP
See Section 31.10.2.3.6, "VBR Protocol-Specific TCTE."
31.3.5.4 Peak and Minimum Cell Rate Traffic Type (UBR+)
To support UBR+ channels, the APC schedules transmission according to PCR and MCR.
For each priority level, the APC maintains a parameter that monitors the traffic load
measured as the time-slot delay between the service pointer (pointing to the current time
slot waiting transmission) and a real-time slot pointer. If the transmission delay is greater
31-14
Freescale Semiconductor, Inc.
the APC parameters, PCR,
PCR [slots] = (155.52 Mbps)/(6 Mbps × 8) = 3.24
3.24 = 3 + 0.24 × 256/256 = 3 + 61.44/256 ~ 3 + 62/256
PCR = 3 PCR_FRACTION = 62
SCR [slots] = (155.52 Mbps)/(2 Mbps × 8) = 9.72
SCR_FRACTION = 185
= (1000 - 2) × ((9+185/256) - (3+62/256)) + (9 +185/256)
= 6477
MPC8280 PowerQUICC II Family Reference Manual
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PCR_FRACTION,
cells are scheduled by PCR or SCR
0+1
cells are still scheduled by
0
cells are always scheduled by PCR.
1
SCR, and
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