Motorola MPC8260 - Manual (Page 793)

29-13 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
Equation D yields the number of slots the user writes to the channelÕs TCT[BT].
29.3.5.3.2 Handling the Cell Loss Priority (CLP)ÑVBR Type 1 and 2
The MPC8260 supports two ways to schedule VBR trafÞc based on the cell loss priority
(CLP). When TCTE[VBR2] is cleared, CLP
0+1
cells are scheduled by PCR or SCR
according to the GCRA state. When TCTE[VBR2] is set, CLP
0
cells are still scheduled by
PCR or SCR according to the GCRA state, but CLP
1
cells are always scheduled by PCR.
See Section 29.10.2.3.4, ÒVBR Protocol-SpeciÞc TCTE.Ó
29.3.5.4 Peak and Minimum Cell Rate TrafÞc 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 trafÞc 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
than MDA (maximum delay allowed), the APC begins scheduling channels according to
the MCR parameter. If the delay, however, drops below MDA, the APC again schedules
channels according to the PCR. Note that in order to guarantee a minimum cell rate for
UBR+ channels, there must be enough bandwidth to simultaneously send all possible
channels at the MCR. See Section 29.10.2.3.5, ÒUBR+ Protocol-SpeciÞc TCTE.Ó
29.3.6 Determining the Priority of an ATM Channel
The priority mechanism is implemented by adding priority table levels, which point to
separate scheduling tables; see Section 29.10.4, ÒAPC Data Structure.Ó The APC ßow
control services the APC_LEVEL1 slots Þrst. If there are no cells to send, the APC goes to
the next priority level. The APC has up to eight priority levels with APC_LEVEL8 being
the lowest. The user speciÞes the priority of an ATM channel when issuing the
ATM
TRANSMIT command; see Section 29.14, ÒATM Transmit Command.Ó
The real-time channels, CBR and VBR-RT, should be inserted in APC_LEVEL1; non-real-
time channels, VBR-NRT, ABR, and UBR should be inserted in lower priority levels.
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
9.72 = 9 + (0.72 ´ 256/256) = 9 + 184.32/256 ~ 9 + 185/256
SCR = 9 SCR_FRACTION = 185
BT [slots] = (MBS[cells] - 2) ´ (SCR[slots] - PCR[slots]) + SCR[slots]
(D)
= (1000 - 2) ´ ((9+185/256) - (3+62/256)) + (9 +185/256)
= 6477