PM73121ꢀAAL1gator II
Data Sheet
PMC-Sierra, Inc.
PMC-980620
,VVXHꢀꢁ
AAL1 SAR Processor
Figure 23 shows how the CSD assigns credits to determine in which frames cells should be sent.
Frame Boundaries
TFTC
CSD
RL_SER(0)
FR_ADVANCE_FIFO
The CSD reads
Set
frame advances
and determines
cells to be sent
RL_FSYNC(0)
The TFTC sees
frame advance and
records this in the
FR_ADVANCE_FIFO
NEXT_
SERV
RL_SER(1)
RL_FSYNC(1)
Figure 23. Frame Advance FIFO Operation
The following is an example of the calculations the CSD circuit performs. This example assumes
a structured line with four channels allocated to one queue.
1. The TFTC writes Line 3 and Frame 4 to the FR_ADVANCE_FIFO.
2. The CSD circuit determines the queue for which a cell is ready by finding a set bit in the
Transmit Calendar. In this example, it is queue number 100.
3. The CSD circuit reads the number of credits for queue number 100. The number of credits is
always greater than 47 because it is ready for service. In this example, QUE_CREDITS =
59.375.
4. The CSD circuit subtracts AVG_SUB_VALU, the average number of credits spent per cell.
(Remember: For structured lines, the average number of credits per cell is 46-7/8. For unstruc-
tured lines, the average number of credits per cell is 47.)
Credits = 59.375 - 46.875
Credits = 12.5
5. The frame differential for the next service is computed from the number of credits needed to
exceed 47 and NUM_CHAN, the number of channels allocated per frame.
47 - 12.5 = 34.5
34.5 ÷ 4 = 8.625
Round 8.625 up, so the frame differential is 9.
6. Therefore, the next cell will be sent nine frames ahead of the current cell.
Next frame = present frame number + 9
ꢂꢄ
PMC [ PMC-SIERRA, INC ]
