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PM73121 Datasheet(PDF) 52 Page - PMC-Sierra, Inc |
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PM73121 Datasheet(HTML) 52 Page - PMC-Sierra, Inc |
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52 / 223 page  PM73121 AAL1gator II PMC-Sierra, Inc. PMC-980620 ,VVXH AAL1 SAR Processor Data Sheet QSPQSD@U6S`Ã6I9Ã8PIAD9@IUD6GÃUPÃQH8TD@SS6ÃDI8Ã6I9ÃAPSÃDUTÃ8VTUPH@ST¶ÃDIU@SI6GÃVT@ Figure 23 shows how the CSD assigns credits to determine in which frames cells should be sent. 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 Figure 23. Frame Advance FIFO Operation FR_ADVANCE_FIFO CSD Frame Boundaries RL_SER(0) RL_FSYNC(0) TFTC The TFTC sees frame advance and records this in the FR_ADVANCE_FIFO RL_SER(1) RL_FSYNC(1) The CSD reads frame advances and determines cells to be sent Set NEXT_ SERV |
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