I want to use the Credit Based Shaping feature to allot a particular bandwidth to Channel 2. How do I program the CBS parameters to achieve the same?

Document created by VineethaThomas Employee on Jun 10, 2015
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Consider an example of operating at 100Mbps, and the bandwidth to be allotted to Channel 2 is 60Mbps.

Parameters to be programmed: 1. Send Slope 2. Idle Slope 3. High Credit 4. Low Credit

  • Channel2 idleSlope

     

  Since bandwidth allocated for Channel 2 is 60%, idleSlope should be 60Mbps.

  Credit accumulation rate = idleSlope = 60 Mbps

  Credit accumulated per cycle (40ns for 100Mbps operation) = 60 Mbps * 40ns = 2.4 bits

  Channel2 idleSlopeCredit Register = 2.4*1024 = 0x99A 

  • Channel2 sendSlope

Since bandwidth allocated for Channel 2 is 60%, sendSlope should be (100-60)Mbps = 40Mbps
  Credit drain rate = sendSlope = 40 Mbps
  Credit drained per cycle (40ns for 100Mbps operation) = 40 Mbps * 40ns = 1.6 bits
  Channel2 sendSlopeCredit Register = 1.6*1024 = 0x666

  • High Credit Value

  This is the maximum value that can be accumulated in the credit parameter, specified in bits scaled by 1,024. The maximum value is maxInterferenceSize, that is, best-effort maximum frame size which is 16,384 bytes or 131,072 bits. The value to be specified is 131,072 * 1,024 = 134,217,728 or 0x0800_0000. 

  • Low Credit Value

This is the minimum value that can be accumulated in the credit parameter, specified in bits scaled by 1,024. The maximum value is maxInterferenceSize, that is, best-effort maximum frame size which is 16,384 bytes or 131,072 bits. The value to be specified is 131,072 * 1,024 = 134,217,728 or 0x0800_0000. The programmed value is 2's complement (negative number), that is, 0xF800_0000.

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