High Scale Ethernet MDA Capabilities
Quality of Service Guide 781
For instance, if the MBS value is defined as 16,800 bytes and the low slope was configures
with a starting depth set to 75 percent, a maximum depth set to 100 percent and a maximum
discard probability set to 80 percent:
• The system takes 75 percent of 16,800 bytes and derives a starting slope at a queue
depth of 12,600 bytes
• The system takes 100 percent of 16,800 bytes and derives an ending slope of 16,800
bytes.
• The system converts the starting, ending and slope duration values to number of
buffers and then calculates the slope duration (end — start).
→ 12,600 / 168 = 75 start-buffer
→ 16,800 / 168 = 100 end-buffer
• The actual step of the slope may be calculated by dividing the maximum discard
probability value with the slope run:
→ 80 / (100 ñ 75) = slope step is 3.2 (for every buffer beyond the start of slope, the
probability rises 3.2 percent, after 25 buffers the slope reaches 80%).
• But the HSMDA uses the inverse slope within its algorithm and a drop probability
random number between 0 and 128. The system calculates the inverse slope by
dividing the slope run by the maximum discard probability multiplied by 1.27
(conversion from 0 to 100 to 0 to 127):
→ (100 — 75) / 80 * 1.27 = 0.396875 inverse-slope
• The system takes the inverse slope step and converts it to an internal HSMDA fixed
point binary notation with the most significant 4 bits representing the whole portion
of the inverse slope (above 0) and least significant 4 bits representing the fractional
portion of the slope (below 0).
→ The inverse slope is less than 0 so the most significant 4 bits is 0000.
→ The fractional portion of the inverse slope is 0.396875 and the closest result in
four bits based on Table 61 is a least significant bit value of 0110 (0.375
decimal).
→ The system concatenates the two results into an 8 bit number resulting in
0000110 as the inverse slope binary value.
The system uses the starting buffer value, the ending buffer value and the inverse slope eight
bit value to populate a slope definition into the HSMDA. Two slopes are populated per slope
policy. Each slope policy is given an HSMDA slope index between 0 and 1023. Since every
packet received on an HSMDA queue is associated with either the high or low slope, the
provisioned MBS value is not required and is not a managed parameter for HSMDA queues.
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