Alcatel-Lucent 7450 Quality Of Service Manual page 779

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 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.
Quality of Service Guide
• 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
decimal).
→ The system concatenates the two results into an 8 bit number resulting in
0000110 as the inverse slope binary value.
High Scale Ethernet MDA Capabilities
Table 61 is a least significant bit value of 0110 (0.375
781