Samsung Ubigate iBG3026, Configuration Manual, Page: 234 / 718

CHAPTER 19. QoS
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Simply put, the average queue size is a ‘smoothened-out’ version of the
instantaneous queue size.
The average queue size is compared against the ‘minimum threshold’ or
‘minth’ and ‘maximum threshold’ or ‘maxth’. When the average queue size is
less than minth, all packets are queued. When it is between minth and maxth,
packets are dropped based on the calculated drop probability. This drop
probability increases in direct proportion with the average queue size.
When the average queue size equals or exceeds maxth, all packets are dropped.
The intention here is to maintain the average queue size between minth and maxth.
The average queue size is calculated using the formula:
Qaverage = (1 - w) * Qa w * Qcurrent
Where, w is the weighting factor. It is a very small quantity and is configured
as a negative power of 2 for efficiency in calculation.
The drop probability when the average queue size equals ‘maxth’ is known as
the Mark Probability. Mark Probability = 1/MPD, where MPD is the Mark
Probability Denominator. MPD is configured as a power of 2 for efficiency in
Average Queue Size Calculation
Average Queue Size Calculation for Interface Queues
The average queue size computation for interface is done for every packet sent
on the interface. Before deciding whether packet needs to be dropped or not,
the average queue size is calculated using the following formula and by
reading the current driver queue size and current average queue size.
Qaverage = (1 - weight) * Qa weight * Qcurrent
Where weight = 1/2ewf, and ewf is configured by the user.
It seems obviously that above expression involves more computation. To get
more efficiency from the implementation perspective the same formula is
regrouped and calculated as show below. The weight is restricted to be a
negative power of 2 so that a shift operation can be used instead of division to
make the calculation more efficient.