Ubigate iBG3026 Configuration Guide/Ed.00
© SAMSUNG Electronics Co., Ltd.
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Qaverage = Qa (Qcurrent - Qaverage) << ewf
This Qaverage ‘average queue size’ is used as an indication of congestion by
RED and is compared against the ‘minth’ and ‘maxth’ to decide whether
packet need to be allowed, randomly dropped or dropped.
Average Queue Size Calculation for Traffic Classes
The average queue size calculation for traffic classes is little bit different than
for interfaces. The packet scheduler services the class queues every scheduling
interval Ts msec(5 msec). In a Ts msec interval, if the queues are serviced for
T ms, during the remaining time(Ts - T) msec, the packets are classified and
queued in to the respective queues. So, the queuing process runs for sometime
followed by the de-queuing process in a periodic manner.
Therefore, even if the incoming rate for a particular queue is less than its CR,
the queue builds up to N packets during the queuing phase and drops to 0
during the de-queuing phase. If the queue size is greater than 0 even after the
de-queuing process has happened, it indicates that the receive rate is greater
than the transmit rate for the queue which is what signals congestion.
Therefore, to get an accurate indication of congestion in the queue, we read
the queue size after the queue is serviced but before en-queuing process, for
every Ts msec. We call this the ‘pending queue size’. It is not very useful for
RED to use the ‘pending queue size’ as it is because it can be very fluctuating.
RED uses an Exponential Weighted Moving Average(EWMA) of this queue
which, simply put, is a smoothed out version of the instantaneous values.
This average queue size is calculated every time the pending queue size is
read. The average queue size computation and applicability is same as
specified in above section.
Drop Probability Calculation
Instead of using drop probability, ‘drop rate’ is used to do random drop of
packets. DropRate = 1/(DropProbability). There are two advantages for using
drop rate. First, it results in a more uniform distribution of drops, which is
more desirable. Secondly, it results in more efficient implementation.
By using drop rate, we avoid making a function call for random number
generation, for every packet.
The following expression shows how drop rate is calculated,
Drop rate = ((maxTh - minTh)/ 2mpd) / (Qaverage - minTh)
Summary of Contents for Ubigate iBG3026
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