Samsung Ubigate iBG3026, Configuration Manual

Page: 233 / 718

Ubigate iBG3026 Configuration Guide/Ed.00
© SAMSUNG Electronics Co., Ltd.
195
Max loaned buffers = (3*BCR / average_packet_size) –QOS_MIN_RES_
CLASS_Q_SIZE
Max loaned buffers ~ (3*BCR / average_packet_size)
Note that a single class can still not consume all the buffers in the common
buffer pool. There will be(BCR / average_packet_size) buffers still remaining
for the other classes(which is enough to support a rate of up to the interface
bandwidth).
The following sums up the maximum buffer limit calculation for a class.
max1 = (3 * bbr) / average_packet_size;
max2 = (10 * bcr) / average_packet_size;
Max Buffers = MAX(max1, max2, QOS_MAX_CLASS_Q_LIMIT_MIN);
Where, QOS_MAX_CLASS_Q_LIMIT_MIN is 10.
It is important to note that, the buffer requirement for traffic classes is(10
*BCR / 100) buffers irrespective of the number of classes.
Congestion Management on Output Queues
In the iBG3026 system, RED(Random Early Detect) will be used to do
congestion avoidance on output queues. RED is the de facto standard for
congestion management in the internet today. The basic philosophy of RED is
to detect the onset of congestion and start dropping packets in a random
fashion before queue overflow leads to tail drops. Random drops not only
improves throughput of adaptive applications using TCP but is also more
suitable than tail drops for voice and streaming audio/video. Random drops
result in less perceptible degradation of voice/audio/video quality than
consecutive drops.
The intent of RED is detect persistent congestion but allow transient or
momentary bursts. In that sense, it behaves like a low pass filter.
RED therefore uses the ‘average queue’ size and not the ‘current queue size’
to determine congestion. The average queue size is calculated as an EWMA
(Exponential Weighted Moving Average) of the current queue size.