Samsung Ubigate iBG3026 Configuration Manual page 230

CHAPTER 19. QoS
For a class to, potentially, transmit up to its BR, it needs to have at least bbr
bytes in its queue to be scheduled. Hence, ideally, all classes should have a
buffering capacity of at least bbr bytes. If classes are created on a bundle with
BR equal to the bundle bandwidth, the total buffer requirement for the bundle
would be(n*BCR) bytes, where n is the number of classes. This requirement
increases in direct proportion with the number of classes created, which can
result in a huge total buffer requirement for the system. To better understand
this buffer/memory requirement, lets take an example in which 20 classes are
created on a bundle with two T1s with each class having a CR = (3072/20)
Kbps and BR = 3072 Kbps.
BCR = ((3072000/8) * 5) / 1000 = 1920 bytes-per-scheduling interval
Total buffering requirement for a class = bbr = 1920 bytes
Let's estimate the average packet size to be 100 bytes.(This would be a very
conservative estimate).
The minimum # of buffers required for one class = bbr/100 = 20 buffers
With 20 classes on the bundle,(20 * 20) = 400 buffers would be required for
the bundle. With 14 bundles on a t3 interface, the total buffers requirement
would be(400 * 14) = 5600 buffers! Each buffer occupies about 2000 bytes of
memory, including all the control information, which results in a total memory
requirement of(5600 * 2000) = 11.2 Mbytes! This is the minimum buffer
requirement and does not provide any burst tolerance. Moreover, this memory
requirement increases in direct proportion with the number of additional
classes.
One would tend to think that having a shared pool of buffers with a capacity
of BCR bytes is enough to transmit line rate of traffic. That will work only if
the traffic entering each queue(classes) is proportional to its CR. This is
mostly not the case and which is why we do bandwidth management. It is
possible for traffic entering a class to be at a much higher rate than other
classes, because of which it will gain access to majority of the buffers and
hence most of the bandwidth.
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