Samsung Ubigate iBG2016, Руководство по настройке

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CHAPTER 10. QoS
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Buffer Management for Class Queues
It is not possible for a class to guarantee bandwidth if it doesn’t have enough
buffers for its queue. Because the packets can be dropped even before they
have a chance to be scheduled. Therefore it is essential to have a good buffer
management scheme along with a good scheduling algorithm. This section
discusses how buffers should be reserved for class queues of an interface.
It may have to be modified slightly or considerably based on memory
availability and the type of interfaces on which CBQ will need to be supported.
Buffer Requirements for Traffic Class
For a class to guarantee the CR, it should have capacity to buffer at least bcr
bytes, since, in the worst case, the class would have to queue bcr bytes before
the scheduler can service it.(Note that, to support a rate of CR kbps, at least
bcr bytes need to be sent every scheduler interval). For the class to transmit at
BR kbps, bbr bytes would have to be buffered. The BR of a class can be equal
to a maximum of the interface’s bandwidth, which means bbr can be a
maximum of BCR(interface bandwidth). A class can transmit more than its
CR and up to its BR if enough unused bandwidth is available. The scheduler
offers unused bandwidth to higher priority classes before the lower priority
ones.
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).