Samsung Ubigate iBG3026, Configuration Manual

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CHAPTER 19. QoS
190
© SAMSUNG Electronics Co., Ltd.
It is important to realize that we will not create separate buffer pools for each
interface or class. Instead we will define ‘logical’ buffer pools for each
interface and class which is nothing but a maximum limit which indicates the
number of buffers that the interface or class can use. The sum of these limits,
plus the limits for other uses, cannot exceed the total number of buffers
available.
Following is an explanation of variables/terms used in the following
subsections.
BCR - ‘Committed rate bytes’. Root-out class parameter. Number of bytes
that can be sent from an interface in one scheduler interval. This depends on
the bandwidth of the interface.
CR - ‘committed rate bytes’. Class parameter. Number of bytes that can be
sent from the class in one scheduler interval. This is configured by the user.
bcr - ‘committed rate bytes’. Class parameter. It is the number of bytes that
a class can send in one scheduler interval, corresponding to its CR.
BR - ‘Burst rate bytes’. Class parameter. Maximum number of bytes that
can be sent from an interface in one scheduler interval. This is configured
by the user.
bbr - ‘Burst rate bytes’. Class parameter. This is the number of bytes a class
can send in one scheduler interval, corresponding to its BR.
Buffer Reservation for Interfaces
When a wan interface is created, a certain amount of queue buffers will be
reserved for it’s transmitting queue from the global buffer pool. Buffer
reservation is only done for wan interfaces and not GE interfaces because we
expect congestion and hence queue build-ups only on wan interfaces.
The number of buffers reserved for a wan interface’s queue is such that it
provides 15 msec of buffering for 100 byte packets at a rate equal to the
interface bandwidth. This amount of buffering is always guaranteed for a wan
interface. A wan interface can use more than its reserved amount of buffers by
borrowing queue buffers from the global pool if available. However, only the
portion allocated for transmit queues can be borrowed. Therefore it is not
possible for traffic, flooding a WAN interface, to consume all buffers and
starve other interfaces(including Ethernet) of receive buffers. The calculation
below shows how the number of reserved buffers is calculated for a WAN
interface.