Samsung Ubigate iBG3026, Configuration Manual

Page: 229 / 718

Ubigate iBG3026 Configuration Guide/Ed.00
© SAMSUNG Electronics Co., Ltd.
191
MAX((NxBCR_BUFS_RED * Bc)/AVG_PKTSIZE_FOR_BUF_CALC, 20) +
(Bc / MIN_PKTSIZE_FOR_BUF_CALC)
Where,
The value of NxBCR_BUFS_RED is 3,
The value of _PKTSIZE_FOR_BUF_CALC is 100 bytes,
Bc is interface bandwidth proportional to scheduling interval(5 msec),
The value of MIN_PKTSIZE_FOR_BUF_CALC 64 bytes.
The first part of expression gives 15 msec worth of packet buffering, and
second part of expression gives extra buffers accounting for buffers which are
not freed right away after completing transmission of packet from the driver
queue.
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.