Samsung Ubigate iBG3026 Configuration Manual page 225

Ubigate iBG3026 Configuration Guide/Ed.00
For understanding of CBQ operation, let us first consider all classes to be of
equal priority. The scheduler runs every 5 msec to service the queues on each
interface.(Each leaf class is associated with a queue). In every scheduling
interval, the scheduler makes two passes on the classes of each interface.
In the first pass, it transmits bcr bytes from each class in the scheduler list,
where bcr of a class is the number of bytes it can send in one scheduling
interval(5 ms) proportional to its CR. If the packet to be sent is greater than
bcr bytes, it is not sent and the class is given a credit of bcr bytes which it can
use the next time it is serviced.(In other words, the class has a deficit of bcr
bytes). Later, when the(credit + bcr) for the class equals or exceeds the size of
the packet(at the queue head), it is transmitted.
Packets are transmitted from a queue as long as the(bytes sent + current packet
size) does not exceed(credit + bcr). When no more packets can be sent, the
credit is updated to(credit + bcr - bytes sent). During the first pass, if a class
has more packets to send even after using bcr bytes, and its BR > CR, the class
is added to the tail of the 'xs-bw-list'. Therefore, the 'xs-bw-list' consists of
classes that need to send bytes in excess of the amount permitted by their CR
and are permitted to do so by their BR. Let us define Bc as the total number of
bytes that can be sent on an interface in one scheduling interval(5 ms)
proportional to its bandwidth. If the total number of bytes sent or credited in
the first pass is less than Bc, it means additional bytes can still be transmitted.
This excess bandwidth can be given to the classes that still have packets to
send and whose BR > CR. For this, a second pass is done, through a 'xs-bw-
list'. In the second pass, the excess bytes are first offered to the first class in
this list, then to the second class if some are still available and so on until the
excess bytes are exhausted or all classes in the 'xs-bw-list' have been satisfied.
The excess bytes available to a class cannot exceed(bbr - bcr), where bbr is the
maximum bytes that a class can send in a scheduling interval(5 ms)
proportional to its BR. Again, packets from a queue can be sent only as long
as(excess bytes sent + current packet size) does not exceed(excess bytes
available + credit). When packets can no longer be sent, any deficit equal
to(excess bytes available + credit - excess bytes sent) is credited to the class.
If all packets from the queue could be transmitted, the credit is set to 0.
The remaining excess bytes(Total excess bytes available - excess bytes offered
to this class) are then offered to subsequent classes in the 'xs-bw-list'.
To sum up, the scheduler makes two passes for each interface every
scheduling interval. In the first pass, enough bytes are sent from each class to
guarantee its CR and a second pass is done to distribute any excess bandwidth
available.
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