Nokia ESB26 User Manual page 224

18. Quality of Service
Figure 18-2 Strict Priority Queuing
Benefits of SP Queuing
SP provides absolute preferential treatment to high priority traffic, ensuring that mission-
critical traffic traversing various WAN links gets priority treatment. In addition, SP provides a
faster response time than do other methods of queuing.
Weighted Round Robin (WRR)
In this scheduling method, a weighting factor for each queue determines how many bytes of
data the system delivers from the queue before it moves on to the next queue. The WRR
mechanism is cycles through the queues. For each queue, packets are sent until the number of
bytes transmitted exceeds the bandwidth determined by the queue's weighting factor, or the
queue is empty. Then the WRR mechanism moves to the next queue. If a queue is empty, the
router will send packets from the next queue that has packets ready to send.
Note that if a packet's length exceeds the queue's allowed bandwidth, the packet is still
transmitted during its time slot, but its quota is overdrawn so that on the next time slot it
receives a smaller allotment. This mechanism guarantees a minimum bandwidth to each
queue, but allows the minimum to be exceeded if one or more of the port's other queues are
idle. However, when all the queues are loaded each is limited to its maximum bandwidth
according to its assigned weight - no queue achieves more than a predetermined proportion of
overall capacity when the line is under stress.
The weighting factors are specified as relative percentages, either as the actual number of
packets transmitted each turn, or as the byte count transmitted, in 256-byte quanta. The values
for all the queues must be positive, and must add up to ten or 100.
If the packet sizes of the queues vary significantly, using byte counts rather than packet values
provides a greater degree of bandwidth fairness. For example, suppose one protocol has 500-
byte packets, another has 300-byte packets, and a third has 100-byte packets. If you want to
split the bandwidth evenly across all three protocols, you might choose to specify byte counts
of 200, 200, and 200 for each queue. However, this configuration does not result in a 33/33/33
ratio of bandwidth usage. When the router services the first queue, it sends a single 500-byte
packet; when it services the second queue, it sends a 300-byte packet; and when it services the
third queue, it sends two 100-byte packets. The effective ratio is 50/30/20 - setting the byte
count too low can result in an unintended bandwidth allocation.
However, very large byte counts will produce a "jerky" distribution. That is, if you assign 10
KB, 10 KB, and 10 KB to the three queues in the example given, each protocol is serviced
promptly when its queue is the one being serviced, but it may be a long time before the queue
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