Nokia ESB26 User Manual page 225

18. Quality of Service
is serviced again. A better solution is to specify 500-byte, 600-byte, and 500-byte counts for
the queue. This configuration results in a ratio of 31/38/31, which may be acceptable.
In order to service queues in a timely manner and ensure that the configured bandwidth
allocation is as close as possible to the required bandwidth allocation, you should cross-check
the byte count resulting from each protocol's packet size, otherwise the results may not match
what you wish to configure.
Figure 18-3 shows how WRR queuing behaves.
Figure 18-3 Weighted Round Robin Queuing
Benefits of WRR Queuing
WRR scheduling prevents the low-priority queues from being completely neglected during
periods of high-priority traffic. By using this scheduling, low-priority queues have the
opportunity to transmit packets even though the high-priority queues are not empty.
Byte-Rate Calculations
To check actual byte counts resulting from packet-based allocation:
For each queue, divide the percentage of bandwidth you want to allocate to the queue by the
packet size, in bytes. For example, assume the packet size for protocol A is 1086 bytes,
protocol B is 291 bytes, and protocol C is 831 bytes. We want to allocate 20 percent for A, 60
percent for B, and 20 percent for C. The ratios would be:
20/1086, 60/291, 20/831 or 0.01842, 0.20619, 0.02407
Normalize the numbers by dividing by the lowest number:
1, 11.2, 1.3
The result is the ratio of the number of packets that must be sent so that the percentage of
bandwidth that each protocol uses is approximately 20, 60, and 20 percent.
A fraction in any of the ratio values means that an additional packet will be sent. Round up
the numbers to the next whole number to obtain the actual packet count. In this example, the
actual ratio will be 1 packet, 12 packets, and 2 packets.
Convert the packet number ratio into byte counts by multiplying each packet count by the
corresponding packet size. In this example, the number of packets sent is one 1086-byte
packet, twelve 291-byte packets, and two 831-byte packets or 1086, 3492, and 1662 bytes,
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