3Com 3C13636 Configuration Manual page 1309

3Com Router 3000 Ethernet Family
Configuration Guide
III. CQ (Custom Queuing)
incoming packets
Figure 3-3 Custom queuing
CQ classifies packets into 17 classes in accordance with certain rules (corresponding
to 17 queues). Based on their own classes, packets will enter the corresponding
custom queues with FIFO policy.
Of these 17 queues, queue 0 is a system queue (not shown) which is not configurable
and queues 1 through 16 are user queues, as shown in the above figure. User can set
the rules of traffic classification and assign the proportions on occupying interface
bandwidth for those 16 user queues. During dispatching, packets in system queue are
sent preferentially till the queue is empty. Then with polling method a certain number of
packets, taken from No.1-16 user queues under the bandwidth-occupying proportion
set in advance, are sent out. In this way, packets of different application can be
assigned with different bandwidth. Therefore, it will not only ensure mission-critical
application to get more bandwidth but also prevent normal application from obtaining
no bandwidth at all. By default, the data flow enters the No.1 queue.
Another advantage of custom queuing is that bandwidth can be assigned according to
the busyness of applications, which is suitable for those applications having special
requirement for bandwidth. Though the dispatching for 16 user queues is polling, the
service time for each queue is not fixed. So when there are no packets of certain
classes, the CQ dispatching mechanism can automatically increase bandwidth
occupied by the packets of current existing class.
queue1
queue2
......
queue15
classify
queue16
3Com Corporation
Chapter 3 Congestion Management
10%
outgoing packets
30%
10%
transmit
scheduler
queue
5%
3-3
interface