3Com MSR 50 Series Configuration Manual page 1651

Congestion Management Overview 1651
CQ (Custom Queuing)
Figure 466 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 according to FIFO policy.
Of these 17 queues, queue 0 is a system queue (not shown in the above figure).
The router always sends packets in this queue with the highest priority. 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.
n
You are recommended not to put common data packets in queue 0, because this
may cause other queues not to be dispatched for a long time.