3Com 3C13636 Configuration Manual page 1311

3Com Router 3000 Ethernet Family
Configuration Guide
The bandwidth-occupying proportion for each traffic is: (priority + 1)/total quota of
bandwidth, i.e. Bandwidth available for each traffic: 1/15, 2/15, 3/15, 4/15, 5/15.
Because WFQ can balance the delay and jitter of every flow when congestion occurs, it
is effectively applied in particular fields. For instance, in the assured services using the
RSVP (resource reservation protocol), generally, WFQ will be used as the dispatching
policy. And also in GTS, WFQ is used to dispatch buffered packets.
V. CBQ (Class-Based Queuing)
CBQ is the extension of WFQ, which supports user-defined classes. CBQ allocates an
independent FIFO reserved queue for each user-defined class to buffer data of the
same class. In case of network congestion, CBQ matches output packets according to
user-defined class rules and enables them to enter corresponding queues. It is
necessary to check the congestion avoidance mechanism (tail drop or WRED) and
bandwidth restriction before the packets enter queues. WFQ is performed to the
packets in the queue corresponding to each class when they go out of the queue.
If CBQ treats queues of all classes in weighted fair mode, delay sensitive data streams
like voice packets may not be serviced timely. This is why the PQ feature is introduced.
This feature provides an emergency queue, which adopts FIFO scheduling mode and
bears no bandwidth restrict and therefore is capable of providing service of strict priority
(SP) to sensitive packets such voice packets. CBQ providing an emergency queue is
called low latency queuing (LLQ).
LLQ combines SP mechanism with CBQ. The user can set a class to use SP service
when defining the class. Such a class is known as priority class. All packets of the
priority class will enter the same priority queue. It is necessary to check bandwidth
restriction of packets before they enter queues. When packets go out of queues, the
packets in priority queue get transmitted first. Then packets in other queues are
transmitted in weighted fair mode.
In order that packets in other queues will not be delayed too long, maximum available
bandwidth can be specified to each priority class when using LLQ. The bandwidth value
is used to monitor traffic in case of congestion. If no congestion happens, the priority
class is allowed to use the bandwidth exceeding the allocated value. If congestion
happens, the packets of the priority class exceeding the allocated bandwidth will be
discarded. LLQ can also specify burst-size.
The system will always match the priority class first and then the other classes when
matching rules for packets. Multiple priority classes are matched in the configuration
sequence. It is the same with other classes. Multiple rules in a class are matched in the
configuration sequence.
VI. RTP (Real-time Transport Protocol) priority queuing
RTP priority queuing technology is used to solve the QoS problems of real-time service
(including audio and video services). Its principle is to put RTP packets carrying audio
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Chapter 3 Congestion Management