3Com MSR 50 Series Configuration Manual page 1653

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.
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.
CBQ provides an emergency queue for emergent packets. This queue adopts FIFO
scheduling mode and bears no bandwidth restriction. Therefore, if CBQ treats
queues of all classes in weighted fair mode, delay sensitive data streams like voice
packets may not be serviced timely. For this reason, PQ is introduced into CBQ,
called Low Latency Queuing (LLQ), which strictly provides preferential services for
voice packets and other delay sensitive data streams.
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 matches rules for packets as follows:
Match the priority class first and then the other classes.
■
Multiple priority classes are matched in the configuration sequence.
■
Match in the configuration sequence for other classes.
■
Multiple rules in a class are matched in the configuration sequence.
■
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 or video into high-priority queue and send it first, thus minimizing
Congestion Management Overview 1653