3Com 5500-EI PWR Install Manual page 674

Figure 1-6 Diagram for SP queuing
SP queue-scheduling algorithm is specially designed for critical service applications. An
important feature of critical services is that they demand preferential service in congestion
in order to reduce the response delay. Assume that there are eight output queues on the
port and the preferential queue classifies the eight output queues on the port into eight
classes, which are queue7, queue6, queue5, queue4, queue3, queue2, queue1, and
queue0. Their priorities decrease in order.
In queue scheduling, SP sends packets in the queue with higher priority strictly following
the priority order from high to low. When the queue with higher priority is empty, packets in
the queue with lower priority are sent. You can put critical service packets into the queues
with higher priority and put non-critical service (such as e-mail) packets into the queues
with lower priority. In this case, critical service packets are sent preferentially and
non-critical service packets are sent when critical service groups are not sent.
The disadvantage of SP queue is that: if there are packets in the queues with higher priority
for a long time in congestion, the packets in the queues with lower priority will be "starved"
because they are not served.
2) WFQ queuing
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