3Com MSR 50 Series Configuration Manual page 1650

1650 C 86: C M
HAPTER ONGESTION ANAGEMENT
PQ (Priority Queuing)
Figure 465 Priority queuing
Priority queuing is designed for mission-critical applications. Those applications
have an important feature, i.e. when congestion occurs they require preferential
service to reduce the response delay. PQ can flexibly design priority sequence
according to different network protocols (e.g. IP and IPX), interface receiving
packets, packet length, source/destination IP address etc. Priority queuing classifies
the packets into four different types: top, middle, normal and bottom, in
descending order. By default, the data flow enters the normal queue.
During queues dispatching, PQ strictly comply with the priority sequence from
high to low, and it will send packets in the high-priority queue first. When that
queue is empty, PQ will begin to send packets in lower priority queue. By putting
the key service packets in the high priority queues, you can ensure that they can
always be served first. At the same time, the common service packets can be put
in the low priority queues and transmitted when there are no key service packets
waiting for transmission.
The disadvantage of PQ is that packets in the lower queues will be neglected if
there are packets in the higher queues for a long time.