3Com 3C13636 Configuration Manual page 1308

3Com Router 3000 Ethernet Family
Configuration Guide
Within each queue, the sending (sequence) of packets is defaulted as FIFO.
II. PQ (Priority Queuing)
incoming packets
Figure 3-2 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. The system then put packets of
mission-critical application in higher priority queue, and packets of normal application in
lower priority queue. It guarantees that the packets of mission-critical application are
sent with priority, and the packets of normal application are sent at the free interval of
operating mission-critical application.
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.
top queue
middle queue
normal queue
classify
bottom queue
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3-2
Chapter 3 Congestion Management
outgoing packets
interface
transmit
scheduler
queue