H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 47)

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FIFO
Figure 11 FIFO queuing
As shown in Figure 11 , the first in first out (FIFO) uses a single queue and does not classify traffic or
schedule queues. FIFO delivers packets depending on their arrival order, with the one arriving earlier
scheduled first. The only concern of FIFO is queue length, which affects delay and packet loss rate. On
a device, resources are assigned for packets depending on their arrival order and load status of the
device. The best-effort service model uses FIFO queuing.
FIFO does not address congestion problems. If only one FIFO output/input queue exists on a port, you
can hardly ensure timely delivery of mission-critical or delay-sensitive traffic or smooth traffic jitter. The
situation gets worsened if malicious traffic is present to occupy bandwidth aggressively. To control
congestion and prioritize forwarding of critical traffic, you must use other queue scheduling mechanisms,
where multiple queues can be configured. Within each queue, however, FIFO is still used.
By default, FIFO queuing is used on interfaces.
Priority queuing
Figure 12 Priority queuing (PQ)
Priority queuing is designed for mission-critical applications. The key feature of mission-critical
applications is that they require preferential service to reduce the response delay when congestion occurs.
Priority queuing can flexibly determine the order of forwarding packets by network protocol (for example,
IP and IPX), incoming interface, packet length, source/destination address, and so on. Priority queuing
classifies packets into four queues: top, middle, normal, and bottom, in descending priority order. By
default, packets are assigned to the normal queue. Each of the four queues is a FIFO queue.
Packets to be sent
through this interface
Classify Schedule
High queue
Middle queue
Normal queue
Bottom queue
Packets sent
Sending queue
Interface