H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 43)

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Congestion management techniques
Congestion management uses queuing and scheduling algorithms to classify and sort traffic leaving a
port. Each queuing algorithm addresses a particular network traffic problem, and has a different impact
on bandwidth resource assignment, delay, and jitter. A port of the switching engine on a WX3000E
wired-wireless switch provides four queues numbered 3, 2, 1, and 0, which accommodate the packets
with local precedence values 6 and 7, 4 and 5, 2 and 3, and 0 and 1, respectively.
Queue scheduling processes packets by their priorities, preferentially forwarding high-priority packets.
This section describes in detail Strict Priority (SP) queuing, Weighted Round Robin (WRR) queuing, and
SP+WRR queuing.
SP queuing
SP queuing is designed for mission-critical applications, which require preferential service to reduce the
response delay when congestion occurs.
Figure 10 Schematic diagram for SP queuing
As shown in Figure 10 , SP queuing classifies eight queues on a port into four classes, numbered 3 to 0
in descending priority order.
SP queuing schedules the eight queues in the descending order of priority. It sends packets in the queue
with the highest priority first. When the queue with the highest priority is empty, it sends packets in the
queue with the second highest priority, and so on. You can assign mission-critical packets to the high
priority queue to ensure that they are always served first and common service packets to the low priority
queues and transmitted when the high priority queues are empty.
The disadvantage of SP queuing is that packets in the lower priority queues cannot be transmitted if
packets exist in the higher priority queues. This may cause lower priority traffic to starve to death.
WRR queuing
WRR queuing schedules all the queues in turn to ensure every queue is be served for a certain time, as
shown in Figure 11 .
Queue 7
Queue 6
Queue 1
Queue 0
……
Packets to be sent through
this port
Packet
classification
High priority
Low priority
Sent packets
Interface
Sending queue
Queue
scheduling