3Com S7906E Configuration Manual page 1759

Each queuing algorithm addresses a particular network traffic problem and which algorithm is used
affects bandwidth resource assignment, delay, and jitter significantly.
Queue scheduling processes packets by their priorities, preferentially forwarding high-priority packets.
In the following section, Strict Priority (SP) queuing, Weighted Fair Queuing (WFQ), and Weighted
Round Robin (WRR) queuing are introduced.
SP queuing
SP queuing is specially designed for mission-critical applications, which require preferential service to
reduce the response delay when congestion occurs.
Figure 5-2 Schematic diagram for SP queuing
As shown in Figure 5-2, SP queuing classifies eight queues on a port into eight classes, numbered 7 to
0 in descending priority order.
SP queuing schedules the eight queues strictly according to 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. Thus, 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
there are packets 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 that every queue can be served for a certain
time, as shown in Figure 5-3.
5-2