3com 2928 User Guide

 

2-7 

SP queuing is specially designed for mission-critical applications, which require preferential service to 
reduce response delay when congestion occurs. 

Schematic diagram for SP queuing 

 

 
A typical switch provides eight queues per port. As shown in 

, 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 (such as Email) packets to the low priority queues to be 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 schedules all the queues in turn to ensure that every queue can be served for a certain 
time, as shown in 

.