3Com Switch 4800G 24-Port Configuration Manual page 878

878 C 68: C
HAPTER ONGESTION
M
ANAGEMENT
The following paragraphs describe strict-priority (SP) queue-scheduling algorithm,
and weighted round robin (WRR) queue-scheduling algorithm.
1 SP queue-scheduling algorithm
Figure 260 Diagram for SP queuing
Packets to be sent through
this port
Packet
classification
SP queue-scheduling algorithm is specially designed for critical service
applications. An important feature of critical services is that they demand
preferential service in congestion in order to reduce the response delay. Assume
that there are eight output queues on the port and the preferential queue
classifies the eight output queues on the port into eight classes, which are
queue7, queue6, queue5, queue4, queue3, queue2, queue1, and queue0. Their
priorities decrease in order.
In queue scheduling, SP sends packets in the queue with higher priority strictly
following the priority order from high to low. When the queue with higher priority
is empty, packets in the queue with lower priority are sent. You can put critical
service packets into the queues with higher priority and put non-critical service
(such as e-mail) packets into the queues with lower priority. In this case, critical
service packets are sent preferentially and non-critical service packets are sent
when critical service groups are not sent.
The disadvantage of SP queue is that: if there are packets in the queues with
higher priority for a long time in congestion, the packets in the queues with lower
priority will be "starved" because they are not served.
2 WRR queue-scheduling algorithm
Queue 7
Queue 6
Queue 1
Queue
scheduling
Queue 0
High priority
Sent packets
Interface
Sending queue
Low priority