3Com Switch 7754, Configuration Manual

Page: 664 / 940

664 C
HAPTER
61: Q
O
S C
ONFIGURATION
In the following section, strict priority (SP) queues and weighted round robin
(WRR) queues are introduced.
1 SP queue
Figure 174 Diagram for SP queues
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 8 output queues on the port and the preferential queue classifies
the 8 output queues on the port into 8 classes, which are queue7, queue6,
queue5, queue4, queue3, queue2, queue1, and queue0. Their priorities decrease
in order.
In the 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 to death” because they are not served.
2 WRR queue
Queue 7
Queue 6
Queue 1
Queue 0
ĂĂ
Packets to be sent through
this port
Packet
classification
High priority
Low priority
Sent packets
Interface
Sending queue
Queue
scheduling