Allied Telesis AT-S63 - Manual (Page 341)

AT-S63 Management Software Menus Interface User’s Guide
Section II: Advanced Operations 341
Note
Scheduling is set at the switch level. You cannot set this on a per-
port basis.
Strict Priority Scheduling
With this type of scheduling, a port transmits all packets out of higher
priority queues before transmitting any from the lower priority queues. For
instance, as long as there are packets in Q3 it does not handle any
packets in Q2.
The value to this type of scheduling is that high priority packets are always
handled before low priority packets.
The problem with this method is that some low priority packets might never
be transmitted out the port because a port might never get to the low
priority queues. A port handling a large volume of high priority traffic may
be so busy transmitting that traffic that it never has an opportunity to get to
any packets that are stored in its low priority queues.
Weighted Round Robin Priority Scheduling
The weighted round robin scheduling method functions as its name
implies. The port transmits a set number of packets from each queue, in a
round robin fashion, so that each has a chance to transmit traffic. This
method guarantees that every queue receives some attention from the
port for transmitting packets.
To use this scheduling method, you need to specify the maximum number
of packets a port should transmit from a queue before moving to the next
queue. This is referred to as specifying the “weight” of a queue. In all
likelihood, you will want to give greater weight to the packets in the higher
priority queues over the lower queues.
Table 8 shows an example.
In this example, the port transmits a maximum number of 15 packets from
Table 8. Example of Weighted Round Robin Priority
Port Egress Queue
Maximum Number of
Packets
Q3 15
Q2 10
Q1 5
Q0 1