Allied Telesis AT-GS950/8 - Manual (Page 168)

Chapter 12: Quality of Service (CoS)
168
The procedure for changing the default mappings is found in “Associate
Ports to CoS Priorities” on page 173. Note that because all ports must use
the same priority-to-egress queue mappings, these mappings are applied
at the switch level. They cannot be set on a per-port basis.
One last thing to note is that the AT-S107 Management software does not
change the priority level in a tagged packet. The packet leaves the switch
with the same priority it had when it entered. This is true even if you
change the default priority-to-egress queue mappings.
Prioritizing
Untagged Packets
CoS relates primarily to tagged packets rather than untagged packets
because untagged packets do not contain a priority level. However, the
AT-GS950/8 switch has a priority associated with each individual ingress
port. By default, each port’s priority is 0. You can redefine this parameter
as described in “Associate Ports to CoS Priorities” on page 173.
Scheduling
A switch port needs a mechanism for knowing the order in which it should
handle the packets in its four egress queues. For example, if all the
queues contain packets, should the packets in queue Q3, the highest
priority queue, be processed through the switch before moving on to the
other queues, or should it instead just do a few packets from each queue
in a sequential fashion and, if so, how many?
This control mechanism is referred to as the scheduling algorithm.
Scheduling determines the order in which a port handles the packets in its
egress queues. The AT-S107 software has two types of scheduling:
 Strict priority
 Weighted round robin priority
To specify the scheduling, refer to “Associate Ports to CoS Priorities” on
page 173.
Note
Scheduling is set at the switch level. You cannot set this parameter
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 of this type of scheduling is that high priority
packets are always handled before low priority packets which is required
for voice or video data.
The problem with this method is that some low priority packets might
never be transmitted from the switch because the algorithm might never