Allied-telesis AT-GS950/16 User Manual download pdf (Page 168)

Chapter 12: Quality of Service (CoS)

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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-S108 Version V1.1.0 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-GS950/16 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-S108 Version V1.1.0 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

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Allied-telesis AT-GS950/16 User Manual Page 168