Allied Telesis AT-S25 - Manual (Page 108)

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Configuring Quality of Service
The AT-8324 and AT-8316F Series Switches support Quality of Service
(QoS) as defined in the IEEE 802.1p standard. QoS can be important in
network environments where there are time-critical applications, such
as voice transmission or video conferencing, that can be adversely
affected by packet transfer delays.
Prior to QoS, network traffic was handled in a best-effort manner, where
packet forwarding was typically performed on a first-in, first-out basis.
File transfer delays did occur, but were mostly transparent to network
users. But with the introduction of time-critical applications, packet
transfer delays can be problematic. For example, transfer delays of voice
transmission can result in poor audio quality.
QoS was designed to address this problem. The IEEE 802.1p standard
outlines eight levels of priority, 0 to 7, with 0 the lowest priority and 7 the
highest priority.
The AT-8324 and AT-8316F Series Switches feature two priority egress
queues: high and normal. Packets with priority values 0 through 3 are
placed in the normal priority egress queue. Packets with priority values 4
through 7 are placed in the high priority egress queue.
When a tagged packet enters a switch port, the switch responds to the
priority in the tag and forwards the packet accordingly. If desired, you
can configure the individual ports on the switch so that the priority level
in a tagged frame is ignored and that the tagged packets received on a
port are automatically assigned to either the normal or high priority
queue, regardless of the priority level in the packet. Consequently, the
switch will forward a tagged frame according to the port priority level
and not to the priority level in the tagged frames. However, the switch
does not alter the priority level in the packet, so that when the switch
transmits the packet, its original packet priority level is unaltered.
Note
The priority value in the packet is forwarded unchanged except in
the rare case when VLAN ID equals 0 (a special priority-tagged frame
with no VLAN ID information). In this case, packets with priority
values 0 through 3 will be mapped to priority 0, and packets with
priority values 4 through 7 will be mapped to priority 7.
The options available are:
❑ Keep or override the default tag value.
❑ Set the priority to high or normal.