Allied Telesis AT-S26 - Manual (Page 116)

Configuring Virtual LANs and Quality of Service
116
Configuring Port Priority Queueing
The AT-9006 Series Ethernet Switches support the IEEE 802.1p standard
and Quality of Service (QoS). QoS can be of importance 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. 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 in voice
transmission can result in poor audio quality.
The QoS feature 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-9006 Series Ethernet Switches have
two priority queues, normal and high. Packets with priority levels 0 to 3
are placed in the normal queue and packets with priority levels 4 to 7 are
placed in the high 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 packet itself. However, the
switch does not alter the priority level in the packet, so that when the
packet egresses the switch, its original packet priority level is
unchanged.
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.