Allied Telesis AT-S39 - Manual (Page 175)

AT-S39 User’s Guide
175
Class of Service Overview
The AT-8000 Series switch supports CoS as specified in the IEEE 802.1p
and 802.1Q standards. CoS 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 CoS, 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 prove problematic. For example, transfer delays of voice
transmission can result in poor audio quality.
CoS was designed to address this problem. The 802.1p standard outlines
eight levels of priority, 0 to 7, with 0 the lowest priority and 7 the highest.
The AT-8000 Series switch has two priority queues, low and high. When a
tagged packet enters a switch port, the switch responds by placing the
packet into one of the two queues according to following assignments:
IEEE 802.1p AT-8000 Series Switch
Priority Levels Queue
7 high
6 high
5 high
4 high
3 low
2 low
1 low
0 low
For example, a tagged packet with a priority tag of 6 is placed in the high
priority queue, while a packet with a priority tag of 1 is placed in the low
priority queue.
These priority-to-queue assignments can be overridden using the
AT-S39 management software on a per port basis.
You can also use CoS to control which priority queue handles untagged
frames that ingress a port. By default, untagged frames (i.e., frames
without VLAN or priority level information) are automatically assigned to
the low priority buffer. But you can configure CoS on a port so that all
untagged frames received on the port are directed to the high priority
queue.