Allied Telesis AT-S60 - Manual (Page 321)

Chapter 17: Class of Service
Section II: Local and Telnet Management 320
Class of Service Overview
When a port on an Ethernet switch becomes oversubscribed, meaning
that its egress queues contain more frames than the port can handle in
an timely and orderly manner, there is the possibility that frames may be
delayed in reaching their destinations. A port may be forced to delay the
transmission of some frames while it handles other traffic. And in some
circumstances, some frames destined to be forwarded to an
oversubscribed port from other switch ports may be discarded.
Minor delays are often of no consequence to a network or its
performance. But there are some applications, referred to as delay or
time sensitive applications, that can be impacted by frame delays. Voice
transmission and video conferencing are two examples. When frames
carrying data for these applications are delayed from reaching their
destination, the audio or video quality may suffer.
This is where CoS is of value. It allows you to manage the flow of traffic
through your switch by having the switch ports give higher priority to
some frames, such as delay sensitive traffic, over other frames. This is
referred to as prioritizing traffic.
CoS applies principally to tagged frames. If you have read Tagged VLAN
Overview on page 251, then you know that a tagged frame contains
information within it that specifies the VLAN to which the frame belongs.
This information is located in the Ethernet header of a frame.
A tagged frame also contains priority data, also within the Ethernet
header, which is used by network switches and other networking
devices to know how important or delay sensitive the frame is compared
to other frames. Frames of a high priority are typically handled by the
switch ports before frames of a low priority.
CoS, as defined in the IEEE 802.1p standard, has eight levels of priority.
The priorities are 0 to 7, with 0 the lowest priority and 7 the highest.
When a tagged frame is received on a port on the switch, it is examined
by the AT-S60 software for its priority. The switch software uses the
priority to determine which egress priority queue the frame should be
directed to on the egress port.