Allied Telesis AT-S82 - Manual (Page 54)

Chapter 7: Class of Service
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Class of Service Overview
When the egress queues on a port in an Ethernet switch contains more
packets than the port can handle in a timely manner, the port may be
forced to delay the transmission of some packets. A port may be forced to
delay transmission of packets while it handles other traffic and, in some
situations, some packets destined to be forwarded from the port are
discarded.
Minor delays are often of no consequence to a network or its performance.
But there are applications referred to as delay- or time-sensitive, that can
be impacted by packet delays. Voice transmission and video conferencing
are two examples. If packets containing data for either of these
applications are delayed in reaching their destination, the audio or video
quality may suffer.
CoS allows you to manage the flow of traffic through a switch by setting
the switch ports to give higher priority to some packets, such as delay-
sensitive traffic, over other packets. This is referred to as prioritizing traffic.
CoS applies primarily to tagged packets. A tagged packet contains
information that specifies the VLAN to which the packet belongs and can
also contain a priority level. Network switches and other networking
devices use the priority level to determine how important that packet is
compared to other packets. High priority packets are handled before low
priority packets.
Mapping Ports to
Egress Queues
CoS, as defined in the IEEE 802.1p standard, has eight levels of priority—
0 to 7, with 0 the lowest priority and 7 the highest. Each port has four
egress queues, labeled Q0, Q1, Q2, and Q3. Q0 is the lowest priority
queue and Q3 is the highest. A packet in a high priority egress queue is
typically transmitted out a port sooner than a packet in a low priority
queue.