Allied Telesis AT-GS950/24 - Manual (Page 194)

Chapter 15: Quality of Service and Class of Service
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Overview
When a port on an Ethernet switch becomes oversubscribed, its egress
queues contain more packets than the port can handle in a timely manner.
In this situation, the port may be forced to delay the transmission of some
packets, resulting in the delay of packets reaching their destinations. 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 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 applications, referred to as delay or time-sensitive
applications, that can be impacted by packet delays. Voice transmission
and video conferences are two examples. If packets carrying data in either
of these cases are delayed from reaching their destination, the audio or
video quality may suffer.
This is where Class of Service (CoS) is of value. It allows you to manage
the flow of traffic through a switch by having the switch ports give higher
priority to some packets, such as delay sensitive traffic, over other
packets. This is referred to as prioritizing traffic.
The various aspects of CoS are:
 “Packet Priority”
 “Egress Queue vs Packet Priority Mapping” on page 195
 “Prioritizing Untagged Packets” on page 196
 “Scheduling” on page 196
Packet Priority CoS applies primarily to tagged packets. A tagged packet contains
information within it that specifies the VLAN to which the packet belongs.
A tagged packet can also contain a priority level. This priority level is used
by network switches and other networking devices to know how important
(delay sensitive) that packet is compared to other packets. Packets of a
high priority are handled before packets 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 packet is received on a port on the switch, it is examined
by the AT-S115 Management software for its priority. The switch software
uses the priority to determine which ingress priority queue the packet
should be directed to on the ingress port.