Allied Telesis AT-S62 - Manual (Page 192)

Chapter 14: Quality of Service
Section II: Advanced Operations 192
Quality of Service 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—the port may be forced to delay the transmission of some
packets. This can result 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 conferencing are two examples. If packets
carrying data for either of these are delayed from reaching their
destination, the audio or video quality may suffer.
This is where QoS 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.
QoS, as implemented in the AT-S62 management software, consists of
the following two parts:
❑ Class of Service
❑ Scheduling
Class of Service
(CoS)
CoS applies primarily to tagged packets. A tagged packet, as explained
in Tagged VLAN Overview on page 395, 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 typically 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-S62 software for its priority. The switch software uses the
priority to determine which egress priority queue the packet should be
directed to on the egress port.