Allied Telesis AT-S63 - Manual (Page 222)

Chapter 13: Quality of Service
222 Section II: Advanced Features
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 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 some 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 your 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-S63 management software, consists of
the following two parts:
❑ Class of Service
❑ Scheduling
Class of Service
(CoS)
CoS applies only to tagged packets. If you read ”Tagged VLAN Overview”
on page 436, then you know that a tagged packet contains information
within it that specifies the VLAN to which the packet belongs.
A tagged packet also contains 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, its priority is
examined by the AT-S63 management software. The switch software
uses the priority to determine which egress priority queue the packet
should be directed to on the egress port.