Quality of Service (QoS)
©2008 Allied Telesis Inc. All rights reserved.
39.2
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Introduction
This chapter introduces the concept of Quality of Service (QoS) with particular reference to
Allied Telesis switches running the AlliedWare Plus
TM
Operating System. For additional
information on using QoS on AT switches see,
How To Configure QoS on x900-24, x900-12, and
SwitchBlade x908 Series Switches
. This How To Note is available from www.alliedtelesis.com/
resources/literature/howto_plus.aspx.
The concept of QoS is a departure from the original networking concept of treating all network
traffic in the same way. Without QoS, all traffic types are equally likely to be dropped when a
link becomes oversubscribed. With QoS, certain traffic types can be given preferential
treatment. QoS is therefore a very useful tool both to control congestion and to meter or cap
data in order to apply pre-agreed service levels
.
Operationally, QoS is applied within the link and network layers. Functionally it provides the
capability to intelligently transport your network traffic in order to provide stable and
predictable end-to-end network performance.
Business benefits
Quality of Service mechanisms enable:
■
network service providers to sell different levels of service to customers, based on what
their customers require, and be confident in their ability to guarantee the reliable delivery
of these services
■
enterprise and educational organizations to actively manage and provide many services
across one network, for example live video streaming and standard data services, with
preferential treatment being given to mission-critical traffic
■
network administrators to manage network congestion as network traffic levels increase
and time-critical applications, such as streaming media, become more widely in demand by
customers and organizations
QoS Operations
Quality of Service is typically based on how the switch performs the following functions:
■
assigns priority to incoming frames (that do not already carry priority information)
■
correlates prioritized frames with traffic classes, or maps frames to traffic classes based on
other criteria
■
correlates traffic classes with egress queues, or maps prioritized frames to egress queues
■
provides minimum and maximum bandwidths for traffic classes, egress queues, and/or
ports
■
schedules frames in egress queues for transmission (for example, empty queues in strict
priority or sample each queue)
■
re-labels the priority of outgoing frames
■
determines which frames to drop or re-queue if the network becomes congested
■
reserves memory for switching/routing or QoS operation (for example, reserving buffers
for egress queues or buffers to store packets with particular characteristics)