Allied Telesis AT-S63 - Manual (Page 298)

Chapter 16: Quality of Service
298 Section II: Advanced Operations
Quality of Service Overview
Quality of Service enables you to prioritize traffic and/or limit the
bandwidth available to it. The concept of QoS is a departure from the
original networking protocols, which treated all traffic on the Internet or
within a LAN in the same manner. Without QoS, every traffic type is
equally likely to be dropped if a link becomes oversubscribed. This
approach is now inadequate in many networks, because traffic levels have
increased and networks transport time-critical applications such as
streams of video and data. QoS also enables service providers to easily
supply different customers with different amounts of bandwidth.
Configuring Quality of Service involves two separate stages:
ˆ Classifying traffic into flows, according to a wide range of criteria.
Classification is performed by the switch’s packet classifiers, described
in Chapter 13, “Classifiers” on page 251.
ˆ Acting on these traffic flows.
Quality of Service is a broadly used term that encompasses as a minimum
both Layer 2 and Layer 3 in the OSI model. QoS is typically demonstrated
by how the switch accomplishes the following:
ˆ Assigns priority to incoming frames, if they do not carry priority
information
ˆ Maps prioritized frames to traffic classes, or maps frames to traffic
classes based upon other criteria
ˆ Maps traffic classes to egress queues, or maps prioritized frames to
egress queues
ˆ Provides maximum bandwidth limiting for traffic classes, egress
queues and/or ports
ˆ Schedules frames in egress queues for transmission (for example,
empty queues in strict priority or samples each queue)
ˆ Relabels the priority of frames
ˆ Determines which frames to drop if the network becomes congested
ˆ Reserves memory for switching/routing or QoS operation (e.g.
reserving buffers for egress queues, or buffers to store packets with
particular characteristics)
Note
QoS is only performed on packets that are switched at wire speed.
This includes IP, IP multicast, IPX, and Layer 2 traffic within VLANs.
The QoS functionality described in this chapter sorts packets into various