Allied Telesis AT-S63 - Manual (Page 338)

Chapter 17: Quality of Service
338 Section II: Advanced Operations
Quality of Service Overview
Quality of Service allows 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 14, “Classifiers” on page 283.
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.