Extreme Networks ExtremeWare XOS 11.3, Manual

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Quality of Service
ExtremeWare XOS 11.3 Concepts Guide
290
NOTE
Policy-based QoS has no impact on switch performance. Using even the most complex traffic groupings has no cost
in terms of switch performance.
Applications and Types of QoS
Different applications have different QoS requirements. The following applications are ones that you
will most commonly encounter and need to prioritize:
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Voice applications
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Video applications
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Critical database applications
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Web browsing applications
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File server applications
General guidelines for each traffic type are given below and summarized in Table 40 . Consider them as
general guidelines and not as strict recommendations. After QoS parameters have been set, you can
monitor the performance of the application to determine if the actual behavior of the applications
matches your expectations. It is very important to understand the needs and behavior of the particular
applications you want to protect or limit. Behavioral aspects to consider include bandwidth needs,
sensitivity to latency and jitter, and sensitivity and impact of packet loss.
Voice Applications
Voice applications, or voice over IP (VoIP), typically demand small amounts of bandwidth. However,
the bandwidth must be constant and predictable because voice applications are typically sensitive to
latency (inter-packet delay) and jitter (variation in inter-packet delay). The most important QoS
parameter to establish for voice applications is minimum bandwidth, followed by priority.
Video Applications
Video applications are similar in needs to voice applications, with the exception that bandwidth
requirements are somewhat larger, depending on the encoding. It is important to understand the
behavior of the video application being used. For example, in the playback of stored video streams,
some applications can transmit large amounts of data for multiple streams in one “spike,” with the
expectation that the endstations will buffer significant amounts of video-stream data. This can present a
problem to the network infrastructure, because the network must be capable of buffering the
transmitted spikes where there are speed differences (for example, going from gigabit Ethernet to Fast
Ethernet). Key QoS parameters for video applications include minimum bandwidth and priority, and
possibly buffering (depending upon the behavior of the application).