Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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110 ExtremeWare 7.2e Installation and User Guide
Quality of Service (QoS)
Overview of Policy-Based Quality of Service
Policy-based QoS allows you to protect bandwidth for important categories of applications or
specifically limit the bandwidth associated with less critical traffic. For example, if voice–over-IP traffic
requires a reserved amount of bandwidth to function properly, using policy-based QoS, you can reserve
sufficient bandwidth critical to this type of application. Other applications deemed less critical can be
limited so as to not consume excessive bandwidth. The switch contains separate hardware queues on
every physical port. The prioritization parameters that modify the forwarding behavior of the switch
affect how the switch transmits traffic for a given hardware queue on a physical port. Up to eight
physical queues per port are available.
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:
• Voice applications
• Video applications
• Critical database applications
•
Web browsing applications
• File server applications
General guidelines for each traffic type are given below and summarized in Table 21. Consider them as
general guidelines and not strict recommendations. Once QoS parameters are 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 wish 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 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 end-stations will buffer significant amounts of video-stream data. This can present a
problem to the network infrastructure, because it must be capable of buffering the transmitted spikes