Chapter 7 Quality Of Service (Qos); Overview Of Policy-Based Quality Of Service; Applications And Types Of Qos; Voice Applications - Extreme Networks ExtremeWare XOS Guide Manual

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. Each hardware queue is programmed by ExtremeWare XOS with bandwidth
management and prioritization parameters. The bandwidth management and 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.
The switch tracks and enforces the minimum and maximum percentage of bandwidth utilization
transmitted on every hardware queue for every port. When two or more hardware queues on the same
physical port are contending for transmission, the switch prioritizes bandwidth use so long as their
respective bandwidth management parameters are satisfied. 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 11. Consider them as
general guidelines and not 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 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.
82 ExtremeWare XOS 10.1 Concepts Guide