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Cisco IOS Software Configuration Guide for Cisco Aironet Access Points
OL-30644-01
Chapter 15 Configuring QoS
Configuring QoS
Rate Limiting
Rate limiting provides control over the data traffic transmitted or received on an interface.The
Class-Based Policing feature performs the following functions:
• Limits the input or output transmission rate of a class of traffic based on user-defined criteria.
• Marks packets by setting the IP precedence value, IP differentiated services code point (DSCP)
value and Quality of Service (QoS) group.
This is used to rate-limit the upstream traffic originating from each of the non-roots to root bridge incase
of P2MP setup. To do rate-limiting on downstream traffic , class-maps are applied at the root-side
router/switch.
Note Rate-limiting can be applied to ethernet ingress only
Adjusting Radio Access Categories
The access point uses the radio access categories to calculate backoff times for each packet. As a rule,
high-priority packets have short backoff times.
The default values in the Min and Max Contention Window fields and in the Slot Time fields are based
on settings recommended in IEEE 802.11 amendment. For detailed information on these values, see the
IEEE 802.11e amendment, 7.3.2.27 or 802.11-2012 standard, 8.4.2.31 (EDCA Parameter Set element).
Cisco strongly recommends that you use the default settings on the Radio Access Categories page.
Changing these values can lead to unexpected blockages of traffic on your wireless LAN, and the
blockages might be difficult to diagnose. If you change these values and find that you need to reset them
to defaults, use the default settings listed in Table 15-1.
The values listed in Table 15-1 are to the power of 2. The access point computes Contention Window
values with this equation:
CW = 2 ** X minus 1
where X is the value from Table 15-1.
Figure 15-4 shows the Radio Access Categories page. Dual-radio access points have a Radio Access
Categories page for each radio.
Table 15-1 Default QoS Radio Access Categories
Class of Service
Min
Contention
Window
Max
Contention
Window
Fixed
Slot
Time
Transmit
Opportunity
Admission
Control
Local Cell Local Cell Local Cell Local Cell Local Cell
Background 4 10 6 0
Best Effort 4 10 2 0
Video <100ms Latency 3 2 1 3008
Voice <100ms Latency 2 3 1 1504