Motorola AP-7131 Series, Reference Manual

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6 - 50 WiNG 5.5 Access Point System Reference Guide
5. Either select the Add button to define a new WLAN QoS policy, or select an existing WLAN QoS policy and Edit its
configuration. Existing QoS policies can also be selected and deleted as needed.
A Quality of Service (QoS) policy screen displays for the new or selected WLAN. The screen displays the WMM tab by
default, but additional tabs also display for WLAN and wireless client rate limit configurations. For more information, refer
to the following:
• Configuring QoS WMM Settings
• Configuring a WLAN’s QoS Rate Limit Settings
• Configuring Multimedia Optimizations
6.2.1 Configuring QoS WMM Settings
WLAN QoS Policy
Using WMM, end-user satisfaction is maintained in a wider variety of environments and traffic conditions. WMM makes it
possible for both home networks and Enterprises to decide which data streams are most important and assign them a higher
priority.
WMM’s prioritization capabilities are based on four access categories. The higher the access category, the higher the
probability to transmit this kind of traffic over the access point managed WLAN. ACs were designed to correspond to 802.1d
priorities to facilitate interoperability with QoS policy management mechanisms. WMM enabled access points coexist with
legacy devices (not WMM-enabled).
Packets not assigned to a specific access category are categorized by default as having best effort priority. Applications assign
each data packet to a given access category packets are then added to one of four independent transmit queues (one per access
category -
voice , video , best effort or background ) in the client. The client has a collision resolution mechanism to address
collision among different queues, which selects the frames with the highest priority to transmit.
The same mechanism deals with external collision, to determine which client should be granted the opportunity to transmit
(TXOP). The collision resolution algorithm responsible for traffic prioritization is probabilistic and depends on two timing
parameters that vary for each access category.
• The minimum inter-frame space, or Arbitrary Inter-Frame Space Number (AIFSN)
• The contention window, sometimes referred to as the random backoff wait
Both values are smaller for high-priority traffic. The value of the contention window varies through time. Initially the contention
window is set to a value that depends on the AC. As frames with the highest AC tend to have the lowest backoff values, they
are more likely to get a TXOP.
After each collision the contention window is doubled until a maximum value (also dependent on the AC) is reached. After
successful transmission, the contention window is reset to its initial, AC dependant value. The AC with the lowest backoff
value gets the TXOP.
To configure a WMM configuration for a WLAN:
1. Select the Configuration tab from the Web UI.
2. Select Wireless.
3. Select Wireless LAN QoS Policy to display a high level display of existing WLANs QoS policies.
4. Select the Add button to create a new QoS policy, or Edit to modify the properties of an existing WLAN QoS policy.
The WMM tab displays by default.