Brocade Mobility RFS Controller System Reference Guide (Supporting software release 5.5.0.0 and later) - Manual (Page 308)

296 Brocade Mobility RFS Controller System Reference Guide
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Configuring a WLAN’s QoS WMM Settings
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 traffic priority.
WMM’s prioritization capabilities are based on the four access categories. The higher the access
category, the higher the probability to transmit this kind of traffic over the WLAN. Access categories
were designed to correspond to 802.1d priorities to facilitate interoperability with QoS policy
management mechanisms. WMM enabled wireless controllers/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 an internal 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(s) 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 interframe 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 Configuration > Wireless > WLAN QoS Policy to display existing QoS Policies.
2. 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.