Wireless Configuration 6 - 47
6.2.1 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 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 wireless controller 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 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
>
Wireless LAN QoS Policy
to display existing WLAN 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.
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