Motorola Solutions WiNG 5.2.6, Справочное руководство

Страница: 308 / 670

6 - 46 WiNG 5.2.6 Access Point System Reference Guide
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.
The WMM tab displays by default.