IEEE 802.11e is an enhancement to the 802.11 standard that adds Quality of Service for wireless LAN applications such as Voice over Wireless LAN (VoWLAN) or IP Streaming Multimedia. This important standard has been added to the published 802.11-2007 standard.

WMM, which stands for WiFi MultiMedia, is also known as WME (Wireless Multimedia Extensions). It is a WiFi Alliance interoperability certification based on the 802.11e standard. WMM/WME provides basic QoS features to 802.11 networks, prioritizing traffic according to four queues, without guaranteed throughput.

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On a wired network, stations use CSMA-CD (Carrier Sense Multiple Access with Collision Avoidance) to improve communications performance. On an 802.11 network, a similar mechanism, known as the distributed coordination function (DCF), is used. When a wireless station wants to transmit, it selects a random backoff time and listens for contention. After a default 3-millisecond interframe space (IFS), the station starts the countdown of its backoff timer; when the backoff timer reaches zero, the station transmits. Using this mechanism, all stations are equal in the competition for the wireless media, which can cause delay-sensitive applications such as VoWLAN to exhibit poor Quality of Service characteristics.

WMM, on the other hand, defines four access category (AC) queues:

• Background
• Best effort
• Video
• Voice

• AIFSN, Arbitration IFS Number, the default time before countdown. This is similar to the IFS in DCF.
• CW minimum (CWmin), the minimum contention window.
• CW maximum (CWmax), the maximum contention window.
Transmit Ops (TXOP), the opportunity to transmit. This is the time during which a station that has won the control of the shared medium can retain it.

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• Radios can place traffic into the four queues according to 802.1p priority. (You can also specify DSCP priority.)
• WMM-enabled stations can use QoS marking to queue traffic as well; otherwise, traffic goes to the Best Effort queue by default.

3.1 Activate WMM
To activate WMM for a WLAN:

1. Go to the web interface of the Wireless Edge Services Module. In the Network Setup > WLAN Setup > Edit screen, go to the Advanced section and choose Automatic/WMM:
   
2. Click OK to accept Automatic/WMM as the Access Category. Wireless traffic is placed in the four queues according to its 802.1p priority (default) or DSCP value. The default mapping is as follows:
   

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3.2 Change WMM mapping
To change the WMM mapping:

1. Go to the WMM menu, select a Queue for a particular WLAN to get to the WLAN Setup window, and click QoS Mappings. You see the QoS Mappings window, where you can change mapping:
   
2. Click OK to accept any changes.
   

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3.3 Modify WLAN prioritization mode and AC queue parameters
You can also modify the prioritization mode and AC queue parameters for the WLAN.

1. For instance, by default the prioritization mode is 802.1p (Layer-2 priority) but you can also set it to DSCP. To modify this setting:
   

2. To modify the WMM parameter for an access category, highlight the category and click Edit at the bottom left of the screen. You see the Edit WMM window:
   
3. From the same window you can also configure the QoS parameters: AIFSN, Transmit Ops, CW Minimum, CW Maximum

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3.4 Configure admission control
Finally, you can configure admission control, to limit the number of wireless stations that can use the same QoS settings at the same time. (This is for Voice and Video categories only.)

To configure Admission Control:

1. Select Network Setup > WLAN Setup and click the WMM tab.
2. Select the queue for which you want to alter the settings, and then click the Edit button. The Edit WMM screen is displayed:
3. Enable the Admission Control check box, and enter the number of stations. Then click OK to accept the new settings.
   

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3.5 General configuration tips

• Low AIFSN and low CWmin will provide low latency, high throughput.
• By default, high-priority queues on the radio port use AIFSN=1ms; high-priority queues on stations use AIFSN= 2ms.
• If Transmit Ops is too high, a station can use the medium for a long time and other traffic may be delayed. Generally Transmit Ops is best set to a maximum of 100ms.
• Increasing CWmax can decrease the number of collisions.
• CWmax must always be higher than CWmin.

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WMM is a popular standard, and has been adopted by most wireless IP telephone vendors, including Cisco, Aastra, Alcatel, Avaya, Siemens, and Spectralink-Polycom.

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U-APSD (Unscheduled Automatic Power Save Delivery) has been adopted by the WiFi Alliance, where it is known as WMM Power Save. This technology allows saving battery power and fine-tuning the power consumption of wireless stations on WMM-enabled wireless networks. WMM Power Save Certification is a part of the WMM certification as defined by the WiFi Alliance.

With U-APSD, data destined to a wireless station such as a PDA or phone can be transmitted in a shorter time period, while allowing the device to stay longer in a low-power sleep mode. When the device wakes up, the client sends a trigger frame to the access point to obtain the data. The application, not the device driver, decides when the client will communicate with the access point, allowing more granularity in adapting the power save mechanism to the application use (e.g., to voice traffic).

To take advantage of U-APSD, both the AP and the client must have WMM enabled. U-APSD can coexist with legacy devices and networks. Improvements in battery life range from 15 percent to 40 percent.

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• For the ProCurve Wireless Edge Services Module Management and Configuration Guide:
  http://cdn.procurve.com/training/Manuals/WESM-zl-MgmtCfg-Feb2008-59918626.pdf
  In particular, be sure to read chapter 4 of this Management and Configuration Guide.
• For other references about WMM:
  http://en.wikipedia.org/wiki/IEEE_802.11e
  
  http://www.wi-fi.org/knowledge_center/wmm

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