Exploring DTPC and 802.11h Transmit Power Control

This article describes several wireless features to control the transmit power of wireless clients, along with some examples.

When designing, surveying or troubleshooting a wireless network, it is ideal to know the wireless clients and specifically their transmit powers.

In some cases, a mismatch of transmit power between an access point and a client can create network performance issues. A classic scenario is an access point with very strong transmit power. A packet sent by the access point will be received by a wireless client but a packet sent by a wireless client with less transmit power may not reach the access point. Matching transmit power could prevent one-way audio in Voice-over-IP.

Moreover, limiting the transmit power of the clients may help to reduce co-channel interference and to improve the network capacity, especially in high density environments.

A radio driver determines the transmit power of a wireless frame based on several factors including:

  • The radio hardware capability.

  • The regulatory domain: one or multiple country may be defined on the radio and will enforce a maximum allowed power for compliance with the regulations of the country.

  • The operational frequency: the transmit power may vary by band (2.4 GHz, 5 GHz) and subband (UNII1, UNII2, UNII2-Ex, UNII3).

  • The data rate and/or the modulation technique: the transmit power usually depends on the modulation technique (DSSS, CCK, OFDM, HT-20, HT-40) and the data rate of the transmission. Lower data rates generally have higher transmit powers than higher data rates.

  • Additional power constraints: Cisco proprietary DTPC feature minimizes power consumption and can save battery by limiting the transmit power of CCX clients to the transmit power of the access point radio. Similarly, the IEEE 802.11h amendment has a Power Constraint element to control the maximum transmit power of a client.

Standard 802.11d – and Cisco world-mode implementation

IEEE 802.11d-2001 is the 802.11 amendment that provides regulatory domain information. When enabled, a Country information element is added to the beacons and probe responses. With this element, the wireless clients can learn about the domain in term of allowed channels and maximum transmit output and can maintain compliance with the regulations of the country.

The Country element (IE number 7) contains the following information:

  • Country string: 3 octets value to indicate the regulatory domain (for example, AU – Australia). The third octet indicates if the regulations are for indoor, outdoor or both environments.

  • One or several consecutive subband triplets:

    • First Channel Number/Operating Extension Identifier: the lowest channel number of the subband.

    • Number of Channels/Operating Class: the number of channels defining the subband, starting from the first channel number.

    • Maximum transmit power level/Coverage Class: this value in dBm is an indication of either the maximum Transmit Power Output send to the antenna (TPO) or the Effective Isotropic Radiated Power (EIRP) depending on the implementation and the value of the country code.

Associated clients will limit their transmit power to the maximum value allowed for the country in the current channel.

On WLC, 802.11d is enabled by default using the country code provided at the initial WLC setup.

To change the controller’s country code:

(WLC) >config country CODE

Where the value CODE is the two-character ISO country code.

To verify the country code on a WLC:

(WLC) >show country

Configured Country............................... AU  - Australia

Using GUI:

In WIRELESS | Country, select the Country Code.

For example, when using the country code for Australia (ISO code AU), the beacon and probe response frames include the following Country element.

On an autonomous Cisco access point, IEEE 802.11d is disabled by default. You can enable it using the world-mode radio interface command:

AAP(config)#interface dot11Radio {0 | 1}

AAP(config-if)#world-mode dot11d country-code CODE {both | indoor | outdoor}

Regulatory information can restrict differently based on the access point’s location (indoor, outdoor or both).

For example, to enable 802.11d with Australia as a country code on 5GHz radio interface:

AAP(config)#interface dot11Radio 1

AAP(config-if)#world-mode dot11d country-code AU both

Selected country Australia

The world-mode legacy command is only useful for Cisco Aironet 350/CB20 NICs using earlier software versions that didn’t support 802.11d yet.

The GUI allows to enable 802.11d but the country code cannot be set (bug?).

In NETWORK | NETWORK INTERFACE | Radio1-802.11N 5GHz | Country Code section, you can only enable Dot11d.

On a captured Beacon frame, the Country element concatenates regulatory information from different subbands (UNII1, UNII2, UNII2-EX, UNII3):

You can verify the work-mode configuration from the running-configuration or the GUI.

Note that since the 1st of January 2015, manufacturers have been required by the FCC that non-SDR wireless clients do not rely on 802.11d for enforcing regulatory domains. [1]

Cisco Dynamic Transmit Power Control (DTPC)

DTPC is a Cisco proprietary feature which is part of the CCX program. When enabled on the global radio settings, Cisco access points advertise their channel and transmit power level in the beacons and probe responses. CCXv2 clients will automatically adjust their transmit power to the same level as the access point. For example, Cisco wireless phones from 7920 to 8821 series support DTPC.

This feature is particularly useful for the following reasons:

  • DTPC saves energy for low-powered devices which increases battery life.

  • DTPC reduces co-channel interference resulting from the client themselves. This is particularly true for high density environment.

  • DTPC can contribute to prevent a transmit power mismatch between the access point and the CCX client. Voice-over-IP will benefit from it.

On a WLC, DTPC is enabled by default. DTPC can be enabled/disabled per band in 2.4 GHz and for 5 GHz global settings. When enabled, it will transmit a vendor specific DTPC element (IE number 150) in the beacon and probe response frames.

In CLI:

config {802.11a | 802.11b} dtpc {enable | disable}

In addition, Aironet Extensions must also be enabled on the WLAN to support DTPC.

config wlan ccx AironetIeSupport enable 1

In GUI:

In WIRELESS | 802.11a/n/ac | Network | Advanced tab, enable Aironet IE.

In WLANs | WLANs | <WLAN ID> | General section, enable DTPC Support.

With DTPC enabled, access points advertise their transmit power in beacons and probe responses. The following picture shows the DTPC Information Element (IE number 150) captured in a beacon frame.

In this example, the access point transmit power was set to 19 dBm = 0x13 in hexadecimal. An associated CCX wireless client will limit its own transmit power to the access point’s transmit power.

To verify if DTPC is enabled:

(WLC) >show {802.11a | 802.11b}

DTPC  Status..................................... Enabled

 

(WLC) >show wlan 1

CCX - AironetIe Support.......................... Enabled

Let’s see an example of DTPC using a Samsung S5 mobile phone (CCX) and a Cisco 2700 access point. During this test, ICMP traffic was transmitted between the two devices. The frequency was set to 5745 MHz (channel 149) and the supported data rate was limited to 24Mbps only. Between each association, the access point transmit power level has been incremented from level 8 (2 dBm) to level 1 (23 dBm).

The RSSI of the access point frames (in blue) shows that the access point transmit power was increasing with time. The averaged RSSI of the mobile phone frames (in red) was constant at -60 dBm while the AP power was between 2 dBm and 8 dBm. When the AP power increased between 8 dBm and 14 dBm, the RSSI of the S5 frames increased up to -53 dBm. When the AP power was above 14 dBm, the RSSI of the S5 was capped at -53 dBm with no growth.

To explain this, the CCX client set its power accordingly to the access point transmit point within the client transmit power capabilities. In this example, when the access point transmit power is greater than 14 dBm, the S5 transmit power has reached its maximum. In other words, there is no more performance gain in the direction to the access point when the access point transmit power is above 14 dBm with a Samsung S5 at a data rate of 24 Mbps.

On an autonomous Cisco access point, DTPC is disabled by default and can be configured with the following radio interface command:

In CLI:

AAP(config)#interface dot11Radio {0 | 1}

AAP(config-if)#power client local

Notice again that Aironet extensions must be enabled to limit the power level on associated client devices. Aironet extensions are enabled by default.

AAP(config)#interface dot11Radio {0 | 1}

AAP(config-if)#dot11 extension aironet

Using GUI:

In NETWORK | NETWORK INTERFACE | Radio0-802.11N 2.4GHz, set Client Power (dBm) to Local.

The DTPC IE will be included and will match the transmit power configured for the access point radio interface.

With the IOS autonomous mode, the power client command has an extra feature. The client power can be set to any power level from -127 dBm  to 127 dBm or to the client maximum transmit power, regardless of the access point transmit power (actually limited to the maximum allowed transmit power of the access point) using the following command:

AAP(config)#interface dot11Radio {0 | 1}

AAP(config-if)#power client {-127 - 127 | maximum}

The feature uses the same DTPC IE but this time, the client power can be changed without matching the access point transmit power. This command offers a better precision in unit of dBm.

In a second example, using the same test conditions (Samsung S5, autonomous Cisco 2700 access point, channel 149, 24Mbps data rate only), the client power has been incremented from 6 dBm to 22 dBm between each association.

The RSSI of the access point frames (in blue) shows that the access point transmit power was constant. The averaged RSSI of the S5 frames (in red) increased and reached a plateau. We can see more precisely that the Samsung S5 reached its maximum transmit power at a data rate of 24 Mbps when the DTPC IE value is above 13 dBm.

802.11h – TPC Power Constraint

802.11h-2003 was an amendment called “Spectrum and Transmit Power Management Extensions in the 5GHz band in Europe” which later became enforced worldwide by both the ETSI and the FCC. Its purpose is to adjust the WLAN in order to co-exist with satellite and radar communications sharing the same frequency range.

802.11h operations consist of 2 mechanisms:

  • Dynamic Frequency Selection (DFS): to detect radar systems and to avoid co-channel operations. If the radio detects a radar signal on its operational channel, it must stop using the channel for 30 minutes and will move to another channel after checking that the new channel is free of radar for 1 minute. This mechanism is mandatory for UNII2 channels (52, 56, 60, and 64) and UNII2-EX channels (100, 104, 108, 112, 120, 124, 128, 136, and 140).

  • Transmit Power Control (TPC): to enforce an additional power constraint in dBm and to reduce the interference with satellite services. The access point and its wireless clients will ensure that they cannot transmit over the maximum regulatory transmit power minus the power constraint. This mitigation mechanism is optional.

The IEEE 802.11h mechanisms are carefully considered for outdoor wireless deployments, especially near airports, maritime ports and weather radars.

Looking at TPC specifically, a power constraint can be used to limit the transmit power of mesh access points associating to root access points for example. 802.11h Power Constraint can be configured for 5GHz radios only. When enabled, it will transmit the Power Constraint element (IE number 32) in the beacon and probe response frames.

802.11h TPC also requires the Country element. In fact the wireless client determines its local maximum transmit power with the combination of the local maximum transmit power in the Country element and the power constraint according to the following formula:

On a WLC, you can set a power constraint of 10 dB with the following command:

(WLC) >config 802.11h powerconstraint 10

 

Power constraint value set.

Note that DTPC and 802.11h Power Constraint are mutually exclusive. They cannot be enabled simultaneously.

If you try to modify the power constraint while DTPC is still enabled, you will see the following error:

(WLC) >config 802.11h powerconstraint 10

 

802.11a Dynamic Transmit Power Control (DTPC) must be disabled before enabling 802.11h power constraint. Use 'config 802.11a dtpc disable' to disable the 802.11a DTPC.

Using GUI, you can configure 802.11h Power Constraint:

In WIRELESS | 802.11a/n/ac | DFS (802.11h) | Power Constraint tab, set Local Power Constraint to a value in dB.

To check the power constraint value:

(WLC) >show 802.11h

 

Power Constraint................................. 10

Channel Switch................................... Enabled

Channel Mode..................................... Quiet

The following picture shows the Power Constraint element captured in a beacon frame.

The access point informs the client of a power constraint of 10 dB. The constraint will be applied from the maximum transmit power allowed by the regulatory domain, and not from the transmit power of the access point (which is unknown by the client with TPC). If the maximum power allowed by the domain is 23 dBm, the client will limit its power to 23 – 10 = 13 dBm.

On an autonomous access point, a default power constraint of 3 dB is set on 5GHz radios when 802.11d is enabled and when the power client {-127 to 127 | local} command is not used.

In the next example, an iPhone 5S is used with the same access point setup with 802.11h Power Constraint instead of DTPC. Between each association with an iPhone 5S, the Power Constraint was decremented from 28 dB to 7 dB. The configuration was the same as the previous tests (24Mbps data rate, channel 149).

Again, the RSSI of the access point frames (in blue) shows that the access point transmit power was constant. The averaged RSSI of the iPhone frames (in red) increased and reached a maximum when the Power Constraint hit 12 dB. To find out the client transmit power, use the formula: the client transmit power is limited to the maximum transmit power level specified for the channel in the Country element minus the power constraint.

In a beacon, the Country element specified that the maximum transmit power level specified for channel 149 was 30 dBm. So with a Power Constraint of 12 dB, the maximum client transmit power of the iPhone 5S for 24Mbps would be 18 dBm.

But let's just be careful with those values. One step back on the interpretation of Maximum Transmit Power Level in the Country element, it can relate to the TPO or to the EIRP. In Australia, the maximum output powers are expressed in EIRP which means antenna gain must be taken into account.

802.11h - Power Capability and TPC Report

802.11h has also introduced two Information Elements, Power Capability (IE number 33) and Supported Channels (IE number 36) which are now in every Association Request and Reassociation Request frame (2.4GHz and 5GHz). The first one specifies the minimum and maximum transmit powers in dBm with which a wireless client is capable of transmitting in the current channel. The second contains the list of channel subbands in which the wireless client is capable of operating.

The following picture shows the two IEs in an Association Request of the iPhone 5S on 5GHz channel 149:

Finally 802.11h includes TPC Report element (IE number 35) which contains the transmit power information. The TPC Report can be in beacon and probe response frames and can also be requested to any station. The reply may be in the form of an Action frame containing the power used to transmit the response. You may see the following TPC Report element with other wireless vendor access points in conjunction with 802.11k:

Conclusion

In addition to technical documentation and FCC ID Search information, benchmarking the wireless clients may help to understand what the client behaviour is under specific requirements (country, radio band, channel, data rate, etc). Packet analysis helps me to find lots of useful information, including:

  • Supported subbands and channels: using Country element and Supported Channels element.

  • Maximum transmit power allowed by the regulatory domain: using Country element.

  • Maximum transmit power supported by the wireless client on the current channel: using Power Capability element.

  • Client transmit power limitation: using DTPC element or 802.11h Power Constraint element. Up to the maximum transmit power supported by the wireless client.

Using benchmarking results, the wireless network can be changed in order to optimise the performance. It is generally recommended to ensure that the transmit power of the access point is not greater than the maximum transmit power of the weakest client for the desired minimum data rate.

However matching transmit power makes sense in a world using same radios and isotropic radiators. Performance may be affected by many other factors such as the antenna gain, the radiation pattern, the radio received sensitivity, the noise, Dynamic Rate Shifting and roaming algorithms to name a few.

Additional resources

Cisco Enterprise Mobility 8.1 Design Guide: http://www.cisco.com/c/en/us/td/docs/wireless/controller/8-1/Enterprise-Mobility-8-1-Design-Guide/Enterprise_Mobility_8-1_Deployment_Guide.html

Overview on 802.11h, Transmit Power Control (TPC) and Dynamic Frequency Selection: http://www.cisco.com/c/en/us/support/docs/wireless-mobility/80211/200069-Overview-on-802-11h-Transmit-Power-Cont.html

TPC and DFS Overview, Cisco Support Community: https://supportforums.cisco.com/document/52376/tpc-and-dfs-overview

TPC vs DTPC vs World mode and 802.11h parameters:

http://wirelessccie.blogspot.com.au/2009/07/tpc-vs-dtpc-vs-world-mode.html

http://wirelessccie.blogspot.com.au/2009/08/80211h-parameters.html

[1] FCC 594280 D01 Configuration Control: https://apps.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=39498&switch=P

QBSS Load element

Let’s begin this first post with a specific part of Wi-Fi Quality of Service (QoS), the QBSS Load element.

I wanted to learn which roles QBSS takes in Voice over IP deployment, and how different the Cisco implementations are.

The actual name of this Information Element (IE) is BSS Load as referred in the IEEE 802.11-2012 standard. The original name QBSS Load (QoS Basic Service Set) comes from the 802.11e amendment which has introduced QoS for wireless application in 2005.

The element is advertised in every beacon and every probe response frame of a QBSS. Which means an access point must have WMM enabled to advertise QBSS Load IE.

The IE defines some QoS information about the load of the access point:

  • Station Count: total number of STAs currently associated with the access point radio.

  • Channel Utilization: percentage of time that the access point sensed the channel was busy, by either the physical or virtual carrier sense mechanisms. The value is linearly contained into 1 octet, from 0 to 255, the highest value meaning 100% busy.

  • Available Admission Capacity: remaining amount of medium time used for admission control (2 octets long, in unit of 32 μs/s).

BSS Load element format. Source: IEEE Std 802.11™-2012

There are 3 different versions of the QBSS Load element which Cisco has implemented on wireless access points. Let’s get into the details for lightweight and autonomous access points.

Cisco proprietary QBSS Load – version 1 – non-CCA

The legacy version of QBSS is a Cisco proprietary feature based on the draft 6 of 802.11e. This feature was created before the ratification of 802.11e in 2005, so before WMM exists.

One difference with 802.11e is that the Channel Utilization value is expressed between 0 and 100, and not normalized to 255 as per-standard.

The QBSS Load V1 element is compatible with the earliest firmware releases of the Cisco 7920 phone. The 7920 is an old 802.11b wireless IP phone which does not support WMM. The 7920 determines the access point to associate, based on the RSSI and the Channel Utilization advertised by the access points.

For 7920 using QBSS Load V1, Cisco recommends to ensure that the Channel Utilization is less than 45. If the Channel Utilization is Channel Utilization is above 45% on all the potential access points, a "Network Busy" message will appear and the phone will not be able to make a call.

On a WLC, the feature is called 7920 Client CAC because the legacy 7920 handles the admission control. The QBSS Threshold can be changed temporarily on the phone via a hidden menu.

Configure QBSS Load V1 on a WLAN of a WLC with the following CLI commands:

At first, ensure that WMM is disabled on the WLAN as it is not supported with QBSS Load V1 (see the explanation later).

(WLC) >config wlan wmm disable <WLAN_ID>

Enable 7920 Client CAC support on the WLAN.

(WLC) >config wlan 7920-support client-cac-limit enable <WLAN_ID>

Using the GUI:

In WLANs | WLANs | <WLAN ID> | QoS tab, set WMM Policy to Disabled and enable 7920 Client CAC.

Notice that WMM and 7920 Client CAC are mutually exclusive. If you try to enable both at the same time, you will get the following error message. You will see the reason in the next section.

After enabling QBSS Load V1, it appears on the beacon and probe response frames in the Information Element 11. On the picture below from the packet capture of a beacon, there are 2 wireless stations associated to the access point, and the Channel Utilization is 1%.

A bigger difference with 802.11e is that the value for the Channel Utilization in QBSS Load V1 ONLY depends on the 802.11 traffic load on the access point. During some tests, when there was no data traffic (Tx Load, Rx Load) on the access point, the QBSS load element showed 1% of Channel Utilization. When an IPerf throughput test was running between the 2 stations to increase the network throughtput, the Channel Utilization went up to 66% (hexadecimal 0x42).

In another test, an IPerf test was done on the same channel via another access point with different stations to consume airtime near the original access point. On the access point with QBSS Load V1 and with no associated stations, the Channel Utilization stays low at 1% while the radio resource on the frequency is severely used. QBSS Load V1 Load does not vary with the physical carrier sense energy detection.

On an autonomous Cisco access point, you can enable the Draft 6 QBSS-Load IE with the following CLI command:

AAP(config)#dot11 phone

Regarding WMM, you can leave it enabled (by default) or disable it. The first option advertises beacons with both the QBSS Load V1 IE-11 and the WME IE-221 element, without QBSS element. Not properly WMM compliant.

AAP(config)#interface dot11Radio 0

AAP(config-if)#no dot11 qos mode wmm

Or via the GUI:

In SERVICES | QOS | ADVANCED tab, set QoS Element for Wireless Phones to Enable.

802.11e standard-based QBSS Load – version 2 - CCA

This second version of QBSS is based on the IEEE 802.11e-2005 amendment, with the format described in the introduction.

The value of Channel Utilization is between 0 and 255 as per-standard: 255 equal 100%. To obtain the Channel Utilization percentage, convert the Channel Utilization value from hexadecimal to decimal and divide by 255.

The percentage is defined in the standard by the formula:

Where:

  • channel busy time is defined to be the number of microseconds during which the CS mechanism has indicated a channel BUSY indication.

  • dot11BeaconPeriod is the interval in number of TUs (Time Unit 1024 µs) between Beacon frames that shall be scheduled by the AP. It is configurable by the AP but usually set to 102 TUs.

  • dot11ChannelUtilizationBeaconInterval represents the number of consecutive beacon intervals during which the channel busy time is measured. The default value of dot11ChannelUtilizationBeaconInterval is 50 in the standard.

Using the default values, the Channel Utilization is computed as a percentage of busy airtime for the last 5 seconds approximately.

More importantly, the Channel Utilization depends on the physical (CCA) or virtual (NAV) carrier sense mechanisms. Any radio energy detected by CCA (incoming Wi-Fi preamble detected by CCA-CS or non-Wi-Fi energy detected by CCA-ED) will be factored in the Channel Utilization.

On a WLC, you simply have to set WMM to Allowed or Required on the WLAN in order to automatically advertise the 802.11e QBSS Load IE.

Using CLI:

(WLC) >config wlan wmm {allow | require} <WLAN_ID>

Using GUI:

In WLANs | WLANs | <WLAN ID> | QoS tab, set WMM Policy to Allowed or Required.

On the picture below captured from a beacon frame, the Channel Utilization value is hexadecimal 0x9d in the third octet, equal to 157/255 = 62%. Remember that using the 802.11e version, the Channel Utilization value is normalized to 255. Wireshark does this nicely for you.

Notice also that the 802.11e QBSS Load element takes exactly the same Information Element (IE-11) as the Cisco QBSS Load V1. This is the reason why they are mutually exclusive. Therefore in the WLC implementation, you can use either WMM or QBSS Load V1.

The 802.11e QBSS Load IE can be used on the Cisco wireless IP phones supporting WMM, such as Cisco 7921G, 7925G, 7526G and 8821.

The recommendation from Cisco deployment guides is to keep the Channel Utilization below 105 (105 normalized to 255 is approximately 41%). Depending on the firmware release, newer Cisco phones may or may not make their roaming decision based on the Channel Utilization (see Cisco bugID CSCtz46981 792x on 2.4 GHz utilizes QBSS for roaming decisions, or CSCvc52145 8821 Network Busy error). Latest versions seem to display only the Channel Utilization.

On an Autonomous access point, the implementation is a bit different from enabling WMM. You can enable the 802.11e QBSS IE with the following CLI command:

AAP(config)#dot11 phone dot11e

Or via the GUI:

In SERVICES | QOS | ADVANCED tab, set QoS Element for Wireless Phones to Enable and check the Dot11e option.

When enabling QoS Element for Wireless phone with Dot11e, both 802.11e QBSS Load IE and Cisco QBSS Load V2 (see next section) are both advertised by the access point.

Cisco proprietary QBSS Load – version 2 - CCA

In mixed environment with 7920 and newer wireless IP phones (7921G, 7925G, 7926G, 8821), QBSS Load V1 and 802.11e QBSS Load issued with WMM cannot work together using the same IE. To support mixed environment with WMM devices, Cisco has developed another proprietary QBSS Load element using vendor-specific IE 221 rather IE 11. The new IE is only supported on latest versions of 7920 firmware (2.01 or later).

The element uses the same information defined by the 802.11e standard. Channel Utilization is based on CCA and expressed from 0 to 255.

An access point can advertise at the same time both:

  • 802.11e QBSS Load IE11: supported by WMM phones

  • Cisco QBSS Load V2 IE221: supported by Cisco wireless IP phones including non-WMM 7920 upgraded with firmware 2.01 or later

On a WLC, you can enable Cisco QBSS Load V2 with the 7920 AP CAC feature using with the following CLI command:

(WLC)>config wlan 7920-support ap-cac-limit enable <WLAN_ID>

 

(WLC)>show wlan <WLAN_ID>

WMM.............................................. Allowed

Dot11-Phone Mode (7920).......................... ap-cac-limit

Using GUI:

In WLANs | WLANs | <WLAN ID> | QoS tab, set WMM Policy to Disabled and enable 7920 AP CAC.

On the picture below, you can see the QBSS Load V2 with the IE tagged 221. The Channel Utilization value is equal to 124/255 = 48%. You can also see a maximum threshold Call Admission Limit that is defined to 105/255 = 41% by default.

The QBSS Load V2 Load element adds this extra benefit. The maximum CU threshold is customizable and advertised by the access point. The AP can handle the admission control by adjusting the Call Admission Limit, hence 7920 AP CAC.

To change the Call Admission Limit on a WLC:

(WLC) >config advanced 802.11b 7920VSIEConfig call-admission-limit <0-255>

However in the event of Network Busy issues with 7920s, you should address a high Channel Utilization issue with troubleshooting and design with best practices (use 5GHz, disable lower data rates , lower the number of SSIDs, reduce CCI, etc) before fine-tuning the Call Admission Limit.

On an Autonomous access point, you can enable Cisco QBSS Load V2 IE using the same command which enables QBSS Load V1 IE. QoS Element for Wireless Phones enables both Cisco QBSS Load V1 and QBSS Load V2:

AAP(config)#dot11 phone

Or via the GUI:

In SERVICES | QOS | ADVANCED tab, set QoS Element for Wireless Phones to Enable.

When QoS Element for Wireless Phones is enabled, the access point will also create dynamic voice classifiers for RTP-based traffic, to allow a higher priority for voice traffic even if there is no QoS.

To change the maximum threshold for Channel Utilization on an autonomous AP:

AAP(config)#dot11 phone cac-thresh <0-255>

Where can we find the Channel Utilization?

You can perform a wireless packet capture. Beacon and Probe Response will display the QBSS Load element if the QBSS Load is enabled on the WLAN.

You can also find the Channel Utilization on a WLC with the following CLI command:

(WLC) >show ap auto-rf {802.11b | 802.11a} <AP_NAME>

...

Load Information

Load Profile................................. PASSED

Receive Utilization.......................... 0 %

Transmit Utilization......................... 0 %

Channel Utilization.......................... 40 %

Attached Clients............................. 1 clients

On an autonomous access point, use the following command:

AAP#show controller dot11radio {0 | 1} | i QBSS

QBSS Load: 0xC5 Tx 9 Rx 13 AP 22

In this case, the Channel Utilization is hexadecimal 0xc5 equal to 197/255 = 77%.

The QBSS Load element is extremely useful as a metric for troubleshooting and performance analysis. Cisco phones in survey mode and other wireless network tools can display the Channel Utilization on screen. The Channel Utilization can also be found with Prime Infrastructure or WLCCA.

What’s next?

Applications are growing fast and wireless networks can quickly reach their throughput capabilities. Channel Utilization can be volatile in a heavily-used environment and admission control could be difficult to manage.

The next step to leverage capacity would be enabling TSPEC with Admission Control on the access point to protect high-priority calls.

References

Jérôme Henry, http://wirelessccie.blogspot.com, writer of the excellent CCNP-Wireless reference guides. He has published videos going through the configuration of QBSS Load in detail:

Wireless QoS, part 2-a and 2-b, Web link: http://www.youtube.com/watch?v=QTeOpW9YLFs

CCNP Wireless 642-742 IUWVN Quick Reference: http://www.ciscopress.com/store/ccnp-wireless-642-742-iuwvn-quick-reference-9781587143113

Cisco Unified Wireless IP Phone 7925G, 7925G-EX, and 7926G Deployment Guide: http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/cuipph/7925g/7_0/english/deployment/guide/7925dply.pdf

Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide: http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/cuipph/8821/english/Deployment/8821_wlandg.pdf