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High Channel Utilization on B/G

We are in the process of implementing a new wireless VOIP solution and have just got done rolling out our new wireless infrastructure.  We have problems in quite a few areas where the calls cut out, the phone loses service, or other add scenarios.  I've been doing a lot of research and trying to get my head around a few things and hopefully someone here can help clarify some of my questions.

To try and troubleshoot these problems I have been doing site surveys with the 7925g phone(s).  We certainly have move issues when leverage 802.11a on the phones versus 802.11g, which seems to lean toward coverage issues (we have lots of cinder block walls and the like).  So my questions:

I did a site survey with 802.11g, then surfed to the phone and examined the output.  I have a few questions:

Almost all of our APs have Channel Utilization listed well over 30, sometimes into 50.  Is this value a percentage?  The reason I ask is sometimes the value on the phone display while doing the survey will bounce well over 100, thus my questioning whether this is a percentage or some other value?

If that is percentage, is there an "easy" way to figure out what is causing that?  We really don't have that many wireless clients, and Clean Air shows very few devices that could be interfering.

Thanks to anyone that can help me understand this.


High Channel Utilization on B/G

CU range is from 0 - 255, depending on your QoS settings used. What are your settings used for security, QoS, etc? Also what firmware on the phones are you using.

Configuring QoS Basic Service Set (QBSS)

There are three different versions of QoS Basic Service Set (QBSS) that the Cisco Unified Wireless IP Phone 7925G Series supports.

The first version from Cisco was on a 0-100 scale and was not based on clear channel assessment (CCA), so it does not account for channel utilization, but only the 802.11 traffic traversing that individual access point’s radio. So it does not account for other 802.11 energy or interferers using the same frequencies. The max threshold is defined on the client side, which is set to 45. This would allow for up to 7 calls at 11 Mbps plus some background traffic.

QBSS is also a part of 802.11e, which is on a 0-255 scale and is CCA based. So this gives a true representation on how busy the channel is. The max threshold is also defined on the client side, which is set to 105.

The second version from Cisco is based on the 802.11e version, but allows the default max threshold of 105 to be optionally configured.

Each version of QBSS can be optionally be configured on the access point.

For the Cisco Unified Wireless LAN Controller, enabling WMM will enable the 802.11e version of QBSS. There are also the “7920 Client CAC” and “7920 AP CAC” options, where “7920 Client CAC” will enable Cisco version 1 and “7920 AP CAC” enables Cisco version 2. See the “SSID / WLAN QoS Settings” section for more info.

For the Cisco Autonomous Access Point, “dot11 phone” or “dot11 phone dot11e” will enable QBSS.
“Dot11 phone” will enable the 2 Cisco versions, where “dot11 phone dot11e” will enable both CCA versions (802.11e and

Cisco version 2). It is recommended to enable “dot11 phone dot11e”.

New Member

High Channel Utilization on B/G

Thank you very much for your response.  In QoS we have WMM as required, we do not have either of the 7920 options checked.  The phone is running 1.4.1SR1 it appears.  So then if I am interpretting what you are saying, as long as the APs are below 105 it should be fine, the max value would be 255.

If I look at Settings -> Device Information -> QoS:

  • DSCP For Call Control: CS3
  • DSCP For Configuration: CS3
  • DSCP For Services: Default

Thanks again for your response I appreciate it.

Cisco Employee

Re: High Channel Utilization on B/G

The quote from blakekrone is from my 7925 Deployment Guide.

Standard QBSS is also advertised to the client when WMM is enabled on the SSID when using the Cisco WLAN Controller.

If 2.4 GHz is too busy and not attributed to 802.11 traffic or an interferer that can be eliminated, then you should look to use 5 GHz.

5 GHz offers more channels with fewer interferers.

Sent from Cisco Technical Support iPhone App

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