WCS and Power Output Of An Access Point

Unanswered Question

Good Afternoon,

Running WCS version 4.1.185, with Cisco LWAPP 1242 access points. Have configured the controllers to perform "Dynamic AP Tx Power", Have found an issue where the power level on the some AP go down to 6 - 8. Removing an AP does not change te power level. We are rolling out wirelss phones, and are having issues, so we have had to custom set the x power to level 1 which is 17db or 50mww.

Has anyone experenced the same issue?

Thanks,

Dave

I have this problem too.
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rseiler Mon, 12/31/2007 - 15:02

So you are probably only using the 2.2 dBi radial 'rubber ducky' antennas?

This is most likely a side effect of using the wrong antennae type for your physical environment.

Note that the default time for auto-rf on the wlc to re-calculate the tx-power level assignment is 600 secs or 10 minutes. So removing an AP will take potentially up to 10 minutes for the other APs to update their signal strength. Just making sure you waited long enough. You can click the 'invoke power update now' button, but that still takes up to a minute to see the results in the browser...

Be advised also that not all 5Ghz channels support 17dB/50mW. The lower 5Ghz channels have a 11dB/12.5mW limit when working with external (non-captured) antennas. There's a Cisco Maximum Transmit Power for Lightweight Access Points doc that details these power levels. The 1242 is most affected by this, as all antennas are external, and they limit the power based on the highest available 9.5dB external antenna, so if you use the standard 3dB 5Ghz omnis you get screwed due to EIRP product of the higher gain antennas, which has to be the regulated worst case. The 1200 RM-21A radio/antenna combo is the exception, as well as the embedded radio/antennas on the 1130s. Since they are captured you can't replace them with another antenna, so you benefit from their higher gain (5 or 6dB) and get 17dB/50mW across all 5Ghz channels.

The aforementioned Cisco doc is somewhat dated only includes the original 12 channels which has since expanded to 23. The lower U-NII band seems to have the greatest power restriction; the middle and upper bands are less so.

I see I didn't explain my issue correctly. The problem is that the WCS auto power settings is not adjusting power correctly. I have several AP's next/above/below to each other and these AP's are all at power level 8. Signal strengh is real low. Using 5.2dbi antennas. With the rollout of the wireless phones we have had to hard code the power to 3 on the WISM controllers. This seems to resolve the issue, but my management doesn't like the fact that we don't have a self healing wireless network in power when an AP fails.

Thanks for the doc. It was helpful.

I see I didn't explain my issue correctly. The problem is that the WCS auto power settings is not adjusting power correctly. I have several AP's next/above/below to each other and these AP's are all at power level 8. Signal strengh is real low. Using 5.2dbi antennas. With the rollout of the wireless phones we have had to hard code the power to 3 on the WISM controllers. This seems to resolve the issue, but my management doesn't like the fact that we don't have a self healing wireless network in power when an AP fails.

We are using some of the rubber duckies, but not at the site we have the issue at.

We are using the 1242 AP's and external antennas are either rubber 'duckies' or 3.2 Omni direction.

This is what shows on all controllers:

Automatic Transmit Power Assignment

Transmit Power Assignment Mode................. AUTO

Transmit Power Update Interval................. 600 seconds

Transmit Power Threshold....................... -65 dBm

Transmit Power Neighbor Count.................. 3 APs

Transmit Power Update Contribution............. SNI.

Transmit Power Assignment Leader............... 00:19:06:33:62:80

Last Run....................................... 268 seconds ago

(WiSM-slot9-2) >show advanced 802.11b txpower

Automatic Transmit Power Assignment

Transmit Power Assignment Mode................. AUTO

Transmit Power Update Interval................. 600 seconds

Transmit Power Threshold....................... -65 dBm

Transmit Power Neighbor Count.................. 3 APs

Transmit Power Update Contribution............. SNI.

Transmit Power Assignment Leader............... 00:19:06:33:62:80

Last Run....................................... 430 seconds ago

Only 8 controllers have mode = off as we have locked down those sites.

montana123 Wed, 01/16/2008 - 16:11

Hi Dave,

We are experiencing similar issue for 2.4Ghz b/g band not only with Dynamic Power Control but with Dynamic Channel Allocation. As you may know, there was a bug with firmware older than 4.1.185 in which APs didn't communicate correctly in order for auto RF to work. We are getting alarms that interference is happening, but no indication what WLC (WiSM in our case) is doing about it. We had to hardcode as temporary measure.

In any case, I'm trying to confirm rumor that a group of 3 or 4 APs are needed for autoRF to work/heal/auto adjust. I hope I'm wrong for everyone's sake!

I share your managements concern about WLAN not healing itself.

Regards,

Dan

Dave,

Thanks for the insight. Do you happen to have the bug ID around this issue?

Auto-RF capabilities are predicated around having at least three neightboring APs establish a -65dB Transmit Power Threshold (a "grid" in effect). This threshold has been modified in later verions of code and recommended to be -71 or lower due to its impact in dense deployments. This is for the power-down function (DCA and DTPC).

Issue the following command from the controller to see how the current grid is configured:

show advanced 802.11a txpower

Issue the following command against an AP (per-radio based), and look at the Nearby APs listed in the output to see their signal strength:

show ap auto-rf <802.11b/802.11a>

Coverage Hole Detection is a power-up function and is client-centric based on an SNR cutoff-value formula:

Client SNR Cutoff Value (|dB|) = [AP Transmit Power (dBm) - Constant (17 dBm) - Coverage Profile (dB)]

The coverage profile can be seen by this command:

show advanced 802.11a profile [global/]

--Bruce Johnson

daviescs Sun, 01/13/2008 - 16:21

Hi Dave,

Whilist I may not be on the ball; typical problems like this with wireless may be due to channel selection. 802.11x generally has 3 non overlap channels (1,6,11) If your running two aps next to each other on same channel; you may reduce your bandwith by 1/2. Also, if your power is too high and your aps' are two close; that can cause the same problems too. Typically when aps' are within 100 ft of each other 30mW does the job. but, this is based upon my usage of the 1230 series ap's with 2dbI antenna. let me know how it goes.

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