Tx Power (FAT) & Tx Power Level (THIN)

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Mar 20th, 2008
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During the site survey, we used the 1230 and assigned tx power based on 100mw, 50, 30, 10, 5.. so on.. on the AP. But when we implemented the controller-based, the transmit power were replaced with 'tx power levels', 1,2,3,4... so on... I read somewhere that 1 is the max and as the level increases, its tx power becomes 50% less. say, 1=100mw, 2=50 mw, 3=25mw, 4=12.5, 5=6.25 and so on...


the confusion is, some AP's tx power were set to 30mw during the site survey. Which level should be the euqivalent as 'tx power level' on the wlc-base deployment? is it safe to assign the 30mw site survey data to the #3 'tx power level' on the controller?


And does anyone has ideas on the effect of using or assigning the country codes of the controller. If US is used, what is the max power level value? is it 100mw? And if US is used as country code, can any AP on any country domain be used. I checked out the chart, but it's quite confusing.


Thanks for anyone who can shed light on this.


Thanks for any ideas.... Argh!


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Rob Huffman Fri, 03/21/2008 - 09:13
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Hi Pirate,


Hope all is well "shiver me timbers....Argh!"


You are really on the right track here. Here is some related info for the Transmit Power portion of RRM;


Transmit Power Control Algorithm


The TPC algorithm, run at an un-modifiable ten-minute interval, is used by the RF Group Leader to determine APs RF proximities and adjust each band's transmit power level downward, as necessary. this is performed in order to limit excessive cell overlap and co-channel interference.


Each AP reports an RSSI-ordered list of all neighboring APs. Provided an AP has three or more neighboring APs, the RF Group Leader applies the TPC algorithm on a per-band, per-AP basis in order to adjust AP power transmit levels downward such that the third loudest neighbor AP is heard at a signal level of -65dBm (default value) or lower.


Power changes are only made when the third loudest neighbor of a given AP is heard at a signal level higher than the default value of -65 dBm.


Note: When all APs boot up for the first time, the APs transmit at their maximum power levels. Also, if the APs do not see each other with a -65dBm (default value), they push the power level up to the maximum in order to achieve this. If you go to the Controller CLI and issue the show ap auto-rf 802.11b command, there is a Nearby APs section near the bottom of the command output. In this section, you find the dBm listing. If this dBm listing has values more than -65, then the AP transmit power is at the maximum. This is tweakable to an extent. You can issue the Config advanced 802.11b tx-power-control-thresh <-50 to -80> command.



From this excellent doc (Thanks Ben!);


http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008072c759.shtm


Note: The 802.11a Cisco Radio supports five transmit power levels: 1 = Maximum

transmit power level allowed per Country Code setting, 2 = 50% power, 3 = 25%

power, 4 = 6.25 to 12.5% power, and 5 = 0.195 to 6.25% power.


So, in your environment, you could certainly go with #3 to acheive a power setting close to the reccommendations from the Site Survey.The WLC can be configured with the -A Americas Country Code but only with AP's from the same (-A) Regulatory Domain. With WLC 4.1.x.x you can also use Multiple Country Codes and AP's on the same Controller.


Hope this makes some sense Matey!

Rob

Rob,


Can you purchase non-US regualatory hardware in the US? I'd love to be able to bypass Cisco's artifically low 5GHz US transmit power levels on the 1242APs for all external antennas up to 9.5dB (half of the UNII-low and upper bands are limited to 11dB). The EU versions let you manually stipulate graduated power levels based on the actual antenna used.

Rob Huffman Mon, 03/24/2008 - 08:47
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Hi Bruce,


Hope all is well with you :) I have read many threads with great input from you, so I should comment on your valuable participation here!


This really a loaded question. Yes, I beleive you can purchase non-US Regulatory Hardware in the US. Whether or not you can legally use it is another question altogether ;-)


Take care,

Rob

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