AP Power

[Moudar]

Hi,

I have 2 APs 2802 running 40Mhz channel width. They are using power as follows:

(Cisco Controller) >grep include "AP34" "show advanced 802.11a txpower"
Press any key to continue..
AP34                 (56,52)*           *1/7 (18 dBm) [18/15/12/9/6/3/2/2]

There are 1 lines matching the pattern AP34

(Cisco Controller) >grep include "AP35" "show advanced 802.11a txpower"
Press any key to continue..
AP35                 (132,136)*         *4/8 (14 dBm) [23/20/17/14/11/8/5/2]

There are 1 lines matching the pattern AP35

These APs are in the EU so the max power on AP34 should be 23 dBm same as (AP35), why is it 18 dBm here? what I know is that UNII-1 max power is 23 dBm!

Both APs are 2802 model and according to the datasheet, they should support 23 dBm as max value!

[marce1000]

 - Note that in a controller based environment power  and channel allocation are optimized for best cell coverage (TPC and DCA algorithms which run periodically). In terms of cell coverage max. power is not always optimal and or client must be able to talk to the AP too within a cell , 

 M.

[Moudar]

I understand that the WLC is controlling the power sent to the APs. What i wonder about is why same APs get different max values.

I am not concerned about why the WLC is choosing a value over another, but my concern is why there is no 23 dBm max power option for one AP, whereas the other have this option (same AP model! )

[Rich R]

Actually it's not as simple as you think - it depends on the EXACT model of the AP (which you have not provided), regulatory domain, antenna type/configuration and channel.  Refer to 
https://www.cisco.com/c/dam/en/us/products/collateral/wireless/access-points/sales-tool-c96-744090.pdf 
and follow the link for your AP model AP2800I / AP2800E / AP2800P and then select the appropriate tab of the spreadsheet for exact matching characteristics to see the power levels for each channel for -E regulatory domain.

Also check what antenna gain you have configured on each AP.

[Moudar]

The Excel files found in that PDF are very large and need some time to understand them.

But the question still why 2 same model APs get different power options, maybe it is the channel assigned to every AP that dictates the power!

APs are AIR-AP2802I-E-K9

[Rich R]

Exactly - there are sometimes variations according to channel!
There have also been a few bugs around transmit power which have been fixed recently so it goes without saying (but worth saying again anyway) that you should make sure your software version (which you did not mention) is up to date.

[Moudar]

WLC

Software Version

8.10.181.3

[JPavonM]

That Power you see in the outputs is the maximum Transmission Power (TX Power) not the maxiumum EIRP allowed to be radiated, which consist in TX Power + Antenna gain.

Looking into datasheet for antenna gain in AP2802 (below) and adding that to TX power in AP34 output you have the maximum EIRP which is 23 dBm for RLAN band 1 sub-band II (equivalent to UNII-2a).

The rules in ETSI domain for RLAN band 2 are different as 30 dBm is the maximum EIRP (again don't confuse this with maximum TX Power from outputs).

BUT notice that whatever TX Power is selected for the AP by the RRM algorythm, that needs to be inside the thresholdds configured in the RF Profile, and always inside the regulatory range, so the AP won't be operating at maximum power if that exceeed the maximum EIRP.

[Moudar]

What i know is that any AP should not exceed the allowed max EIRP!

Do you mean that the WLC shows the max allowed Txpower without any antenna gain?

If that is right, 18 + 5(antenna gain) = 23 dBm that is right amount of allowed dBm in RLAN band 1 sub-band 1 and 2.

But in the case of AP 35, 23 + 5 does not equal 30 which is the max allowed in RLAN band 2?! 

[Scott Fella]

I'm assuming you have looked at this spreadsheet that shows the max TX for various countries.
https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/www.cisco.com/c/dam/en/us/td/docs/wireless/access_point/channels/reference/2800-3800-I-pwr-chn.xlsx.xml
This is normal to see the lower bands or channels to have a lower max to meet the countries regulations.  That is one reason folks tend to adjust the max TX level to keep ap's in a specific power range.  If you want to have a higher TX, then change your rf profile so that your ap's don't use the channels with lower power.

[JPavonM · Accepted solution]

First WLC is selecting a power for AP35 which is below the maximum permited (30 dBm - 5 dBi antenna gain), secondly the WLC is using the maximum TX Power certified for that AP model in the current configuration (VHT with 4-antennas and @40MHz; column BZ on the Excel @Scott Fella referenced before), and lastly, WLC is selecting a TX power inside your Max and Min thresholds in the RF Profile you are associating to them. At the end, it's all about automatic decitions based on regulations, AP certifications, and configuration, and adding all of them you end up with the maximum transmission power for that AP model, so nothing you can do to increase it other that selecting another AP model with more TX Power.

But why do you want your AP to operate at a higher TX Power? Notice that most if not almost all end-user devices do not operate at such higher TX Power levels, so you can end up suffering power impairment issues, where the AP is transmitting, the device hearing the AP, but unable to be heard by the AP (imagine a concert where you can hear the band from the very end of the auditorium because they use amplifiers, but they cannot hear you).

HTH

[Moudar]

My goal was not to increase the AP power, but I wanted to understand why the same model AP gets different power settings. Now I got it. The best is that I was introduecd to MCSindex which is important to understad.