Wireless Access

Hello,

I think I have a simple question, but because I can not find it in the documentation, I ask it here to know for sure.

In a warehouse we use the Aruba IAP-304 (version 8.3.0.6) with external anttenas (AP-ANT-1). At the access point settings I have entered the gain on both frequencies.

For 2.4 GHz 3.8
For 5GHz 5.8

For the entire system we have set the maximum transmission power to vary between 15 and 18 dB.

If I understand correctly, the APs will not exceed 18dB with these settings?

Because I have entered the gain on the AP, will it automatically turn down the transmis power with 5.8dB for the 5GHz, for example? So that the maximum transmitpower on this AP will be 18dB, or am I seeing this wrong?

The access point will adjust power based on the gain of the external antenna.

Edit. Wrong Post, Sorry.

Hi cjoseph,

Thanks for your reply, but that did not make it clear to me yet.

So with the external antennas with a gain of 5.8dB on the 5GHz, and the maximum transmit power on my system of 18dB, the maximum the access point can transmit is 23.8dB.

But with the gain settings set in the External antenna field in the settings of the Access Point, what will the AP transmit? will he transmit 18dB or 23.8dB on the 5GHz?

I think it will transmit 18dB, but I am not 100% sure.

The access point is adjusted to the expected transmit power, taking the gain of the antenna into account.

thanks for the clarification, that was what I expected.

To add on to Colin's answer, we want to elaborate a bit as it's a bit more complex if you're wanting to understand the underpinning with the new FCC calculation requirements.

Focusing on the AP-304, note it's a 3x3 AP, so we get MIMO/TxBF benefits to the total EIRP of the 'system' to the amount of "10 log (N)" where N is the number of chains. When you see on the data sheet a 'per chain max of 18dBm', that is representing one of the chains of the radio, of which the 304 has three total (other APs will have different chain counts identified in the NxN nomenclature). The math works out as follows:

1x1 AP = No benefit, the per-chain is the max

2x2 AP = 3.01 (which we will use 3dB for easy math)

3x3 AP = 4.77 (which we will use 4.5dB for easy math)

4x4 AP = 6.02 (which we will use 6dB)

So the formula for EIRP calculation is 

Radio Power + Antenna Gain + MIMO/TxBF gain = Total EIRP

NOTE: These gain values are regardless of the client, but 2x2 and 3x3 clients benefit the same. MRC (maximal ratio combining) follows the same math, but since it's on the receive side it's not relevant to transmit. But it's the MIMO/TxBF and MRC benefits of modern WiFi clients that negate the argument that AP power should be lower because clients 'cannot reach back'. If you want to learn more, let me know and I can dig up one of Eric Johnson's talks on this topic.

Now, when you set the power on the Aruba controller, you are setting MAXIMUM EIRP allowed by the AP (this is required by FCC). So when you set the AP max to 18dBm, on the AP-304 with 5.8dBi antennas, the math works out to:

18dBm - 5.8dBi ant - 4.5dB TxBF/MIMO = 7.7 radio transmit power

In addition, the FCC defines the maximum allowable transmit power, defined by a number of factors (what the AP tested to where it remains within the thresholds for spurious emissions, what is allowed on that band, the channel width, whether it's indoor or outdoor, etc). So this value can slightly vary based on the channel in use. You can see this on your controller with the following command:

(7005-1) #show ap allowed-max-EIRP ap-name Garage-f9:a6

Warning: This command has been deprecated.

Max EIRP setting for AP-305
---------------------------
Channel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 36 40 44 48 52 56 60 64 100 104 108 112 116 120 124 128 132 136 140 144 149 153 157 161 165 169 173
------- - - - - - - - - - -- -- -- -- -- -- -- -- -- -- -- -- -- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
b 25.0 25.0 25.0 26.0 26.5 26.5 26.5 26.5 26.5 26.5 26.5 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
g/a 22.0 23.0 23.0 24.0 25.0 26.0 26.0 25.0 24.0 23.5 22.5 * * * 29.0 29.0 29.0 29.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 27.5 27.5 27.5 27.0 29.0 29.0 29.0 29.0 29.0 * *
HT 20 20.0 20.0 20.0 21.0 22.0 22.0 22.0 22.0 22.0 22.0 22.0 * * * 28.5 28.5 28.5 28.5 27.5 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 28.0 27.5 27.5 27.5 27.0 28.0 28.0 28.5 28.5 28.5 * *
HT 40 19.0 19.0 19.0 19.0 21.0 22.0 22.0 22.0 21.0 20.0 19.0 * * * 27.0 27.0 27.0 27.0 24.0 24.0 24.0 24.0 24.5 24.5 24.5 24.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 27.0 29.0 29.0 29.0 29.0 29.0 * *
VHT 80 * * * * * * * * * * * * * * 25.0 25.0 25.0 25.0 21.5 21.5 21.5 21.5 21.5 21.5 21.5 21.5 21.5 21.5 21.5 21.5 21.0 21.0 21.0 27.0 28.5 28.5 28.5 28.5 28.5 * *
(7005-1) #

It's an ugly table formatted on this page, but cut and paste to a text editor and it looks much better, or check out the screenshot attached. In this case of my 305, you can see the max EIRP allowed by FCC is 25dB on Ch1-3, and 26dB on Ch4, and 26.5 on Ch5-11, and on the 5Ghz it's between 27.5dB up to 29dB depending on the channel.

So can a AP-304 with 5.8dBi antennas get higher than 23.8? Sure.

18dBm per chain + 4.5dB TxBF/MIMO + 5.8dBi ant gain = 28.3dB maximum. 

The controller (THANKFULLY) takes all this in to account automatically, so you don't have to be aware of this. If you set 20dB max EIRP, the controller adjust the radio transmit power accordingly ,and automatically takes in to account the TxBF/MIMO benefit and the antenna gain.

Hi jhowerd,

Thank you for your time explaining. It was interesting to read and helps me to better understand the power settings.

I am interested in Eric Johnson's explanation about lowering the AP power for whether or not clients cannot reach back to the access point. There are a lot of discussions about it and I would like to know his point of view on this topic..

You can read Keith Parson's blog post about Eric's explanation of AP and Client Tx power balancing here - https://blogs.arubanetworks.com/solutions/openness-is-critical-for-finding-and-remediating-targeted-attacks-2/