Wireless Access

I am hoping the community can help with some "pre-sales" information that is hard to come by. 

Basically I hoping someone out there with an AP-114 can confirm the lowest possible transmit power of the 2.4GHz radio on this model - i.e. the EIRP as shown on 'show ap bss-table'. Note it will likely not be the same as an AP-115.

I have a specialty application that requires very low power settings. To date we have been using AP-104s. Unfortunately after delivery we discovered the lowest EIRP is 10.5dBm. Even though you can set it lower in software, this is apparently the lowest supported by the chipset... as shown on 'show ap bss' (the value shown in the bss table does not appear to take into account passive gain from the antenna, as changing the provisioning settings do not change the EIRP displayed). By contrast all the integrated antenna APs I've tested go as low as 2.5 or 4. 

If anyone can assist would be much appreciated.

BGC IT,

You should be able to get the actual EIRP by looking at the driver log of the AP:

(MyHost) #show ap debug driver-log ap-name Office-225 | include EIRP
62          2.924745 wifi0: EIRP (Config: 18.0dBm, Set: 18.0dBm, AP Max: 22.50dBm, Reg Max: 33.0dBm), Conducted Power Set: 13.50dBm, Gain: 4.50
67          3.990335 wifi1: EIRP (Config: 18.0dBm, Set: 18.0dBm, AP Max: 21.50dBm, Reg Max: 32.0dBm), Conducted Power Set: 14.50dBm, Gain: 3.50

 The Gain  (antenna gain) plus the Conducted power should total the EIRP.

EDIT:  The ARM TX power of this AP was 18 on both bands.  The internal antenna on the 225 has gains of 3.5 and 4.5 respectively.  That means the conducted power was lowered to 14.5 and 13.5 so that the total EIRP is 18 on each band.

Thanks, I don't have one of these APs, that's why I'm asking the community.

The controller/AP combination is designed so that the AP/Antenna combination totals the EIRP that is set VIA ARM or other means.  Whatever the antenna gain that is input, power is added or subtracted to make it whatever the ARM TX power is.  I sent you the command above so that you can confirm yourself with the APs you mentioned could not achieve the power you wanted....  I do not have an AP114  right now to check.

It seems that the minimum power would be the gain of the connected antennas.  I just set the TX power on the AP225 to 3 and this is what I got:

(MyHost) #show ap debug driver-log ap-name Office-225 | include EIRP
[38623.841039] Mon Apr  7 02:06:46 2014:213     38527.325021 wifi0: EIRP (Config: 3.0dBm, Set: 4.50dBm, AP Max: 22.50dBm, Reg Max: 33.0dBm), Conducted Power Set: 0.0dBm, Gain: 4.50
213     38527.325021 wifi0: EIRP (Config: 3.0dBm, Set: 4.50dBm, AP Max: 22.50dBm, Reg Max: 33.0dBm), Conducted Power Set: 0.0dBm, Gain: 4.50

 The internal antennas on the AP225 is 3.5 and 4.5dbm.  When I set the EIRP to under that, it set the Config to the power of the antenna.  So it looks like the minimum power for the AP114 would be the gain of the antenna you have connected to it.

There are a couple of restrictions when trying to minimize the RF transmit power:

- The lowest level that can be set in the UI is zero dBm

- The lowest level that the software can control is zero + antenna gain (for some AP platforms adjusted by an internal cabling loss factor)

- The lowest antenna gain that can be provisioned is zero dBi

With all that, the minimum you'll be able to configure is the gain of the attached antenna (sometimes a bit lower if there's an internal loss factor).

For the AP-104, if you configure 0dBm, the actual (reported) transmit power level should equal the provisioned antenna gain minus 0.5dB. I am not sure why you see 10.5dBm, unless if you provisioned the platform with 11dBi gain. I'll verify this when I have access to and AP-104 (tomorrow).

For the AP-114 you should see the provisioned gain adjusted by 1.5dB (2.4GHz) or 3dB (5GHz).

/Onno Harms, Aruba PLM

Hi Onno,

I double-checked and that's exactly right, all the APs were provisioned with 11dBi which is why see no variation. Some of the APs should have been provisioned with 7dBi gain as they have a different antenna, but were not.

So I think this now makes full sense..

• There's no minimum power restriction on the radios themselves
• Displayed EIRP is already adjusted for antenna gain as per the provisioned value
• To get a value lower than [antenna gain less internal loss] I guess a good option is to simply use attenuators on the antenna cable

Thanks very much for the explanation.

Exactly. The only way to reduce power further (i.e. below [antenna gain minus internal loss]) is by adding external attenuators.

Note that in that case there may be a mismatch between the power (set and) reported in the UI and the actual transmit EIRP power.

/Onno

@oharms wrote:

 

Note that in that case there may be a mismatch between the power (set and) reported in the UI and the actual transmit EIRP power.

 

 

Can we just lower the provisioned antenna gain by the value of the attenuator to correct this?

Would there be any unwanted side effects to doing this?

Yes you can.

The typical approach would be that the number that is provisioned as antenna gain includes any (cabling/connector/attenuator) losses between the AP connector and antenna plug. The actual antenna gain should be offset by any losses that are introduced.

However, this approach only works if the resulting number is positive.

/Onno