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Cisco 7925G wifi phones only showing 4 dBm transmit power

Greetings,

A customer of mine has deployed these 7925 phones in many stores around the country.  In the locations where we can provide 802.11a service, the phones work great and show a ~12 dBm transmit power.  In the stores where we can only provide 802.11g service, the phones suffer from weaker performance.  We have worked with the engineers from Aruba to resolve everything we can from a wireless side, but the phones still only show the low transmit power.  Is that simply a display bug, or is there a way to manually set the transmit power statically or possibly even a static range (maybe 2-14 dBm)?

What are the determining factors that the phone uses to arrive at a transmit power?  These phones never change, regardless of distance to AP or SNR/RSSI.  Always just 4 dBm.

5 REPLIES
Cisco Employee

Cisco 7925G wifi phones only showing 4 dBm transmit power

Hi William.

Sounds like you are encountering the following issue, which is specific to the max tx power advertised by Aruba APs for UNII-3 channels (149-161).

CSCtf34839      792x phone uses lower tx power when 802.11d max power is 36 dBm

Workaround is to disable 802.11d (world mode) or not use UNII-3 channels if using Aruba APs.

Other 5 GHz channels when using Aruba APs do not have this issue as you mentioned as they advertise a max tx power lower than 30 dBm.

Below is what you should see in the 792x site survye mode for

Channels 36 40 44 48 52 56 60 64 149 153 157 161 165 
Power 23 23 23 23 29 29 29 29 4 4 4 4 4

As far as how the 792x makes the decision of what tx power to use, with Cisco APs the 792x phones utilize DTPC from CCX.  The 792x phones can also utilize TPC (power constraint) from 802.11h to limit the tx power.

With Aruba APs and without power constraint disabled, the 792x phone should be able to use a max tx power of 16 dBm for 5 GHz depending on the channel and data rate.

You may want to check this to ensure that power constraint is not enabled.

Also in the advanced section of the network profile in the 792x phone webpage, you can only specify the max tx power, but can not specify min tx power.

New Member

Cisco 7925G wifi phones only showing 4 dBm transmit power

These phones reporting a low TX power are connected to 802.11g only  radios.  The APs in this area are not capable of 802.11a.  I did change  the Advertised Regulatory Max EIRP tx rate from the Aruba controller  default of 0 to 15.  Their system does have 802.11D/H enabled due to  some connnectivity challenges they experienced with Zebra wireless  printers and Motorolla Symbol hand-held scanners prior to my involvement  with this support contract.

Cisco Employee

Re: Cisco 7925G wifi phones only showing 4 dBm transmit power

I checked my Aruba WLC and I can see the issue on 2.4 GHz if Advertise regulatory max EIRP inside the radio profile is set to 0.
If I set it to 20 then I can see the tx power in WLAN info screen shows as 11 dBm assuming the current RSSI is < -50 dBm.
I can also get 4 dBm if I set Advertise regulatory max EIRP to 36.
So ensure this is set between 17-31; never 0, 32 or greater.

However, a power constraint of 14 dBm via 802.11h TPC is being advertised by the Aruba APs and I can't figure out how to disable that / set it to 0 other than disabling Advertise 802.11d and 802.11h capabilities also in the radio profile.
I tried reconfiguring Transmit EIRP, TPC Power, etc. but there is no difference.
Do you know how to manage the 802.11h TPC value?

Fyi, I am using an AP105 connected to an Aruba 620 WLC running version 6.1.3.1.


Sent from Cisco Technical Support iPad App

New Member

Re: Cisco 7925G wifi phones only showing 4 dBm transmit power

In the Radio profile, there is a check box to enable 802.11h, and next to that are boxes for Transmit EIRP and TPC power.

Cisco Employee

Re: Cisco 7925G wifi phones only showing 4 dBm transmit power

Yes I tried to change the value for both Transmit EIRP and TPC Power in the radio profile already, but did not make any difference as far as 802.11h TPC value advertised as it always shows 14 dBm.

Maybe you can ping the Aruba engineers and ask how the 802.11h power constraint can be managed?

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