carl9170 driver - reducing the output power

Hi all,

I'm engaged in a research project that requires us to 
reduce the range of an ad-hoc wifi network. However, when 
I change the txpower (either using iwconfig, or iw 
command), I can only see the difference when I display the 
current configuration (e.g., with iwconfig). I cannot see 
any difference in the actual tx power observed on a 
spectrum analyzer.

Thus, my question is: is the function of changing the 
txpower really supported? If it is, what steps should I 
take to make it work? I tried with ifconfig up/down when I 
change the txpower, but it does not change anything. Maybe 
I do something wrong? Do I use the correct versions of the 
firmware and the driver?

I've noticed a patch related to this issue, published on 
2012-01-27 by Christian Lamparter. It seems that it is 
included in my driver version, but maybe I'm wrong and I 
should apply some additional patch?

We use NetGear WNDA3100(v1) hardware [Atheros 
AR9001U-(2)NG] and carl9170 driver 
(driverversion=3.0.0-21-generic firmware=1.9.2).

Best regards,
Michal Kudelski

On Monday, June 25, 2012 12:38:50 PM Michal Kudelski wrote:

Thus, my question is: is the function of changing the 
txpower really supported? If it is, what steps should I 
take to make it work? I tried with ifconfig up/down when I 
change the txpower, but it does not change anything. Maybe 
I do something wrong? Do I use the correct versions of the 
firmware and the driver?

No special firmware or driver is required. Just as long
as the driver supports the feature.  

I've noticed a patch related to this issue, published on 
2012-01-27 by Christian Lamparter. It seems that it is 
included in my driver version, but maybe I'm wrong and I 
should apply some additional patch?

We use NetGear WNDA3100(v1) hardware [Atheros 
AR9001U-(2)NG] and carl9170 driver 
(driverversion=3.0.0-21-generic firmware=1.9.2).

That's probably too old. The patch you are talking about
is not a -stable patch so it's not backported to 3.3.x
(or older) kernels.

So, either you can upgrade to 3.4, use compat-wireless
<http://linuxwireless.org/en/users/Download/#Getting_compat-wireless_on_Ubuntu>
or backport the patch yourself (however, this could be diffcult).

Regards,
	Christian

Dear Christian,

Thank you for the response! Indeed, when I used the latest 
compat-wireless, I was able to control the output power.

Now, another question arises: what is the lowest supported 
power level? Do you have any hardware specifications of 
AR9170 that would say what is the lowest power level 
supported by the hardware?

With iw/iwconfig commands, the lowest achievable power is 
1 dbm. But I'm thinking about modifying your code in order 
to write fixed values to the corresponding registers. And 
I need to know what to write into the registers in order 
to obtain the lowest possible power :-) . I know that 
there exist wifi cards that accept -12 dbm (e.g., Intel 
Pro-Wireless 2200 with ipw2200 driver under linux), so it 
would be really nice to know what I can get with 
Atheros...

Cheers,
Michal



On Mon, 25 Jun 2012 19:33:10 +0200
  Christian Lamparter [off-list ref] wrote:

On Monday, June 25, 2012 12:38:50 PM Michal Kudelski 
wrote:

quoted

Thus, my question is: is the function of changing the 
txpower really supported? If it is, what steps should I 
take to make it work? I tried with ifconfig up/down when 
I 
change the txpower, but it does not change anything. 
Maybe 
I do something wrong? Do I use the correct versions of 
the 
firmware and the driver?

No special firmware or driver is required. Just as long
as the driver supports the feature.  

quoted

I've noticed a patch related to this issue, published on 
2012-01-27 by Christian Lamparter. It seems that it is 
included in my driver version, but maybe I'm wrong and I 
should apply some additional patch?

We use NetGear WNDA3100(v1) hardware [Atheros 
AR9001U-(2)NG] and carl9170 driver 
(driverversion=3.0.0-21-generic firmware=1.9.2).

That's probably too old. The patch you are talking about
is not a -stable patch so it's not backported to 3.3.x
(or older) kernels.

So, either you can upgrade to 3.4, use compat-wireless
<http://linuxwireless.org/en/users/Download/#Getting_compat-wireless_on_Ubuntu>
or backport the patch yourself (however, this could be 
diffcult).

Regards,
	Christian

On Tuesday, June 26, 2012 02:10:25 PM Michal Kudelski wrote:

Thank you for the response! Indeed, when I used the latest 
compat-wireless, I was able to control the output power.

I'm glad I could help.
 

Now, another question arises: what is the lowest supported 
power level? Do you have any hardware specifications of 
AR9170 that would say what is the lowest power level 
supported by the hardware?

No, I'm sorry, but I don't have such a thing (Maybe Adrian
knows more). From what I knot, the lowest possible value
for the "tpc" registers and the tx descriptor tpc is "0"
(there's no "sign" bit so I'm afraid anything lower than
0 is not possible).
 

With iw/iwconfig commands, the lowest achievable power is 
1 dbm. But I'm thinking about modifying your code in order 
to write fixed values to the corresponding registers. And 
I need to know what to write into the registers in order 
to obtain the lowest possible power :-) .

The power is set by:
carl9170_tx_rate_tpc_chains in tx.c (for most outgoing frame -
except control frames like ACKs/ERP/... which are
controlled by carl9170_set_mac_tpc in mac.c)

Regards,
	Christian