carl9170 driver - reducing the output power

carl9170 driver - reducing the output power

[Michal Kudelski]

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

Re: carl9170 driver - reducing the output power

[Christian Lamparter]

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

Re: carl9170 driver - reducing the output power

[Michal Kudelski]

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 <chunkeey@googlemail.com> wrote:
> 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

Re: carl9170 driver - reducing the output power

[Christian Lamparter]

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