Raspberry Pi 3: wrong tx-power in iwconfig

Question

On my Raspberry Pi 3 running Raspbian iwconifg produces the following output:

wlan0     IEEE 802.11bgn  ESSID:"ssid of my network"  
          Mode:Managed  Frequency:2.452 GHz  Access Point:*******   
          Bit Rate=72 Mb/s   Tx-Power=1496 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=39/70  Signal level=-71 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

lo        no wireless extensions.

eth0      no wireless extensions.

According to the definition of dBm (see https://en.wikipedia.org/wiki/DBm) such a high tx power is not possible due to energy conservation :-).

Thus I have the following questions:
* What do I have to do, to see the right tx-power?
* Is it a bug of iwconfig or the broadcom kernel driver?

Additional Information:
The Raspberry Pi uses for wlan the BCM43438 (see www.raspberrypi.org/magpi-issues/MagPi43.pdf, page 10 of pdf). I see exactly the same tx power on a Raspberry Pi 1 running ArchLinux ARM with the official Raspberry Pi Wifi Dongle (www.raspberrypi.org/products/usb-wifi-dongle/), which has the BCM43143 chipset. Also other users with the same hardware seem to have this problem: Does BCM43438 Wireless chip in Raspberry Pi 3 support ad-hoc networking?

Answer 1

As explained in the iw documentation iwconfig is deprecated and iw should be the first choice for wifi diagnostics, if the hardware is not very old. iw phy lists for all connected wifi adapters the enabled frequencies and related maximum tx-powers. The output of this tool is realistic :-). The output of iw phy depends not only on your wifi hardware but also on the set regulatory domain. In order to get the regulatory domain, you can use iw reg get.

Answer 2

The same issue occurs on my RPI 3 too. It's not clear whether the value od 1496 reported by iwconfig is in mW, or in dBm (1496 mw = 31.75 dBm). According to the only specs of the BCM43143 wireless chip I could find, the maximum transmission power for this chip is 15 dBm:

http://www.broadcom.com/blog/raspberry-pi/want-wi-fi-for-your-raspberry-pi-broadcom-adapter-brings-connectivity-with-less-power/

This applies to the Wi-Fi USB dongle mentioned there, I'm not sure if it applies also to the builtin BCM43143 on RPI3 (but I suppose the max transmission power should be in the similar range).

This given, it seems odd that the txpower is set to 31.75 dBm (which is the double of the chip's maximum).

After further investigation, it seems like the value of 1496 is some kind of default or uninitialized variable value inside the driver which is set when you don't set the value yourself. I drew this conclusion form a similar issue with another wireless chip:

http://www.snbforums.com/threads/power-at-1496-mw-on-powerup-reboot-ac66u-asuswrt-merlin-rt-ac66u_3-0-0-4.13568/

Also, after playing a little bit with various txpower values I came to the conclusion that it can be set to any possible value, but that doesn't affect much the quality of the link:

# iwconfig wlan0 txpower 1000
# iwconfig wlan0
wlan0     IEEE 802.11bgn  ESSID:"******"
          Mode:Managed  Frequency:2.417 GHz  Access Point: *********
          Bit Rate=54 Mb/s   Tx-Power=1000 dBm
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Encryption key:off
          Power Management:on
          Link Quality=51/70  Signal level=-59 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

# iwconfig wlan0 txpower 40
# iwconfig
wlan0     IEEE 802.11bgn  ESSID:"******"
          Mode:Managed  Frequency:2.417 GHz  Access Point: *********
          Bit Rate=54 Mb/s   Tx-Power=40 dBm
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Encryption key:off
          Power Management:on
          Link Quality=52/70  Signal level=-58 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

I also tried pinging IP of my wireless router with both values (40 and 1000) and the results were the same.

This was just a five minutes quick test, so further investigation is needed to find out whether and how the txpower affects the output power and link quality on RPI 3.