Where can I find information about the txpower and txpowmax values from ifconfig?

[Ryan J]

I've been having trouble figuring out what the txpower and txmaxpow
values shown by ifconfig actually represent.  I was asking about them
on the pfSense forums previously, but I think this list may be more
appropriate now.

http://forum.pfsense.org/index.php/topic,48814.0.html

I have an Atheros (AR2413) card that should max out at 26dBm.  Running
"ifconfig -v ath0_wlan0" shows "txpowmax 50.0" and "txpower 30" using
"regdomain FCC country CA indoor".  I'm confused about what those
values actually mean.  I know they're input in increments of .5dBm and
stored (internally) as integers, but the values I have don't make
sense.

Having a txpowmax of 25dBm (50.0 * .5) would make sense for my card
since it has tolerances of +1dBm / -1.5dBm, but I don't understand why
txmaxpow would be shown in increments of 2xdBm when txpower appears to
be using 1xdBm.  Also, why would my card be allowed above 26dBm?

I've also seen mailing list posts where people have a value for
txpower that is greater than txpowmax.  For example:

http://lists.freebsd.org/pipermail/freebsd-drivers/2008-July/000742.html

In that post txpowmax is 18.0 and txpower is 31.5.  Since it doesn't
make sense to me, I'm guessing txmaxpow and txpower don't mean what I
think they do.

Would anyone be able to explain what kind of values I should be seeing?

Ryan