[CUWiN-Dev] Transmit power control / RSS measurement

[David Young]

On Thu, Apr 27, 2006 at 04:19:15PM +0900, Jeongkeun Lee wrote:
> > On Thu, Apr 27, 2006 at 03:16:55PM -0500, Paul Smith wrote:
> > > David Young wrote:
> > >
> > > >Transmit power is controlled by these sysctls,
> > > >
> > > >hw.ath0.tpscale = 0
> > > >hw.ath0.tpc = 0
> > > >hw.ath0.tpack = 63
> > > >hw.ath0.tpcts = 63
> > >
> > > So, just to ask the naive question, if you, say, turn on TPC, with:
> > >
> > > $ sysctl -w hw.ath0.tpc=1
> > >
> > > then you can turn the other knobs and have coarsed-grain (i.e., not
> > > at the packet-level) control over the power output of the device? And
> > > this currently works? What values should I feed to these sysctls if
> > > say I wanted to make my radio quieter in a lab space?
> > 
> > I'm not sure.  Documentation is lacking in NetBSD.  Maybe it is better
> > in FreeBSD, I haven't checked recently.  Anyway, I believe higher values
> > of tpscale translate to lower power.
> > 
> > Dave
> > 
> 
> In my CUwireless build based on the sourceforge source code distribution,
> the output of '$ sysctl hw.ath0' does not contain 'hw.ath0.tpack' and
> 'hw.ath0.tpcts'.
> 
> And the tpcscale option accepts only five integers 0 to 4 as a valid
> argument. The default is 0 and it is the highest. As I increased the
> tpcscale value one by one at the transmitter, the RSSI at the receiver
> (which was very close to the transmitter) showed 2~3 dB degradation.
> 
> However, the 'tx power' field of tcpdump did not show correlated changes.
> It was 60dBm with tpcscale 0, and changed to 54dBm, 48 dBm, 63dBm, and
> 47dBm.
> Another tcpdump test with a different radio card (but same CM9 model)
> resulted in a similar phenomenon. 

The tx power field in the radiotap header is copied from the ni_txpower
field of an ieee80211_node; in ad hoc mode, the ieee80211_node is the
802.11 layer's representation for one of its peers.  It looks to me
like ni_txpower is only set when an ieee80211_node is created, so the
information that the tx power field contains is probably stale.

Aha!  Here is a great discussion of the fields and their meaning:

http://www.mail-archive.com/madwifi-tickets@lists.sourceforge.net/msg00981.html

ath_update_txpow() should probably iterate over all of the ieee80211_nodes
and set their ni_txpower equal to ic->ic_bss->ni_txpower.

Dave

-- 
David Young             OJC Technologies
dyoung at ojctech.com      Urbana, IL * (217) 278-3933