Modify Txpower

G.W. Haywood ged
Thu Apr 28 00:38:03 PDT 2005

Hi there,

On Thu, 28 Apr 2005 Jar wrote:

> > I need to reduce the transmission power for simulate the
> > handover, but when I try to do it i receive these message:
> >
> > Error for wireless request "Set Txpower" (8B26):
> >   SET failed on device wlan0; Operation not supported.
> currently commmented out.

Not all cards support setting the transmit power.  Some seem to have
firmware faults which prevent it even though they were designed to
support it.  Looking at the HostAP changelog, it seems that TX power
control was removed in 0.0.4 for good reasons.  You might be able to
patch HostAP to enable the feature, see for example:


> But instead you can try to use iwpriv to do the same:
> To e.g. max. power:
> # Turn off alc.
> /sbin/iwpriv wlan0 alc 0
> # Set TX power (0...255) min=127, max=128
> /sbin/iwpriv wlan0 writemif 62 128

There may be reliability issues if the transmit power is increased
beyond that normally used by the card manufacturers.  I have heard of
devices being destroyed within a few days when set manually for
maximum output power.  Apparently there are hardware differences in
the RF output circuits for cards intended for use in areas where the
local regulations permit higher powers - instead of using the output
direct from the chipset, power transistors boost the RF output to the
antenna.  Note that to increase range there's probably no point in
increasing the output power in one device only, since communications
are bi-directional.

I know of no reason to avoid reducing the output power.  If they are
controlling transmit power automatically, I do not know what the cards
do with it, so I do not know what might constitute a reduction.

I should be very pleased to hear from anyone who can shed more light
on this topic.


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