ipw3945 wakes up too much

According to this page:
http://www.lesswatts.org/projects/powertop/known.php

ipw3945 and ipw2200 power save mode

(this tip is contributed by Steinar H. Gunderson)
There is currently no userspace support for enabling the power saving functionality of ipw3945. The driver doesn't support the standard "iwconfig ethX power" interface, but you can turn down the power usage with:

iwpriv eth2 set_power 1

which seems to save almost 100 mA in idle state over the default setting. The value has the following meaning

Power save level: 5 (Timeout 25ms, Period 1000ms)
Power save level: 1 (Timeout 350ms, Period 400ms)

A setting of 5 will save you more power (at the expense of performance).

In many networks, you'll see about 10 wakeups per second due to the wireless router's Beacon Internal of 100ms. Many AP's allow customization of that interval to 1000ms; this will bring the number of ipw interrupts down to 1/s.

A beacon is a packet broadcast by the Router to synchronize the wireless network. We did not observe any performance degradation because of this change.

Thanks for that, I seem to have enabled some kind of power saving mode anyway as I now get ~25 wakeups per second.