[maemo-users] Power consumption and WLAN APs

From: Andrew Daviel advax at triumf.ca
Date: Tue Aug 5 21:27:31 EEST 2008
I have done some more tests with the Proxim AP4000, but I still cannot 
see a problem in the protocol. I have put an ammeter inline between the 
N810 battery and connectors so I can see the current consumption in real 
time - it is pretty impressive that it can draw virtually no current at 
all (<10mA on a 1A scale) and still respond to incoming network traffic 
or a cron job.

I took an AP4000 home and modified the configuration to work without
Radius authentication. At work, we have multiple AP4000's with 3 SSID's 
each, using (mostly) non-overlapping channels 1,6,11. But there may still 
be some in-channel interference from our or foreign APs.

With the AP4000 broadcasting one SSID (which the N810 is associated 
with), power consumption is low in PS mode. An incoming packet, e.g. ICMP 
ping, gives a brief kick on the ammeter.
My Linksys AP broadcasting on the same channel makes no difference - 
there are two beacon frames, one for each SSID, but power consumed is 

With the AP4000 broadcasting 2 or 3 SSIDs on different VLANs, then power 
consumption rises to an average 100mA. If "broadcast unique beacon" is 
disabled for all SSIDs, then the power consumption is low. The AP 
broadcasts only one beacon, with no identifier. But if it is enabled for 
one or more SSIDs, the consumption can rise:

With beacon SSID enabled on "triumf" but disabled on "test" - <10mA
With beacon SSID enabled on "triumf" and enabled on "test" - 100mA
With beacon SSID disabled on "triumf" but enabled on "test" - 100mA
With beacon SSID disabled on "triumf" and disabled on "test" - <10mA
(The N810 is associated with "triumf")

Capture files on

I can see nothing in the capture file (madwifi libpcap -i ath0, no 
filters) from the tablet after it responds to the ping, but it is 
consuming power. I might be able 
to try a spectrum analyzer, but not today.

This is driving me crazy .. I'd give up if there weren't such dramatic 
power savings when it works properly.

Andrew Daviel, TRIUMF, Canada
Tel. +1 (604) 222-7376  (Pacific Time)

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