[maemo-users] Power consumption and WLAN APs

From: Andrew Daviel advax at triumf.ca
Date: Sat Jul 26 21:51:24 EEST 2008
On Thu, 24 Jul 2008, Kalle Valo wrote:

(packets from N810 captured with airodump-ng show as malformed)

> Usually missing FCS causes this. There's a setting in Wireshark under
> IEEE 802.11 to enable and disable FCS calculation. Try reverting that
> option.

Yes. Edit/Preferences/Protocols/IEEE 802.11. Also lets me set WEP keys. 
Not sure I have that working right, but I turned off WEP/WPA to debug, 

I was able to get the IBM laptop into monitor mode using the madwifi
instructions (thanks) - airodump did not do it as it does on the tablet.
# wlanconfig ath0 destroy
# wlanconfig ath0 create wlandev wifi0 wlanmode monitor
# ifconfig ath0 up
# echo '801' > /proc/sys/net/ath0/dev_type

then I could use tcpdump or ethereal/wireshark. If I don't suppress 
prism headers then I can't refilter with "tcpdump wlan host <node>"
which makes it hard to sift through the traffic at work.

I have placed some capture data on my server at 
(password test/test to keep out robots and the clueless)
I may be being dumb, but I can't see a problem. I can see the tablet 
doing a power-save poll and saying it is going to sleep after the 
transactions, and the more-data bit appears cleared. I can see the DTIM 
bits being set in the beacon before the data transactions and cleared 

There is a lot more traffic at work (duh), including lots of ip broadcast 
traffic on a VLAN connected to our wired LAN. But that's on a different 
SSID. We recently created a separate VLAN & SSID with encryption on, 
which is what I normally use on the tablet, but here I used our visitor 
SSID. These don't see the ip broadcasts from the wired LAN, but in any 
case as I understand it these should not make a substantial difference in PS mode.

A battery graph is at

I may be able to get a spare Proxim AP to take home. There's a limit on 
what I can mess with on our production WLAN :-7, and I'd have a less
noisy environment.

I'm not sure what I can tell Proxim. Send them the capture files, I 
guess. Setting the AP to closed mode made no difference.

If I want to see ping broadcasts, I can do
  echo 0 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

I wondered whether setting /proc/sys/net/ipv4/tcp_frto helps, as per 
kernel ip-sysctl.txt ("particularly beneficial in wireless environments")
Not for the power, just for transmission errors.

I thought that unicast data was buffered till the next beacon in PS mode,
with broadcast/multicast buffered till the next DTIM interval, but at 
least on my Linksys router it looks like unicast is buffered till DTIM 
too. It lets me adjust both the beacon interval and DTIM; the Proxim 
allows only DTIM. Defaults are 100ms and 1x.
If the delay (beacon interval x DTIM) exceeds about 10 seconds, the 
tablet won't respond to incoming data. My desktop says "network not 
reachable". I thought that was a tunable parameter but the only one I 
found was a SYN timeout at 60x.
If the tablet is in power-save mode, doing "iwconfig wlan0 power off"
will momentarily revert it to non-PS, and incoming data is received.
Outgoing data is unaffected.
Doing a broadcast ping was interesting with DTIM > the ping interval;
wired stations respond every second but the wireless ones are bunched up
in a group. As expected; it just looks weird.

Andrew Daviel, TRIUMF, Canada

More information about the maemo-users mailing list