Hi,

when I do
/etc/init.d/network restart
on my Buffalo Airstation with openwrt 29614 and a connected device I get the following:

root@router:/etc/config# /etc/init.d/network restart
ifconfig: SIOCGIFFLAGS: No such device
ifconfig: SIOCSIFADDR: No such device
command failed: No such device (-19)
Configuration file: /var/run/hostapd-phy0.conf
20/40 MHz operation not permitted on channel pri=10 sec=6 based on overlapping BSSes
Using interface wlan0 with hwaddr 4c:e6:xx:xx:xx:xx and ssid 'net1'
Using interface wlan0-1 with hwaddr 4e:e6:xx:xx:xx:xx and ssid 'net2'
Configuration file: /var/run/hostapd-phy1.conf
Using interface wlan1 with hwaddr 4c:e6:xx:xx:xx:xx and ssid 'net1'
Using interface wlan1-1 with hwaddr 4e:e6:xx:xx:xx:xx and ssid 'net2'

Can anyone explain me what does
"20/40 MHz operation not permitted on channel pri=10 sec=6 based on overlapping BSSes" mean? Do I or do I not have 40MHz activated? If it is not activated, how can I override this and activate it? My location is in a city and a scan shows >30 2.4GHz networks in my neighbourhood.

And what does the first ifconfig-errormessage mean?

If there are over 30 other networks it isn't allowed to use 40mhz on 2.4ghz. If you really need 40mhz get a 5ghz wifi.

If your router is mac80211 type,
add "option noscan 1" to your /etc/config/wireless
it will skip overlap scan test and force dual band 40Mhz.

It would probably violate regulatory and your local law, and also interfere your neighbor.
But who care.

Thank you!

That's the point. No one of the neighbours interests the channel-problem, so I don't care, too.

Another problem I've seen:

root@router:~# iwconfig
lo        no wireless extensions.
eth0      no wireless extensions.
eth1      no wireless extensions.
br-lan    no wireless extensions.
wlan0     IEEE 802.11bgn  Mode:Master  Frequency:2.457 GHz  Tx-Power=15 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:on

mon.wlan0  IEEE 802.11bgn  Mode:Monitor  Frequency:2.457 GHz  Tx-Power=15 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:on

wlan0-1   IEEE 802.11bgn  Mode:Master  Tx-Power=15 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:on

br-guest  no wireless extensions.
pppoe-wan  no wireless extensions.
wlan1     IEEE 802.11an  Mode:Master  Frequency:5.24 GHz  Tx-Power=17 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:on
          
mon.wlan1  IEEE 802.11an  Mode:Monitor  Frequency:5.24 GHz  Tx-Power=17 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:on
          
wlan1-1   IEEE 802.11an  Mode:Master  Tx-Power=17 dBm   
          RTS thr:off   Fragment thr:off
          Power Management:on

I've set txpower to "20", but as you can see it is only 15dBm and 17dBm. Why is the setting ignored?

Transmit power level limits are complicated, but I believe that maximum power in 5.2 GHz range (channels 36 to 48 IIRC) is 23 dBm, including antenna gain. So if your wireless card assumes a 6 dBi antenna (pretty typical) then the transmit power limit is 17 dBm. Likewise the limit for the 2.4 GHz band is 20 dBm, so if your card assumes 5 dBi antenna gain, then the Tx limit is 15 dBm.
I hope someone will correct me if I'm wrong.

The other comments regarding your 40 MHz attempt are spot on.

15 and 17 dBm are the hardware limits for your Buffalo router. It probably has a power amplifier which the driver does not know about, which makes the real transmit power higher.

Using 2.4 GHz and 40 MHz in an environment like that is pretty idiotic. You don't get any improvement in the transfer speeds due to interference, and the transmit power is spread into a twice as wide a bandwidth, giving you 3 dB lower SNR.

So what's your advice to get more than the possible maximum on 20MHz? Normal distance from server to client is about 5-7m.

Its hard to exceed the possible maximum

snk is right, with 30+ 2.4Ghz networks in your area, the only sure solution to get more bandwidth is to move to 5 GHz.

You could try directional antennas. If you are handy, you can build a few 'biquad' ones fairly easily. Use on both ends for maximum effectiveness, but it can still help if used only on the base station.

Another thing you can try is to switch to horizontal polarization instead of vertical. This literally means folding your (typical dipole) antennas so that they are horizontal rather than vertical. You must do this on both ends for maximum effectiveness since the polarization must match for good signal transfer. Make sure the antennas do not point at the other end; the other end should "see" the side of the antenna, not the tip. If you are using a laptop on the other end, it is probably using an internal patch antenna and you can't do anything to change the polarization. But you can still try this out; it may reduce interference from other base stations (since almost everyone uses vertical antennas). This may not be effective since every time a signal bounces off of a surface, its polarization can change, depending on the angle of the surface.