Setting power levels...

John Dowd jdowdster
Wed Sep 2 08:00:26 PDT 2009

My demo system has the following h/w and s/w:
hostapd version 0.6.6 running on SuSE 11.0 with kernel version 2.6.28-rc5-wl-
default. I needed to grab the kernel from git in order to use the wireless USB 
dongle for a rt2x00 chipset. The AP is configured to use WPA2 security.

There is only one client who is going to connect to the AP. It is a Openmoko 
Freerunner phone which uses the Atheros 6000 chipset for it's wireless access.

This setup has been shown to work. The most solid part of the system is the 
wireless encryption handshake. The Openmoko phone finds the AP and the 
subsequent handshake from wpa_supplicant to the AP works fine and it can be 
seen refreshing the connection over some interval. No errors have been logged 
or detected.

The next item is the DHCP server which is running behind the AP (on the same 
machine). When the client issues its DCHPDISCOVER, the message is seen on 
servers log and the sending of its DHCPOFFER is also seen. When the everything 
is working, the next item in the log is the DHCPACK and the client device has 
an IP address and there is much joy. BUT, usually there is no joy for quite 
some time. Although the DCHPOFFER message is seen the next message that is 
usually seen is the DHCPDISCOVER message and the client did not see the offer. 
This can go on for quite some time (hours). And then for no discernible reason 
the client issues the DHCPACK message.

When this was brought up to the Openmoko people they asked for the power 
levels on the client device using the iwconfig command. This is the output:

eth0 AR6000 802.11g ESSID:"Asterisk"
Mode:Managed Frequency:2.412 GHz Access Point: 00:22:B0:5E:50:FF
Bit Rate=54 Mb/s Tx-Power=15 dBm Sensitivity=0/3
Encryption key:off
Power Management:off
Link Quality:204/94 Signal level:-147 dBm Noise level:-97 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:3

Their comment was that the Signal level of -147 dBm was way too low and should 
really be -90dBm. Here is the iwconfig output on the AP side:

wlan0 IEEE 802.11bg Mode:Master Frequency:2.412 GHz Tx-Power=22 dBm
Retry min limit:7 RTS thr=2347 B Fragment thr=2346 B
Power Management:off
Link Quality:0 Signal level:0 Noise level:0
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0

The client device is currently within about 1 meter of the AP.

Does their comment make sense and is there a way of adjusting the power levels 
on the AP side?

"To Err is human, to forgive is NOT company policy."

John Dowd
S/W Contractor
email: jdowd at gmail dot com
Cell: (613)316-7884
Home: (613)234-7884
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