Dealing with wireless links

Never be complacent about wireless. With 802.11, simply moving the antenna by a few centimeters can dramatically change performance, as can rotating the antenna by a few degrees. In one case, we found that the same Soekris box had a factor of 100 difference in performance depending on orientation! When you do not get good wireless performance, first double check the pig tails and make sure the ends are not touching on the wireless card. In addition, to debug wireless, your best bet is to use a combination of two commands: iwconfig and iperf.

iwconfig

The iwconfig command with no arguments will tell you

whether a node is associated with another node
the signal strength
the link quality

If you find that the signal strength is too low, or the link quality is poor, adjust the antenna configuration

iperf

iperf will allow you to test the amount of data that can be transferred on a wireless link.

Make one end the server and run iperf -s
Make the other end the client and run iperf -c <server ip>
After a few seconds, hit Control-C
iperf will print out its summary statistics

You should be aiming for a throughput of at least 5Mb/s on a wireless link. If necessary, you can manually set the link data rate (lower, for more noisy links) by using the following command:

iwconfig rate 18M
- this sets the rate to 18Mbps
- on a noisy link, you may need to use a rate as low as 6Mbps.

Some useful commands

The following assume that the wireless card is called ath0. Use the iwconfig command to find out which interface is actually active (it can be ath1, ath2, etc.)

Switch a wireless card to master mode/access point