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Tux Machines (TM)-specific
If network manager does not solve the problem, the first step should be to see which driver your wireless card needs. Do a search for your card on Google and in the Ubuntu Forums to find out which driver you need. Many of the drivers are already included in Ubuntu, but some newer drivers may not be present.
Next, you need to find out if the driver is loaded. As an example, if you have an Intel Centrino and it uses the ipw2200 driver, run this command:
sudo lsmod | grep ipw2200
Replace ipw2200 with the relevant driver for your card. If you get some lines returned, the driver is loaded and working. If nothing is returned, your card is either not supported or the driver is not included in Ubuntu. You should refer to the Ubuntu Forums for further support.
With the card identified, you now need to get connected.