Fresh from my success with my Thinkpad T420, I tackled my Dell Latitude E6410, which had its own interesting quirks. So, here are some basic wireless troubleshooting steps for Ubuntu.
Step 1: Make sure the hardware switch is not set to off.
Step 1a: Is there another “hardware” switch? My HP2501p had an extra firmware switch for the wireless, accessible only from Windows. (Good thing I hadn’t deleted that partition…)
Step 2: Check the permissions: System -> Administration -> Users and Groups -> Advanced Settings; make sure “allow to connect to ethernet and wireless networks” is checked.
Step 3: Use lpsci to make sure your machine can see your card. Should look something like this (output filtered for brevity):
02:00.0 Network controller : Broadcom Corporation BCM43224 802.11a/b/g/n [14e4:4353] (rev 01)
Step 4: Check the drivers: System -> Administration -> Additional Drivers. You should see a driver appropriate for your card there, e.g. I have the Broadcom STA Wireless Driver. It should show green and say “activated”. If not, click the “Activate” button. (I needed to reboot the Dell in order to get this change to take.)
Step 5: Find your ethernet interface:
lo no wireless extensions.
eth0 no wireless extensions.
eth1 IEEE 802.11 Access Point: Not-Associated
Link Quality:5 Signal level:0 Noise level:163
Rx invalid nwid:0 invalid crypt:0 invalid misc:0
…and enable power*:
sudo iwconfig eth1 txpower on
* This page: http://www.cs.bham.ac.uk/~axs/laptop/#wireless helped me figure this out.