Intel Outside (of my own office)
One thing I've never had too much of a problem with on my Ubuntu 12.04 LTS install is networking. There's been the odd time when it's refused to connect to the network, despite everything else being fine. This has always been fixed with a reboot, though.
The problem I experienced today was a lot more serious. I'd travelled to another office to work and tried to connect to their wifi. I knew the network should have been OK as no-one else seemed to be having an issue, and I'd entered the WPA password correctly.
What I saw was that the wifi showed as connected, but I couldn't connect to anything. Occasionally, I could try and ping a server, and when I bothered to wait long enough to see anything, what I saw was connection timeouts, horrible packet loss, and ping times of several seconds. (Pings are normally measured in milliseconds; I was seeing results of thousands of milliseconds.)
I'd never seen this problem before. It quickly became apparent that this was specific to my hardware and my Ubuntu install. When I booted into Windows, it connected immediately without any problem, and a colleague was using a later version of Ubuntu on different hardware without a problem at all.
It's certainly difficult to try to fix such a vague problem. Running searches like "wifi not connecting on Ubuntu" inevitably brings up much noise relating to every wireless problem out there and not anything much of relevance, but I didn't have a lot more to go on. Having to do lots of searching and skimming through web pages using a phone to search isn't ideal either, but it did eventually lead me to the right answer.
Fun with iwlwifi
What worked was searching for issues relating to the wifi driver
(iwlwifi). A common theme that came up was 11n_disable=1 which is a
modprobe option that disables the wireless N feature of the Intel
wireless adapter. (lshw -C network states the model is "Centrino
Wireless-N 2230".)
By running the commands
sudo rmmod iwldvm
sudo rmmod iwlwifi
sudo modprobe iwlwifi 11n_disable=1
sudo modprobe iwldvm
I got normal wireless access back (minus wireless N speeds, but the choice between fast, but no wifi and slow wifi is not a difficult one to make).
The rmmod commands remove the modules from the kernel, while
modprobe adds them. We need to readd the wifi module with wireless N
disabled. (The offending module is iwlwifi but I couldn't rmmod it
until I'd removed iwldvm as trying to do so showed that iwldvm was
using iwlwifi. There is a force option for rmmod but it didn't seem
sensible to use it when there was an alternative.)
Making this fix stick
If you want this fix to apply every time you boot, you can just create a
new .conf file:
/etc/modprobe.d/wireless-n-fix-iwlwifi.conf, adding just this line to
the file:
options iwlwifi 11n_disable=1
(Thanks to this nice summary.)
You can choose any valid filename you like, provided it ends in .conf;
you'll also need to edit this using sudo and your favourite text
editor.
Why hadn't I seen this problem before?
What seems to be the issue is that I think that the office was using a wireless N router, and there's some kind of issue with this for the Intel Centrino wireless network adapter on my laptop under Ubuntu.
Nowhere else I've been with this laptop seems to be using wireless N, so that explains why it was an issue for the first time today. It's pretty poor that this seems to have been a long-running problem and yet it's still present… but at least there's a workaround.