I have one laptop running over night sometimes. That requires a stable wifi connection which I found out Ubuntu 12.04 with iwlwifi could not offer. Often I returned to the desktop in the morning and network-manager disconnected hours ago without establishing a new connection. So no down and uploads since then. Manually trying to reconnect doesn’t work either. One either has to switch off and on wifi or unload and reload the iwlwifi module.
I experimented with iwlwifi options 11n_disable and swcrypto as some people on the net suggested but no luck. Wifi kept disconnecting unpredictably. Then I stumbled upon one bug addressing this misbehavior to network-manager suggesting to remove it and install wicd. That was what I did last week and had not one disconnection since then!
sudo apt-get install wicd
sudo apt-get remove --purge network-manager network-manager-gnome
Caution: you will lose your network indicator and wicd will not show anywhere in Unity. Doesn’t matter to me since I monitor my network along with other information with Conky.