Is it true that the network manager service turns off the network when there is no activity?

I just discovered that my desktops lost connection to the authentication server. So, screen locks, gdm logins, remote ssh just stopped working.  Only when I logged in as  root on the console did I notice that the network was disabled and when I clicked on the the panel icon, it reconnected.

Is there some special config for the network manager to stop it from doing this?  Is there a kickstart option for this as well?

I have, for the moment, disabled the network manager service and edited the ifcfg-ethX files so they are no longer controlled by NM.  I did this for my headless servers, but I was surprised (annoyed is more accurate) that this also affects "standard" client desktops.