I am getting disconnected from DSL modem and can't connect unless I restart.

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http://askubuntu.com – I am using Ubuntu very recently. My DSL connection with the modem is working fine but after certain idle time the modem connection is getting disconnected and I am not getting the icons to reconnect. If I go to System Settings--> Network, it is showing "Network cable is unplugged". It works fine if i reboot. This is a sudden problem and no configuration has been altered. I am using Ubuntu 12.04 with latest updates. (HowTos)