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But after some look around, I guess I found what is going on. My computer power management is setup to go to sleep mode in every couple of minutes and my cable modem power management setup is also setup to turn it off to save the power.
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Here's how to remove these power management setup.
In windows 7,
- click on Control Panel > Hardware and Sound > Device Manager
- Then expand the Network Adapters
- Right click on your device (Cable Modem, Router etc.) and select properties.
- click on Power Managment tab
- Remove the check mark from "Allow the computer to turn off this device to save power" option.