Various methods, apparatuses, and systems in which a back up power supply is provided to a network interface device are described. In one embodiment, an apparatus includes a network interface device (NID) located outside a building and a power supply unit located inside the building. The power supply unit has one or more ports that couple via a phone line to the NID to provide a power signal to the NID. The apparatus further includes a direct current (DC) back up power unit located in the NID. The DC back up power unit provides back up power to the NID. The NID is configured to periodically switch between receiving the power signal from the power supply unit and the DC back up power unit even when no fault condition exists in the power supply unit.