In addition to data communications between the two devices, Power over Ethernet PoE enables an Ethernet Switch / Hub to power a remote device over an existing LAN cable. Without PoE, any remote device needs to be powered independently. Typically, device power is derived from an AC/DC adapter or at board level inside the device.

This adds cost and requires locating the device near a source of AC power. Device installation is further complicated by different safety codes, AC voltages and wall plugs around the world.

PoE avoids these problems by supplying low-voltage DC power directly to the Powered Device (PD) from the Power Sourcing Equipment (PSE) that is located in the Ethernet Switcher or mid-span Hub and eliminates the complexity of AC power sources.
To ensure that all equipment is protected and devices operate properly when connected, the IEEE established a universal standard (IEEE802.3af). This specifies the power requirements of the PSE and the PD, and the communication protocol between the two - regardless of manufacturer or configuration.