PoE over Gigabit Ethernet
Power over Ethernet delivers electrical power to end devices over the same UTP Ethernet cable that delivers data connectivity. PoE over FastEthernet uses two pairs of the UTP cable for data and the other two pairs for sending power.
GigabitEthernet on the other hand uses all four pairs for data. So how is PoE power sent over a UTP cable that connects to a GigabitEthernet port?
Both electrical power as well as data are sent over the same pairs of wires at the same time. This can be done due to the nature of the way PoE sends power, and the nature of the encoded bits on the wire.
PoE transmits DC power to devices by maintaining a constant voltage across the wire pairs over time. Conversely, data signals cause variations in the voltage across the wires. Due to these distinct characteristics, it is possible to transmit both power and data over the same pair of wires and separate them at the receiving end. Upon receiving the signal at the destination, one can “subtract” the voltage level provided by the PoE to obtain the original data signal.