Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.
The IEEE standards for PoE require category 5 cable or better for high power levels but allow using category 3 cable if less power is required. PoE delivers 44-57v of DC power over unshielded twisted-pair wiring for terminals consuming up to 25 watts, depending on the version of the standard in use. There are several common techniques for transmitting power over Ethernet cabling two of them have been standardized by the IEEE 802.3 committee. Power may be transmitted on the unused (spare) conductors of a cable, since only two of the four pairs are needed for the commonly used 10Mbit/s–100Mbit/s physical layers (Alternative B) or power may be transmitted on the data conductors by applying a common-mode voltage to each pair (Alternative A). PoE integrates data and power on the same wires, while keeping the structured cabling safe and not interfering with concurrent network operation.
