Editing Power over Ethernet (section)

==Techniques==
There are several common techniques for transmitting power over Ethernet cabling, defined within the broader [[Institute of Electrical and Electronics Engineers (IEEE)]] [[IEEE 802.3|802.3]] standard since 2003.

The three techniques are:
*''Alternative A'', which uses the same two of the four [[Balanced line|signal pairs]] that [[10BASE-T]] and [[100BASE-TX]] use for data in typical [[Cat 5|Cat&nbsp;5]] cabling.  
*''Alternative B'', which separates the data and the power conductors for 10BASE-T/100BASE-TX, making troubleshooting easier.
*''4PPoE'', which uses all four twisted pairs in parallel, increasing the achievable power. 

''Alternative A'' transmits power on the same wires as data for common 10 and {{nowrap|100 Mbit/s}} Ethernet variants. This is similar to the [[phantom power]] technique commonly used for powering condenser microphones. Power is transmitted on the data conductors by applying a common voltage to each pair. Because twisted-pair Ethernet uses [[differential signaling]], this does not interfere with [[data transmission]]. The common-mode voltage is easily extracted using the [[center tap]] of the standard Ethernet [[pulse transformer]]. For [[gigabit Ethernet]] and faster, both alternatives ''A'' and ''B'' transmit power on wire pairs also used for data since all four pairs are used for data transmission at these speeds.

''4PPoE'' provides power using all four pairs of the connectors used for twisted-pair Ethernet. This enables higher power for applications like [[pan–tilt–zoom camera]]s (PTZ), high-performance [[wireless access point]]s (WAPs), or even charging [[laptop battery|laptop batteries]].

In addition to standardizing existing practice for common-mode data pair (''Alternative A''), spare-pair (''Alternative B''), and four-pair (''4PPoE'') transmission, the IEEE PoE standards provide for signaling between the ''power sourcing equipment'' (''PSE'') and ''powered device'' (''PD''). This signaling allows the presence of a conformant device to be detected by the power source and allows the device and source to negotiate the amount of power required or available while avoiding damage to non-compatible devices.