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Power over Ethernet technology describes any system that transmits electrical power, in addition to data, to remote devices over standard twisted-pair wire in an Ethernet network. This technology is useful for powering IP telephones, wireless LAN access points, webcams, hubs, and other appliances where it would be inconvenient or infeasible to supply power separately. The technology is comparable to POTS telephones, which also receive power and data (although analog) through the same cable. It works without requiring modification to the existing Ethernet cabling infrastructure. Power over Ethernet is standardized in IEEE 802.3af. There are several ad-hoc standards for supplying power over Ethernet cable that predate IEEE 802.3af, however future solutions will probably adhere to the IEEE standard.
IEEE 802.3af compliant power sources supply 48 volts DC to devices at a maximum current of 350 mA. This allows devices to draw about 15.4 watts. A twisted-pair Ethernet wire contains four pairs. The Ethernet protocol only uses two of these pairs for data. The 802.3af standard allows power transmission over the two unused pairs, or the two data pairs. To prevent damage to devices that don't accept power over Ethernet, an 802.3af power source won't begin sending power until a remote device acknowledges that it can accept power.
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