PoE

PoE (Power over Ethernet, IEEE 802.3af/at/bt) sends electrical power over the same wire pairs that carry network data. A device such as a camera or a Wi-Fi access point then needs only one Ethernet cable, no separate power supply and no nearby outlet. Power comes from the switch port itself or from a PoE injector placed inline.

In practice there are three common tiers: 802.3af, or PoE, delivers up to 15.4 watts at the port, 802.3at, PoE+, up to 30 watts, and 802.3bt, PoE++, substantially more depending on its type. Some power is lost along the cable, so a device receives somewhat less than the port supplies. A managed switch with PoE often shows, per port, exactly how much power is currently being drawn.

A common mistake is connecting a power-hungry device — a PTZ camera or a high-performance access point — to a port that only supports a lower PoE standard; the device then either fails to start or drops out under load. It's also easy to overlook that a switch's total PoE budget is limited: several PoE devices running at once can exceed that budget even if each individual port could deliver enough on its own.