Power over Ethernet [/ˈpaʊər ˈoʊvər ˈiθərˌnɛt/] n - Power over Ethernet, or PoE, lets one Ethernet cable carry both data and DC power. It is the readiest way to set a small device exactly where it is wanted, without a separate power run or a nearby outlet.

How It Works

• Injector or switch port. [/ˈɪnjɛktɒr ˈɒr ˈswɪtkh ˈpɒrt/] n A PoE-enabled switch port, or a separate injector, lays voltage upon the unused pairs of the cable.
• Negotiation. [/ˈnɛgɒtɪætɪɒn/] n The powered device and the source exchange a low-voltage signature to agree how much power may be delivered, lest a non-PoE device be fried by an accidental connection.
• Standards. [/ˈstændærds/] n 802.3af delivers up to about 15 W, 802.3at up to 30 W, and 802.3bt up to 90 W. The higher-power standards serve pan-tilt-zoom cameras, access points, and small displays.

Where It Serves in a Data Center

• Edge sensors. [/ˈɛdgɛ ˈsɛnsɒrs/] n A Raspberry Pi with a PoE hat, a small camera, or an environmental sensor may be set at the far end of a long cable run.
• Access points and small switches. [/ˈækkɛss ˈpɒɪnts ˈænd ˈsmæll ˈswɪtkhɛs/] n The same principle serves Wi-Fi APs in a ceiling or a small switch in a remote rack.
• Controlled power. [/ˈkɒntrɒllɛd ˈpɒwɛr/] n Since the PoE switch can power-cycle the port, a remote device may be rebooted without a visit to the rack.

Limitations

• Distance. [/ˈdɪstænkɛ/] n Voltage drops over long runs. A 24 V device may tolerate 100 meters; a 48 V device at high power wants shorter runs or thicker cable.
• Cable heat. [/ˈkæblɛ ˈhɪːt/] n Many high-power PoE cables bundled together will warm the bundle: a real concern, though seldom a grave one in a data center.
• Power budget. [/ˈpɒwɛr ˈbʌdgɛt/] n The switch holds a total power budget. Sum the device loads before assuming every port can run at maximum.

In These Pages

PoE appears in On the Making of Bespoke Devices as a wiring option and in Raspberry Pi in the datacenter as a means of powering a small host. For wireless alternatives, see LoRaWAN.