Field Guide · concept

Also known as: PoE

Power over Ethernet (PoE) delivers electrical power to a device over the same twisted-pair cable that carries its Ethernet data, so the device needs no separate power supply.1

PoE switch (PSE) access point (powered device) data ⇄ + DC power → one cable carries both — no outlet needed at the device
PoE runs power and data down the same cable: a power sourcing equipment switch (the PSE) injects DC alongside the Ethernet signal, and a single run reaches a powered device — here a mast-mounted access point — that needs no mains outlet of its own.

Overview

Standardized in IEEE 802.3 (af, at/PoE+, and bt/PoE++), PoE lets a power sourcing equipment (PSE) device — a PoE switch or an inline injector — feed a powered device (PD) anywhere from about 13 W up to roughly 90 W over the twisted pairs. The two ends first negotiate: the PSE detects a compliant device and its power class before applying full voltage, so it never over-drives gear that cannot accept power or damages a plain Ethernet device.

That single-cable delivery is ideal for equipment mounted where a power outlet is awkward: wireless access points on ceilings, IP cameras on walls, VoIP phones, and small computers all run on one cable that carries both data and power. Because the power rides the same infrastructure, a centralized PoE switch can also power-cycle or monitor its attached devices remotely.

Later standards raised the ceiling — from the original ~13 W to ~90 W — to feed hungrier devices like pan-tilt-zoom cameras and multi-radio access points that early PoE could not support.

Power classes

Successive standards lifted the wattage available at the device:

Standard Name At the device Typical loads
802.3af PoE ~12.95 W VoIP phones, basic APs
802.3at PoE+ ~25.5 W PTZ cameras, dual-radio APs
802.3bt Type 3 PoE++ ~51 W Multi-radio APs, small PCs
802.3bt Type 4 PoE++ ~71–90 W Displays, high-power devices

Where it fits

PoE collapses two cables into one, which is exactly the appeal for an antenna-side node. A GopherTrunk capture box built on a PoE-capable Raspberry Pi HAT can sit on a mast or in an attic with only a single network cable run to it — data down, power up — instead of needing mains wiring at the antenna. That keeps the install clean, removes a mains supply from near the front end, and lets the whole node be power-cycled from the switch in the rack below.

Sources

  1. Power over Ethernet — Wikipedia, on carrying power alongside data on Ethernet cabling and the 802.3af/at/bt power classes. ↩

See also