Power over Ethernet

showstack / protocols / poe

Power over Ethernet

Also called PoE, PoE+, PoE++, IEEE 802.3af, IEEE 802.3at, IEEE 802.3bt.

DC power delivered down the same cable as the data, negotiated between the switch and the device before any current flows. It is why a wireless access point, a camera, an intercom panel or a small DMX node needs one cable rather than two.

Network transportOpenIEEEReported

Not a traffic flow

Power, not traffic. It shares the cable with a flow rather than being one.

What goes wrong

Budget is per switch as well as per port. A 24-port switch rarely delivers the maximum on all 24 at once, and the shortfall shows up as devices rebooting under load rather than refusing to start.

The class negotiation means power is allocated before it is drawn. A device that mis-declares its class can be granted less than it needs.

Passive "PoE" injectors that put voltage on the pairs without negotiating are not this standard and can damage equipment that is expecting it.

Power is delivered at the switch and consumed at the far end, so the cable loses some along the way. A long run near the class limit is a marginal run.

Typical use

Wireless access points, IP cameras and intercom panels
Small DMX and network nodes at truss height
Anywhere pulling a separate mains feed costs more than the device

The words for it

Glossary entries that name Power over Ethernet, with the false friends that catch people out.

PoE (Power over Ethernet)乙太網路供電Powering a device over the same Cat cable that carries its data. Graded by type: Type 1 up to 15.4 W at the s…

Sources

IEEE 802.3 — Ethernet, including Clauses 33 and 145 (DTE Power via MDI) — IEEE primary
Power over Ethernet standards and compatibility — NETGEAR

Understand the mechanism

Show networks Power, earth and the things that trip

Something wrong, or missing?

Known gaps on this entry: default_ports, first_published, implementations. If you can source one of them, that is a ten minute pull request.

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