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PowerOverEthernet.com is the definitive resource for Power Over Ethernet news, information and articles on IEEE802.3af (PoE), IEEE802.3at (PoE+), and emerging PoE techniques.

PoE101

Ethernet CAT5 and CAT6 cables in their traditional role of transmitting high frequency data were missing a trick. It's electronically easy to separate DC from a high frequency signal, so why not use the existing wires to provide a DC potential at the other end of the cable? No mains power supply needed, ideal for cameras and many remote devices already connected via Ethernet cable. The 8 wires in the cable aren't very thick though and 100M is a long way. There's all sorts of specs to do with heating effect due to the resistance of the wires which need to be formally observed so the net result is two official standards, IEEE802.3af and IEEE802.3at, able to deliver 12.95W and 25.5W respectively at the PD.

There are a number of proprietary extensions to these specifications and some manufacturers don't use them at all. They may still offer the ability to transmit DC over the cables so it's still Power over Ethernet, just not necessarily compatible with the standards. Some products claim to be compatible with the 802.3at standard and offer up to 51 W by utilising all 8 wires in the cable, but strictly the 2009 standard prohibits the use of all 4 pairs in this way.

PSE / PD / Midspan

Generally there are 3 types of devices used in a PoE environment. A PSE, a PD and a Midspan. Poe functionality can be incorporated into most Ethernet devices. The PSE (Power Sourcing Equipment) is simply a specially configured power supply designed to provide power safely to a valid PD (Powered Device).

A simple handshake is used between the PSE and PD before power is allowed to flow from the PSE. The PSE is most often part of a switch or hub, or a variety of up/down converters which include an RJ45 connection and can usefully include a PoE facility. A Midspan is a type of PSE and in simple terms is typically a pass through RJ45 box which adds PoE power and acts as a PSE.

How Much Power?

Perversely, the relatively low powers that can be drawn through the thin wired cable are far more useful today because devices are becoming more energy efficient.

IEEE802.3af - maximum 12.95W at the PD.
IEEE802.3at - maximum 25.5W at the PD.

Stepping outside the standards brings a large increase in power for special applications, with jumps to 51W, 60W and beyond with the current max at 100W. Mention should also be made of the HDBaseT Alliance who have developed the HDBaseT specification aimed at multimedia and offering up to 100W of power via PoE.

Many higher power PSEs or midspans also include outputs that meet the lower power level standards.