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The new Power-over-Ethernet (PoE) standard, IEEE-802.3af, will free wireless access points from ac outlets and allow a central uninterruptible power supply (UPS) to service an entire IP telephone installation. It also means bringing Ethernet into applications dominated by proprietary or entrenched older technology, such as telephony, burglar alarms, and video surveillance, as well as completely new applications.
Common devices that normally use wall warts—such as IP phones, HVAC thermostats, wireless access points, PDA docking stations, and anything else that consumes less than 12.95 W—can be powered through the CAT-5 cable already carrying Ethernet data. PoE not only eliminates wall transformers, it enables a whole new set of devices with a combined data and power interface. It also is backward-compatible with existing 10-, 100-, or 1000-Mbit/s Ethernet equipment.
IEEE-802.3af is essentially a power-transmission (not data-transmission) protocol that's predominantly analog. The standard operates entirely with common-mode signaling between the twisted pairs without disturbing differential data transmission.
PoE begins with a power-enabled switch (known as power-sourcing equipment, or PSE) detecting a device that requires power by measuring its common-mode termination. A valid powered device (PD) must have a 25-kΩ common-mode resistance "detection signature."
With a second measurement, called classification, the PSE can determine the PD's peak power requirements. With this data, the PSE can apply power to devices that need it without damaging those that don't, while efficiently allocating available power.
Once the PD is receiving power, it starts its own circuitry, consuming up to 12.95 W. If the PD is ever unplugged or turned off, the PSE stops sending power and again tries to detect the 25-kΩ signature of a valid PD. The PSE entirely controls the PoE link.
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