[Design View / Design Solution]
Banish Those "Wall Warts" With Power Over Ethernet
By taking advantage of the new IEEE-802.3af PoE standard, anything that uses less than 12.95 W can be powered through the CAT-5 cable already carrying Ethernet data.
DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.
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.
HIGHLIGHTS:
Powered Devices
The requirements for an 802.3af PD begin with a detection signature of 25 kΩ and less than 120 nF. This specific common-mode termination differentiates the PD from Ethernet devices that do not expect power. The PD only needs to display its detection signature while the link is in detection mode.
Power-Sourcing Equipment
The brains of the PoE operation are in the power-sourcing equipment. It detects, classifies, and controls power to compliant 802.3af PDs on the network.
Endpoints And Midspans
Endpoints typically send power over data pairs, even though the 802.3af standard allows them to alternatively use the spare pairs. Midspans apply power only to the spare pairs while the data pairs pass straight through.
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