Power Over Ethernet

Nov. 15, 2004

What is the basis for Power over Ethernet (PoE)?

The IEEE 802.3af standard for PoE enables all data-terminal equipment (DTE) to receive power over the same cabling used for data. It specifies the protocol for delivery of 48 V dc over unshielded twisted-pair cables, such as Category 5. This eliminates the need for local power sources.

What does IEEE 802.3af cover?

The IEEE 802.3af standard presents the requirements for providing and receiving power over existing cabling. It involves power-sourcing equipment (PSE), which provides the power on the cable, and the powered device (PD), which receives the power. This standard defines the interface between the PSE and PD as it relates to the associated detection and classification protocol. The figure shows a simplified typical PoE system.

How does the PoE System know when power is required?

Look for a valid PD. The PSE looks for a valid PD by sending out 2.8 to 10 V across the power lines. A valid PD detects this voltage and places a resistance of 23.75 to 26.25 kž across the power lines. Upon detection of the resulting current, the PSE concludes that a valid PD is requesting power.

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About the Author

Sam Davis 2

Sam Davis was the editor-in-chief of Power Electronics Technology magazine and website that is now part of Electronic Design. He has 18 years experience in electronic engineering design and management, six years in public relations and 25 years as a trade press editor. He holds a BSEE from Case-Western Reserve University, and did graduate work at the same school and UCLA. Sam was the editor for PCIM, the predecessor to Power Electronics Technology, from 1984 to 2004. His engineering experience includes circuit and system design for Litton Systems, Bunker-Ramo, Rocketdyne, and Clevite Corporation. Design tasks included analog circuits, display systems, power supplies, underwater ordnance systems, and test systems. He also served as a program manager for a Litton Systems Navy program.

Sam is the author of Computer Data Displays, a book published by Prentice-Hall in the U.S. and Japan in 1969. He also authored the book Managing Electric Vehicle Power. He is also a recipient of the Jesse Neal Award for trade press editorial excellence, and has one patent for naval ship construction that simplifies electronic system integration.

You can also check out additional articles on his other author page.