Emerging Standard Drives High-Power PoE Design

Mar 23, 2007 2:35 PM
By Daniel Feldman, Product Line Manager, Microsemi, San Jose, Calif.; IEEE 802.3 Working Group, Voting Member; and Ethernet Alliance PoE/PoE+ Technical Committee, Chairman

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Power over Ethernet (PoE) is about to go through a transformation. The increase in the power available to powered devices (PDs) from 12.95 W to 59 W will transform the RJ45 connector into the first truly universal power plug. With the IEEE 802.3at standard expected to be ratified in the second quarter of 2008, and prestandard equipment already available, the full deployment of high-power PoE is rapidly approaching.

To create systems that meet this emerging IEEE standard, designers must have guidelines for cable selection; voltage levels, power levels and classification for power-sourcing equipment (PSE); and power allocation values, as well as the enabling of backward compatibility with the existing IEEE 802.3af PoE specification.

In November 2004, the IEEE 802.3 Working Group approved the creation of the PoE Plus Study Group, with the intent of exploring ways to enhance the successful IEEE 802.3af-2003 PoE standard. The PoE Plus Study Group work culminated in September 2005, with the formation of the IEEE 802.3at Task Force, which is currently drafting the IEEE 802.3at-2008 standard.

Trying to increase the expanding market for PoE systems, the IEEE 802.3at Task Force has taken upon itself several objectives. First, 802.3at should operate on Cat 5e and higher infrastructures, unlike 802.3af, which had to account for the limitations of Cat 3. Also, 802.3at should follow the power safety rules and limitations pertinent to 802.3af. A High Power 802.3at PSE must be backwards compatible with 802.3af, being able to power both 802.3af and high-power 802.3at PDs.

Furthermore, the 802.3at standard should provide the maximum power to PDs as allowed within practical limits — at least 30 W — and 802.3at PDs, when connected to a legacy 802.3af PSE, will provide the user an indication that an 802.3at PSE is required.

The group will research the operation of midspans for 1000BASE-T and the operation of midspans and endspans for 10GBASE-T. It will also create a PD management information base.

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