802.3af: Ethernet's Power Play

Computerworld - The idea behind the IEEE's Power Over Ethernet (POE) standard, which delivers in-line power to networked devices over unshielded twisted-pair cabling, isn't new.

IP telephony system vendors stole a page from the private branch exchange vendors' playbook when they developed POE technologies several years ago in order to provide a competitive, centralized power source for IP telephones. In-line power gave users more flexibility in placing IP phones and made the task of providing backup power easier. But these proprietary systems typically locked users to one vendor's equipment.

The draft POE standard from the Institute of Electrical and Electronics Engineers Inc.'s 802.3af task force should not only bring much-needed interoperability but also deliver in-line power to a wide range of devices, from wireless LAN access points to IP security cameras. "POE is going to be one of those things where people won't even remember when you couldn't get power from an Ethernet jack," predicts Steven Carlson, president of Portland, Ore.-based High Speed Design Inc. and chairman of the IEEE 802.3af task force.

It's also the first international power-delivery standard. "With 802.3af, the RJ45 jack becomes the only power outlet standard that's supported globally," says David Passmore, an analyst at Midvale, Utah-based Burton Group. While that's not important for access points and IP phones, business travelers may one day be able to trickle-charge handheld and notebook computers, or even run them without lugging power converters and power supply bricks.

The IEEE's 802.3af specification calls for power source equipment (PSE), which operates at 48 volts of direct current, to guarantee 12.95 watts of power over unshielded twisted-pair cable to data terminal equipment (DTE) 100 meters away—the maximum distance supported by Ethernet. That's enough power to support IP phones, WLAN access points and many other DTE devices. Two PSE types are supported: Ethernet switches equipped with a power supply module, called end-span devices, and a special patch panel, called a midspan device, that sits between a legacy switch and powered equipment, injecting power to each connection.

Power travels over two pairs that are unused in 10/100 Ethernet but do carry data in Gigabit Ethernet LANs. End-span PSEs will work over Gigabit Ethernet, but midspan devices will not, says Amir Lehr, a member of the IEEE 802.3 standards committee and vice president of business development at PowerDsine Ltd., an Israeli company that supplies midspan devices and components.

To prevent damage to noncompliant network devices, the PSE requests a special electronic "signature" from the end device before supplying power. It also stops power flow within 10 milliseconds after a device is disconnected from the network, so a user swapping out devices on a port can't accidentally damage the second end device.