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Ethernet's power play

Ethernet's power play

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, centralised 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 into one vendor's equipment.

The draft POE standard from the Institute of Electrical and Electronics Engineers' 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 High Speed Design 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 Burton Group. While that's not important for access points and IP phones, business travellers 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 metres 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 mid-span 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 mid-span 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.

The cost to support 802.3af in end devices is negligible, Lehr says, and many vendors offer devices they claim are compliant. Analysts say the specification adds 20 per cent or more to the cost of an Ethernet switch, although Passmore expects prices to drop as volumes increase. But even paying the premium may be cost-effective, Carlson says, because IT doesn't need to run a separate power cable, and Ethernet cabling installations don't typically require the services of a licensed electrician. "It's very easy to put in and very low-cost," he says.

Buying time

"Most larger enterprises will insist on POE in their new Ethernet switches within two to three years," Passmore predicts. But should you buy now? Current devices are based on a draft specification. Most vendors expect full ratification in July and say any further changes will be minor. But Chris Cullin, product marketing manager at Cisco Systems, says the standard is still preliminary. He doesn't foresee ratification until "later this year".

Cisco wasn't in attendance when six members of the POE Consortium performed interoperability tests earlier in the year. Cullin says interoperability is "not really that complicated". But Doug Hyde, product marketing manager at 3Com, expects problems right from the start. "Many vendors will read the specification differently; they always do. And you'll get incompatibilities along the way," he says.

An industry group formed to perform interoperability testing on early POE devices, the POE Consortium has no plans for independent certification testing along the lines of what the Wi-Fi Alliance is doing for wireless Ethernet. And the Wi-Fi Alliance doesn't plan to add POE testing to its certification for WLAN access points, says spokesman Brian Grimm. Nonetheless, Passmore says it's probably OK to buy early products, as long as vendors guarantee that their products will be upgradable to the final standard.

Gartner analyst Rachna Ahlawat says the stakes are too high for vendors not to address interoperability issues. Within a year of ratification, she says, POE will "just become another standard feature".


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