The 802.3af standard may increase interoperability with new products, but organizations with nonstandard power supply equipment may want to check for backward compatibility before buying the new equipment. For example, Cisco Systems Inc.'s proprietary in-line power technology uses a different signature scheme and a slightly different "power envelope," says Bob Veliles, product marketing manager at Cisco. The company's 802.3af-complaint switches will provide backward compatibility with its existing IP phones that support Cisco's proprietary in-line power scheme, the company says, but competing products may not.

3Com Corp. users face a different issue. The vendor's proprietary in-line power scheme used 24 volts vs. the 48 volts specified in the new standard. As a result, the company's new, 802.3af-compliant switches won't support its older phones without adding a $30 converter module to each line. That's disappointing for Bill Miller, desktop services manager at the County of Nevada, Calif. He needs to buy new switches after a fire but already has older IP phones for some 60 users. "Which way am I going to go? I don't want to go backwards," Miller says, noting that any new 802.3af-compatible IP phones won't work if he buys legacy power supply equipment. "People that adopted [Power Over Ethernet] early lose out because their equipment is obsolete," he says.