Power over Ethernet is catching on with organizations looking to cut costs on wireless LAN deployments. The technology also can keep IP phone users talking even if the lights go out.

Some users already have discovered that powering Wi-Fi access points with PoE can save hundreds, even thousands, of dollars per installed device by eliminating costly power-outlet installations. Meanwhile, early adopters of IP PBXs and phones say PoE is an indispensable way to provide dial tone during power outages - a common feature on legacy PBXs that the voice-over-IP market was slow to adopt.

PoE, an IEEE standard since June, is available on switches from almost every major LAN infrastructure vendor, and IP telephony and WLAN equipment companies. Most vendors offer PoE switches that are based on the IEEE 802.3af standard. Cisco offers a proprietary PoE technology for its IP phones, switches and Wi-Fi gear, and 802.3af-based products.

The IEEE 803.3af standard defines two methods for delivering up to 48 volts of DC power to PoE-compliant devices over eight-wire Category 5 and 6 cabling. One is called mid-span, which involves running power over unused wire pairs in a LAN cable. (Only two of the four pairs are used to deliver data in Ethernet and Fast Ethernet.) Mid-span products from the company PowerDsine often are built into patch panel-like devices that can add PoE to existing LAN infrastructures.

The other, increasingly popular version of 802.3af is called end-span. It runs DC power signals over the same wire pairs used for data transmission. Industry experts say end-span devices are becoming popular because they usually are built into new switches with PoE, which users often buy for IP telephony or WLAN rollouts. End-span also allows Gigabit Ethernet and PoE to co-exist as 10/ 100/1000M bit/sec links become more popular. (Netgear last week announced a triple-speed Layer 3 switch with PoE at Comdex.)

PoE has become a hot complementary technology for WLANs. Several makers of wired and WLAN gear - 3Com, Airespace, Aruba Wireless Networks, Cisco, Extreme, HP and Nortel - now offer 802.3af-compliant access points and switch ports.

One benefit of using PoE with Wi-Fi is that it eliminates the need to have electricians install new power outlets to support large-scale WLAN deployments. This benefit becomes quickly obvious in very large-scale WLAN access point deployments, such as on big university campuses.