Designers and customers alike are just beginning to appreciate the far-reaching effects of Power-over-Ethernet (PoE) technology. The products that meet the IEEE 802.3af PoE standard may revolutionize the installation of any device that's based on the Internet Protocol (IP). Imagine if wireless access points (APs), Voice-over-IP (VoIP) phones, cameras, and other devices were instantly ready for use. The user would merely have to plug the devices into a traditional Category-5, Ethernet-cable wall socket (FIG. 1).

Many wireless designers may only view PoE technology as a boon to the proliferation of wireless access points. But IP phones were the first drivers in the development of Power-over-Ethernet systems. One of the early developers of IP-based phones was Cisco Systems (www.cisco.com). Its goal was to bring voice communication into the same network as data structures, according to Steve Shalita, Cisco Systems' Senior Manager.

Customers found those early IP-based phones difficult to install. They required a connection to an AC power source as well as to the network. One way to simplify these installations was to include power through the existing Ethernet cable. Shalita notes that these same problems were faced years later when wireless access points first became popular.

Many vendors shared these challenges, so an IEEE standard was developed to universalize the creation of Power-over-Ethernet devices. This standard, which was published in June of 2003, is known as 802.3af. Not surprisingly, Cisco was one of its main participants.

What is involved in the design of PoE products? Do the benefits of PoE technology outweigh any potential risks? First of all, many obvious as well as subtle benefits can be derived from standardized PoE technology. Perhaps the most important benefit—at least to information-technology (IT) departments—is cost savings. PoE-enabled devices don't require any accompanying AC power source, such as a wall socket. This aspect reduces the need for certified electricians to install conduit, wiring, and outlets throughout a facility.

Instead, engineers with low-voltage licenses can perform installations, observes Nigel Ballard, Wireless Director for Matrix Networks (www.mtrx.com). The use of less expensive labor translates into cost effectiveness and faster deployment times. Ballard explains that the installations are much neater in both the ceiling space—a typical location of access points—and the LAN-switch cupboard.

Keith Hopwood, Vice President of Marketing for Phihong USA (www.phihong.com), says that although cost savings may vary, an electrician who installs power to an access point may charge around $100 per unit. Others place the cost of access-point power installations at several hundreds of dollars per unit. Either way, the return-on-investment (ROI) for PoE-enabled devices can be significant.

Power-over-Ethernet technology also offers flexibility. Instead of locating APs near existing AC power outlets, wireless-LAN designers can place them where they maximize coverage or increase bandwidth. A less obvious benefit is PoE's capability to provide power through data cables to dangerous locations. For example, IP-based security cables sometimes need to be routed to "wet locations," such as a building's exterior. Running a separate power supply to such locations can be a safety hazard. IEEE 802.3af-certified PoE cables can reduce those hazards.

PoE products also boast power reliability. For applications that use AC wall power, PoE offers a source of backup power when outages occur. This characteristic could prove extremely valuable to IP phones. They would still work even if the main supply line goes down.

PoE has even produced benefits outside of the usual venue of wireless-LAN office deployments. One application area that's gaining interest is in tower tops for wireless-Internet service providers (WISPs). According to Mike Tadros of Antenna Systems & Supplies Company (www.antennasystems.com), it's often much more efficient to run 200 ft. of CAT-5 Ethernet cable instead of running a 1/2 in. or 5/8 in. of hardline coaxial cable. Tadros cautions that by "efficient," he's referring to the energy (measured in dBm) that's being broadcast from the antenna(s). As he explains, "There are plenty of online and commercial microwave-link calculators available that will help an end user determine the theoretical distance achievable utilizing the two previously mentioned scenarios."

Almost all LAN-switch vendors endorse the 802.3af standard. In fact, Cisco—one of the largest manufacturers of LAN switches—recently announced support for the IEEE 802.3af PoE standard across almost all of its switching products.

The 802.3af PoE specification details all of the requirements for designing PoE equipment. Two types of devices are specified in the standard: Power-Sourcing Equipment (PSE) and a Powered Device (PD). The PSE provides 48-V DC power, with a current limit of 350 mA, to the PD—be it a VoIP phone or wireless access point. The PSE is limited to a continuous maximum power output of 15.4 W.