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 Even Fewer Wires: Power-over-Ethernet (PoE) By Jim Geier
July 15, 2002
When locating access points, system designers often use the availability of AC (alternating current) electrical outlets to base decisions on where to install access points. In some cases, companies only locate access points near AC outlets and within reach of a typical six foot electrical cord. Or, they'll look for a convenient location to install new outlets at points where it's suitable to run conduit and mount outlet boxes. All of these situations limit the location of access points and can incur significant costs if new outlets must be installed. PoE to the RescuePower-over-Ethernet (PoE) solves these problems. A PoE solution only requires technicians to run one Ethernet cable to the access point for supplying both power and data. With PoE, power-sourcing equipment detects the presence of an appropriate "powered device" (e.g., an access point or Ethernet hub) and injects applicable current into the data cable. An access point can operate solely from the power it receives through the data cable. Because of avoiding the need for AC power cords, PoE solutions have the following benefits:
Standards for PoE are still underdevelopment within the IEEE (Institute for Electrical and Electronic Engineers). The IEEE P802.3af DTE Power via MDI Task Force should finalize PoE standards by the end of 2002. This seems doable because there's agreement on all major technical elements of the standard. With a standard, you should be able to use different vendor access points with the same PoE equipment. Implementing PoEPoE technology generally resides in mid-span equipment ("power hubs") that reside between the Ethernet switch of the wired side of the network and the access points. Many access point vendors sell power hubs that are rack mountable to combine the AC power and network data onto the same Cat 5 unshielded twisted pair cabling that runs from the switch to each access point. Power hubs often have multiple ports to operate up to 12 access points. In some cases, Ethernet switch vendors implement PoE within each switch port (referred to as "end-span"), which avoids the need for separate power hubs. With this configuration, you can plug the data cable from PoE-enabled access points directly into the Ethernet switch. This is the optimum solution, but most companies installing wireless LANs today already have existing switches that don't implement PoE. As a result, the use of power hubs is the most common solution. Most access point vendors also sell inline power devices that only support one access point. These single-port devices are best for home and small office deployments because in these situations you're likely to have very few access points. A multi-port power hub, which can cost hundreds of dollars, generally wouldn't be practical in very small deployments. In addition, you might be able to easily locate a single access point in a home or small office near an AC outlet, which weakens the benefits of PoE. The primary tip for implementing PoE: use it. The benefits of PoE in enterprise solutions generally far outweigh the limitations. Because most companies use the same vendor for the access points within a facility, the lack of standards isn't too important. PoE doesn't impact the interoperability between radio NICs in user devices and the access points. The bottom line is to maximize the use of PoE. In most cases, there's really no reason not to. By eliminating AC power cords, you'll reap tremendous benefits. Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.
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