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Using 802.3af (Power Over Ethernet) as a Sysadmin Tool

By using Power over Ethernet (802.3af) to provide power for devices such as WiFi Access Points, VoIP desktop phones, and webcams, companies can save on A/C wiring costs and UPS provisioning. Daniel P. Dern tells you how you can gain additional sysadmin capabilities, including remote off/on/reboot and power provisioning management.

Although we're used to thinking of Ethernet cabling as a medium for data, the wires can also be used to deliver a useful amount of DC power to connected devices. This is called Power over Ethernet (PoE). In addition to saving money on installation, PoE can sometimes be a useful tool for sysadmins. Here's how.

Power Over Ethernet: A Little Bit Of Background

Similar in principle to the way that the wire pair that carries your POTS (Plain Old Telephone System) telephone signals into your house also carries enough juice to power the phone's core innards of headset, dial, and ringer, PoE delivers DC power (up to about 15 watts per wire) over standard Ethernet cabling (CAT5 and better, and some CAT3)—enough to power a growing number of commonly-used useful devices.

PoE has been around for a number of years, with proprietary (and often noninteroperable) implementations from Cisco, PowerDsine, and others. In June 2003, however, the IEEE ratified its 802.3af Power over Ethernet standard, which has, in turn, spurred standard-compliant 802.3af products.

In addition to powering WiFi Access Points (APs) and Voice over IP (VoIP) phones, PoE is also seen as a way to power things such as networked security webcams and smoke/fire detectors; along with RFID scanners, keyless entry and time card systems, "smart signs," audio/video jukeboxes, EPOS and retail Point of Information systems, vending and gaming machines, switch and display panels, and so on. PoE can also be used to recharge PDAs and cell/mobile phones (through docking stations or chargers), and partially recharge notebooks. (There's also a PoE electric razor and a PoE electric guitar, FYI.)

Power is "injected" into the cabling at the wire-closet side. This can be done either through "end-span" LAN/WAN switches, which incorporate the power supply circuitry, or (if you don't want to replace existing LAN switches or only want to power some segments) "mid-span" power patch panels or injectors, which are connected in-between the switch and the segment. The 802.3af standard defines details such as which wire pairs are used, signaled to the device to determine its power needs, and so on. And, of course, the end devices have to support PoE.


Pre-standard PoE devices may be incompatible with 802.3af equipment.

Sending power over the Ethernet cables avoids the need to install A/C wiring and outlets; for example, for hard-to-reach Access Points (often located in ceilings or otherwise hard-to-reach places, or in areas in which you'd rather not be running electrical cabling). Doing the wiring can also be far less expensive—no power cabling or jack needed, and running power cables often requires union electricians (Ethernet cabling may not).

Powering devices such as Access Points, VoIP phones, and security cameras also simplifies provisioning backup power because a single UPS can be located in the wiring closet, rather than needing one at each device location.

In case you're wondering, Power over Fiber (PoF?) is on its way—still a few years out. But for now, there's an interesting hack approach for runs too long to use Ethernet for data: running Ethernet cabling in parallel to the fiber, just to provide power to the fiber-connected remote device.

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