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POE for wireless

Communications News, Sept, 2007

To facilitate wireless access point (WAP) coverage and throughput capability, placing wireless access points in the most advantageous locations in a building is important. In order to broadcast a clear and uninterrupted signal, these locations are typically in ceilings or out-of-the-way elevated positions where power outlets are not normally available.

Installing multiple WAPs would require numerous conduits, electrical wiring, outlet boxes, and the time and labor of a qualified electrician. Employing an electrician to install an electrical outlet near the WAP will usually cost $200-$300 and could approach $1,000 in more difficult installations.

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Because of these high costs, power over Ethernet (POE)is becoming a more accepted medium. PoE supports wireless access point installation by providing DC power over existing low-voltage data cables to PoE-enabled WAPs. Power is provided by endspan or midspan power sourcing equipment to the powered device.

In a PoE application, DC power and Ethernet data are transmitted over four-pair unshielded twisted-pair or shielded twisted-pair cable, typically using R J-45 Ethernet connectors. This simplifies the installation by providing both data and safe, reliable power (15.4W, 48V) on a single cable connection.

Without the need for AC adapters, installation of wireless access points is simplified and deployment speed significantly improved. This increased flexibility allows for easier relocation and slight adjustments to access point position, which ensures maximum coverage. In addition, with no AC adapter, power to the WAP is less likely to be disrupted by people inadvertently unplugging the connection.

PoE technology ensures that non-PoE devices such as network interface cards and legacy peripherals will not be damaged. To accomplish this, the PoE power-sourcing equipment first determines if an end device is PoE enabled, thereby ensuring that power is never provided to a non-powered device.

Power is typically provided in one of two ways: either using the data pairs (pins 1 and 2, 3 and 6) sometimes called "phantom power," or by using the normally unused pairs (pins 4 and 5, pins 7 and 8) in a typical 10/100BASE-T environment. The PoE standard (IEEE 802.3af) allows powered pairs to be used for data transfer as would typically be required when using Gigabit Ethernet (1000BASE-T). For maximum efficiency, a power backup strategy leveraging uninterruptible power supplies is recommended.

PoE power-sourcing equipment is available in two configurations: Ethernet switches with built-in PoE capability, or midspan power hubs. Ethernet-capable switches with PoE capability have the advantage of consolidating both technologies into one device, thereby simplifying installation and conserving rack space. Each switch port contains PoE capability integrated into the switch and connects directly to each wireless access point in order to provide both electrical power and data over the Ethernet cable.

Midspan power hubs are normally installed between the Ethernet switch and a patch panel, and have the advantage of being able to allocate power to only those work areas associated with a powered device, saving on installation time and costs. Unlike the use of a switch deployment, the user is only paying for PoE capability where it is needed. This allows the system designer to provide power to multiple access points, which are scattered around the network, while only using a single power outlet. Midspan power hubs can also be easily installed in retrofit situations and generally are available in 1-, 6-, 12- and 24-port configurations.

For more information from Leviton Voice & Data, Bothell, Wash.: rsleads.com/709cn-255

Technology Industry

POE for wireless

Communications News, Sept, 2007

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