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Tech Abstract
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Application of Power over Ethernet and PoE Plus

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Summary: This 7-page white paper examines the Power-over-Ethernet (PoE) standard, IEEE802.3af (PoE), and the update to it, the IEEE 802.3at (PoE Plus). This technology provides power to difficult-to-reach areas such as remote network switches, security cameras and outdoor WLAN access points. The paper shows the 802.3af’s limitations with providing enough power to certain devices, and how the updated 802.3at answers those limitations. Also includes a comparison of PoE versus PoE Plus, and an application story on PoE Plus for video surveillance.

As useful as PoE has proven to be, it has become clear that the IEEE 802.3af standard falls short when it comes to certain demanding scenarios. Power sourcing equipment (PSE) with 802.3af PoE technology can only provide a maximum of 15.4 W of power to powered devices (PD). There are many Ethernet-connected devices, such as the following, that simply need more power:

PTZ/Outdoor Cameras: Low-power IP cameras are well-served by existing 802.3af PoE technology. However, cameras with Pan/Tilt/Zoom (PTZ) functionality are increasingly popular, and these cameras use more power in order to operate their motors. Cameras that are ruggedized for operations outdoors, especially in harsh conditions, also need more power in order to use their heating units. It would be nice to be able to use 802.3af PoE technology to supply power to each outdoor camera, but 802.3af just isn’t up to the task of providing enough power for these devices.

Long Distance LAN Devices: Basic wireless access points can get by with 15.4 W of power. However, high-performance wireless devices that provide wider coverage use high gain antennas and multiple radio frequencies, all of which demand much more power. These high-powered wireless devices are particularly common in widely distributed, remote applications. This is precisely the type of application where power supplies and cables are a major cost—yet the power supplied by 802.3af-compliant PSEs is too weak to offer a viable alternative to conventional power sourcing.

PoE Plus
In September 2009, the IEEE ratified 802.3at (“PoE Plus”), which dramatically improved PoE’s ability to meet the requirements of more demanding applications. With PoE Plus, it’s possible to use just an Ethernet cable to deliver power to more power-hungry devices. PoE Plus offers the following key advantages over conventional PoE—it’s more powerful, and more efficient.

IEEE 802.3at technology increases the power delivered to PDs to 34.2 W, which is nearly double the power delivered by the old PoE technology. With this extra power, it is possible to use the Ethernet cable to deliver power to devices such as outdoor cameras, LED display boards, WiMax devices, and even some embedded computers or terminal computers. This capability alone unlocks the possibility of using PoE in applications that were previously too demanding for PoE technology.

PoE Plus is not only more powerful, it is also more efficient. IEEE 802.3at defines a new method of communication and identification between PSEs and PDs. In 802.3af, classification on the hardware layer (Layer 1) was optional. In 802.3at, this identification is mandatory. What’s more, 802.3at adds an additional data-layer (Layer 2) classification mechanism, known as the Link Layer Discovery Protocol (LLDP). LLDP allows the PSE to repeatedly request status information from the PD and dynamically allocate power in response. This means that PDs will use more power efficiently and only request maximum power when it is needed. Power efficiency is particularly useful for applications that must be frugal in their power consumption, such as remote applications powered by batteries or solar cells. However, less power consumption will translate into an improved bottom line no matter the application, simply by reducing the power bill.

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COMPANIES IN THIS ARTICLE: Moxa Americas, Inc.
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