IEEE 802.3at: Power over Ethernet plus

802.3af Power over Ethernet (PoE) has been widely adopted for powering VoIP phones, wireless access points and a number of other network-connected products. But it provides a limited amount of power. In this tip, David Jacobs investigates the latest developments in 802.3at PoE, designed to deliver more power to each device.

Although it became standard only in 2003, IEEE 802.3af Power over Ethernet has been widely adopted. It is used to power Voice over IP phones, wireless access points and many other network-connected products. The main barrier to even wider adoption has been its limitation to 12.95W for each powered device (PD).

Devices such as videophones and dual-band wireless access points require more than 12.95W. Network connected surveillance cameras limited to 12.95W can photograph a fixed scene, but more power is required to pan, tilt or zoom. The IEEE quickly recognized the need to extend the standard and launched the 802.3at, Power over Ethernet Plus Task Force late in 2004.

The primary goal of the 802.3at Task Force is to develop a standard capable of delivering a minimum of 30W per PD and, if possible, more than 30W. Meeting the 30W goal will allow 802.3at to support videophones and dual-band access points. It will also enable support for devices such as RFID readers, industrial sensors and outdoor access points.

Higher network speeds are a secondary goal. The Task Force is required by its charter to investigate the feasibility of technology that will enable midspans to support 1000Base-T. The charter also requested that the Task Force investigate support for 10GBase-T endpoints and midspans. Of course, 802.3at must retain compatibility with 802.3af, so an existing 802.3af compliant PD can be powered by 802.3at power sourcing equipment (PSE).

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The Task Force is currently working on a series of design issues: whether to supply power by using two of the four pairs in an Ethernet cable, or all four; how a PSE can sense the power requirements of a PD; how much current a CAT5 cable can handle; and how to deal with excessive heat in bundles of cable. The Task Force Web site contains presentations from members discussing each of these issues.

Excessive heat in the wiring closet can be a problem with existing 802.3af products. Consider a closet filled with switches totaling potentially hundreds of ports. Additional air conditioning and air flow are required, and 802.3at will add more heat, necessitating a further increase in cooling capacity.

In reality, most of the PDs supported will not require the full 30W. In many cases, the majority of the PDs will be phones that are easily supported within the limitations of 802.3af. There will be a much smaller number of access points or surveillance cameras that need more power. This makes accurate sensing of each PD's power requirements a critical issue. If the PSE can gauge each PD's requirements, it can limit the amount of power made available and thereby limit generated heat.

The problem of heat in cable bundles in existing 802.3af installations was recently reported to the Task Force. Cables are often grouped in thick bundles in the area where they emerge from the wiring closet. Wiring closets are normally air conditioned and the temperature carefully monitored, but the ambient temperature in the area just outside the closet may not be monitored so carefully. In some cases, heat has built up in a cable bundle, resulting in a breakdown of insulation and sparking between cables.

The Task Force continues to work through these problems. Competing solutions are presented and discussed at each meeting. The problems will undoubtedly be overcome, but the jury is still out on how each problem will be resolved. It is unknown at this time how much power the standard will support or when it will be completed.

About the author:
David B. Jacobs has more than 20 years of networking industry experience. He has managed leading-edge software development projects and consulted for Fortune 500 companies, as well as software startups.

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