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Power over Ethernet for network engineers

This introduction to Power over Ethernet (PoE) is directed toward network engineers and discusses how the technology works, its power capabilities and some benefits it provides for the network administrator or manager.

Power over Ethernet (PoE) technology was developed to reduce the cost of deploying low power network-connected devices such as Voice over IP (VoIP) phones and wireless access points. In addition, PoE provides new techniques for monitoring and managing networks.

IEEE standard 802.3af defines the voltages, currents, power levels and management facilities used by power sourcing equipment (PSE) to supply power to powered devices. Many network switches designed since the standard was finalized in 2003 support the PSE function. Typical powered devices are VoIP phones, wireless access points and network-connected surveillance cameras. The standard supports 10Base-T, 100Base-TX and 1000Base-T via standard, unmodified CAT5 cable.

In cases where new switches will not be purchased, midspans can be used to supply power. Midspans are placed between the switch and the powered device. The midspan supplies power to the device through the network cable while transparently passing data between the switch and the device.

The standard defines two different methods of supplying power. The two unused sets of twisted pairs in a CAT5 cable can be used or the two signal pairs can be used. Both methods cannot be used on the same port at the same time. Most PSEs will support one method or the other, but not both. A powered device must be capable of receiving power through either method. A powered device will sense which method is used and adapt automatically. No configuration is required.

Prior to development of the standard, Cisco developed a proprietary method for providing power via the ethernet cable. Other switch vendors also adopted the Cisco method. Most vendors, including Cisco, have now adopted 802.3af, but many also retain support for the earlier, non-compatible method. PSEs that support both methods sense which one a powered device requires and automatically adjust to whichever is needed.

802.3af defines three power levels: 15.4W, 7.0W and 4.0W. Power is dissipated in the cable, so the maximum power available to a powered device is 12.95 watts. The voltage is nominally 48 VDC at the output of the PSE, but may vary from 44 VDC to 57 VDC. A DC-to-DC converter in the powered device reduces the voltage to levels required by its components. Maximum current at the lowest allowed voltage is 350 milliamps.

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A PSE performs a sequence of tests at each port before applying power to avoid damaging devices that are not powered devices. A mixture of powered devices and non-powered device devices can be connected to a multiport PSE.

The 802.3af standard defines a set of management functions that provide network managers or administrators new capabilities in controlling networks. RFC 3621 defines the SNMP MIB used to access these controls. Using these controls, a network manager located at a central management station can power cycle a device at a remote location or shut off all the powered devices in a facility, for example, to save power overnight or on weekends.

A network manager can also monitor and control the amount of power provided by each port. A manager can configure a port to provide power at one of the power levels defined by the standard, but not at a higher level. This can prevent someone from connecting a different type of device than the one intended.

The IEEE has recently begun work on an extension to 802.3af called 802.3at. The goal is to develop a way to increase maximum power per port to at least 30W and, if possible, more. Increased power is needed by devices such as dual radio wireless access points and motorized cameras. The new standard will also provide fine-grained monitoring and control of power levels, so a port can be configured to supply exactly the power required by the device that should be connected and prevent any other type of device from being powered by the port. These improvements will probably not be available for several years, but the current 802.3af standard offers a wealth of capabilities to aid in deploying and managing today's network devices.

David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.

This was last published in March 2006

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