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Power over Ethernet

February 1st, 2003 · No Comments

Rafe Needleman writes about an interesting idea:

A few companies, like PowerDsine, make components that allow power to be sent over Ethernet data cables. Applications include wireless access points, Web security cameras, and Internet phones — devices for which putting in a power line can add greatly to the cost and hassle of installation.

The PowerDsine specification calls for 48 volts of direct current (DC) at about 13 watts, which falls below the Underwriters Laboratories safety cutoff and therefore doesn’t require you to call in an electrician to string network cables. Also, the power isn’t actually sent down the wire until the device signals that it can accept it.

This is not, however, a revolutionary concept. On the world’s first network cables — telegraph wires — power and data were one and the same. For a century this was also true for telephone lines. Ethernet and serial cables were designed without a power component, because when the standards were being written, the computing devices that were to be connected required more juice than the cables could safely carry.

The whole idea behind PowerDsine highlights the anachronistic nature of electrical power today. While it’s easier to work with high-voltage alternating current when you’re transmitting energy over long distances, most modern electronic devices use direct current at low voltages, and most of the AC-to-DC power converters for these devices are very inefficient, not to mention a pain in the neck. But with the power demands of new computing products dropping (especially for network equipment and peripherals), it makes sense again to consider running the power through the data wires. In fact, when it comes to modern cabling specifications — specifically USB and FireWire — the data cables do carry power.

On a slightly different note, a friend, Prakash Vaidya, made the suggestion that computer departments need provide a 48 V supply for equipment, just like they do in telco central offices. I think such an idea would make sense for emerging markets, where getting reliable supply at 230 V is a big challenge.

Tags: Emerging Technologies

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