Known officially as IEEE specification 802.3af, Power over Ethernet (PoE) is a technology standard for transmitting up to 13 Watts of power over any length of Cat 5 Ethernet cable. It is one of the enabling technologies which will allow the current network to expand to the type of system we think of when we say “ubiquitous computing”. PoE is a vital core technology for IP telephony and next-generation LAN peripherals like remote webcams, web radios, wireless access points, and other network-dependant accessories.

The Power over Ethernet standard applies to gear installed into switches, or as stand-alone midspan hubs that allow terminal devices to be power-enabled without upgrading the existing switches. PoE uses the data or spare pairs in Cat 5 cable to carry -48 Vdc. On the data pairs, pins 3 and 6 supply one side of the dc, and pins 1 and 2 supply the other. On the spare pairs, pins 4 and 5 are paralleled for one side of the dc supply, and pins 7 and 8 are paralleled for the other side.

The need for PoE can be found in our first example, the IP telephone. In order to compete with traditional telephone for the mainstream, IP telephony has to be more than just cheap, it has to be as easy to use as a regular telephone. One reason it is not is that it is not only has to be tied to a LAN connection, it must also have a power source. A regular telephone also needs power, but it gets that power from the phone line. PoE puts the power into the signal line, as with a regular telephone, making an IP telephone a one-line deal. (Frankly, I think thin client devices will be the killer app, and not IP telephony, but I could be wrong.)

This also means that PoE blurs the line between power and signal connections for LAN devices in the same way the USB interface does for user peripherals (it also transmits power over a signal line.) By reducing the number of lines needed by half, PoE makes wiring low-power networks as simple as a run of Cat 5 cable.

This is not to be confused with the Powerline technology, also known as the HomePlug standard. That is a way to transmit data at up to 14Mbps (with 56-bit data encryption) over standard 110-V power lines. This is a competing technology for some data applications that don’t actually require a LAN connection (like remote speakers and monitors) or for LAN devices that need more than 13 W (although industry professionals already predict that PoE will push the limits of the cable it runs on and may provide as much as 30 W in newer installations with more robust cable.)

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