A technology blog for The Economist Group IT team

Thursday, July 10, 2003

Wireless power

Well not quite, but this BBC article has a good overview of the 802.3af standard for carrying power over twisted pair Ethernet cable, known as power over LAN (PoL). This standard has virtually been ratified by the IEEE and lots of manufacturers have products that support power over ethernet, such as Cisco, 3COM and PowerDsine.

It works by using the spare pairs of wires in Ethernet 10baseT cabling. For those that don't know, the generic term for the cabling that is widely used for Ethernet these days is unshielded twisted pair (UTP). A UTP cable consists of eight wires which are twisted together in pairs at different twist densities so that interference between pairs is reduced. The greater the number of twists per inch for the whole cable the more data it can carry. So, for example category 5 cable (CAT5) is suitable for for 100Mbps Ethernet, whereas CAT4, with a lower twist density (and therefore cheaper) is only suitable for 10Mbps Ethernet. The IEEE specification for Ethernet over UTP specifies that two pairs of the four are needed, leaving two spare. If you look at an RJ45 jack on the end of some CAT5 cable you'll see that it has eight pins. Pins 1 & 2 (striped orange & orange) and pins 3 & 6 (striped green and green) are used for 10baseT. The other pins are connected to the spare pairs which the 802.3af standard uses.

So what's the catch? Well, as you may have guessed you can't push 240V down a 10baseT cable (well you may be able to but it's not advisable). 802.3af specifies up to 5V DC and 12W - not enough to power your PC and monitor, but enough to power an IP 'phone, for example or a wireless access point. In other words, PoL is useful for when you need to install something where you wouldn't already have power (i.e. a security camera) or where you wouldn't want the expense of running additional power. An important point to remember with digital and IP 'phones, too is that they don't work when there's no power. Typically if you have an installed base of digital 'phones (which are powered from the switch at around 50V) you need to have 10% in number of normal analogue direct exchange lines for use in an emergency in case of a power failure (your normal home 'phone line carries around 12V). PoL can help with this if you have your core network protected by UPS and generators.

If you're interested in reading more, Slashdot has had dicsusssions on the technology in the past. I'm looking forward to power over WLAN.
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