More on power-over-ethernet

I was keeping my eye on the IEE802.2 spec for POE (see a previous post here) and was a tad bemused for the reasons I'd mentioned in April - however, the new 802.2af version is now out and is a specification that embraces gigabit and hence putting the packets as a carrier on a DC voltage - each of the four pairs can supply 13W at an operating potential of 45v. It's a similair arrangement to how your Sky LNB works or how power is sent down a camera triax. Better than this though, and for legacy compatability, a network switch must first "test the water" by measuring the impedance of each pair, and if the device at the other end looks as if it's POE compliant it can start ramping a voltage. It has to test a few more times before it can lift it up to the 45v level and so should never wind up frying old 100baseT NICs or even other deives that are using the structured cabling (telephones etc.) - a few more details on the excellent Power over Ethernet website.