I'm trying to learn about PoE and a friend told me the 100 meter limitation on a PoE circuit is because of the DC voltage drop on the power channel, not the data. I had understood the opposite, that the limitation is due to the data signal degrading beyond that point too much to be reliable. He claims the data uses an "extremely low voltage" so it doesn't have the same problem as, say, a 48V power channel. I suspect he has things backwards and it is actually the lower voltage that would have more problems due to voltage drop.
In addition to this question, how would one stretch the limits if you needed a little more distance? My friend said he had cable runs of over 500 feet and never had trouble, but he relied on careful planning (avoiding EMI sources and keeping cables free from bends).
He also mentioned some kind of boosters you could put midspan to boost the signal if needed. What are these, and would they also work for the power channels on a PoE run?