There are a number of different systems, and more under development. Many
are radio and microwave interrogated. 2.4 GHz is popular as one band. Some
communicate over the wires.
Our rural Coop installed a remote reading system early last year, when their
Chief Engineer was a friend of mine. Unfortunately, he moved on to a better
job shortly afterwards, and the only information I got from him was that
this system "modulates" the zero crossings of the 60 Hz to transmit and
receive data. Funny thing, when this system went on line, several of my old
line connected digital clocks started gaining time. In examining their
circuits, I found that the capacitor specified by the chip manufacturer on
the input line was left out of these designs. Adding the specified .001
eliminated the time gaining problem, but it confirmed, at least to me, that
indeed, there are now more zero crossings for the clock to count than there
used to be.
Remove N O and S P A M imbedded in return address