I'm NOT going to give a detailed course on here explaining how the conversion from longitudinal currents on the power line can be used to see DATA transmitted in a modem LONGITUDINALLY.  POTS (Plain Old Telephone Service) used to be strictly Metallic Current.  Now, in the age of VOIP (Voice of Internet Provider), the voice is also transmitted LONGITUDINALLY; NOT METALLICALLY through CLEC's (Competitive Local Exchange Carriers) but metallically through most ILEC's.