Abstract: A spread spectrum protocol in a power line communication (PLC) system uses Harmonic Modulation (HM) as true spread spectrum modulation made up of a plurality of modulated tones spaced in frequency with equal distance between adjacent tones. The protocol incorporates the following modes:"Conventional"--Master/Slave relation of meters to a common hub."Leap Frog"--performing data relay through locally elected/designated meters for relay to a central hub or a distributed set of concentrators."Alarm Channel"--reserving an HM signal set for meters to telemeter their configuration in a subnetwork after a power distribution network reconfiguration."Probing"--using HM polling to diagnose the location of a feeder fault."Derived Timing"--master derived timing for inbound message framing."Rate Adapting"--allocating a mix of number of HM channels to the outbound/inbound ratio according to message traffic control requirements and power line distribution topology.