Abstract: A communication system is disclosed for transmitting digital information between a central station and a large number of remote stations. Digital information is identified by the relationship between a simple pulse marking signal and a time reference signal which is common to all stations, that is, the time reference signal synchronizes the operation of all the stations. The relationship of the marking signal, which does not in itself carry any digital information, to the time reference signal is initially determined by the content of the digital information. Thus, this digital information can only be subsequently determined by evaluating the relationship between the marking signal and the time reference signal. In this communication system, digital radio wave transmission is used to transmit marking signals between stations and the power lines of a power distribution system are used as the source of the time reference signal.