1. A hierarchical data transmission method for a system comprising: a transmitter for providing a plurality of pre-group data trains, each pre-group data train composed of a main-data signal and a sub-data signal multiplexed with the main data signal; a receiver for reception of a main group data train and for reproduction of the pre-group data trains; and a transmission line connected between the transmitter and receiver, the method comprising the following steps: (a) forming, in the transmitter, a plurality of pre-group data trains such that the pre-group data trains contain therein individual sub-data, the sub-data including an individual signal pattern identifying each corresponding pre-group data train; (b) multiplexing, in the transmitter, the pre-group data trains into a main-group data train to be provided to the transmission line, by applying a parallel-serial conversion to the pre-group data trains; (c) receiving, in the receiver, the main-group data train and demultiplexing the received main-group data train into a plurality of pre-group data trains, by applying a serial-parallel conversion to the received main-group data train; and (d) detecting, in the receiver, the individual signal patterns of each of the demultiplexed pre-group data trains and discriminating the pre-group data trains from each other in dependence on the individual signal patterns.