A data transmission system in a communication network comprising a data modulator comprises: a first part at the level of the collection point part comprising: an NCR module taking account of an external absolute time reference to reconstitute a network clock, a transmitter module receiving the value of a time reference to be inserted into the data packet to be transmitted and transmitting to the NCR module an information item on an instant of synchronization for the receiver, a second part at the level of the radiofrequency part of a gateway comprising; a receiver module, a module for reconstructing a clock locally, a module for inserting a time reference into a data packet received by the receiver before transmission of the data via the satellite, the first part and the second part exchange encapsulated data.