A single tunable filter WDM Bragg cell is employed to both wavelength demultiplex and detect incoming light beams by the application of a DC tuning signal for wavelength selection, together with an AC component for facilitating detection of data impressed upon selected incoming demultiplexed beams, all in the optical domain, in contrast with the electrical domain, to enhance processing speed. The transmitting element is capable of communicating broadcast content of the broadcast service to a plurality of receiving terminals. This member comprises a frustoconical annular skirt and at least one fin arranged in its internal volume and able to modify the path of the flow fraction entering this volume through the opening. The seal member is fitted to the opening portion by screwing means from the lower surface side. The database is used to sign the user onto one domain when the user is working in the other domain.