This ETS is the baseline specification for the provision of return channel based on PSTN and ISDN to DVB systems. The DVB does not intend to specify a return channel solution associated to each broadcast system because the interoperability of different delivery media to transport the return channel is desirable. The PSTN/ISDN solutions for the return channel apply to satellite, cable, SMATV, terrestrial, MMDS or any future DVB broadcasting system.