An OLD GMDSS explanation video (from 1992). Please ignore the errors from19m 23s to 19m 36s, showing MF/HF and VHF radios that do NOT have DSC capabilities and note the old external DSC controller shown from 20m 17s to 20m 24s.
Also ignore the references to the defunct INMARSAT-E beacons.
Further take note of the old "2182khz Watch Receiver", which haven't been used in almost 20 years (at 20m 54s), shown listening to static (which they didn't do, as they were typically squelched and listening for a two-tone alarm generator alert (NOBODY monitors 2182khz anymore!)
Even with the above errors and 20+ year old technology shown, it does give a good overview to how the GMDSS was conceived and how it works!

And, please have a look at the HF-DSC Playlist here...
   • HF-DSC Communications  

As well as the other Maritime communications playlists here..
   / captainjohn49