What is Global Maritime Distress and Safety System or GMDSS and its details?

Basic of DSC :-

DSC of technique of transmitting digital codes, which allows suitable equipped stations to :-

1. Transmit and receive distress alert
2. Transmit and receive distress alert acknowledgements.
3. Announce urgency and safety calls
4. Relay distress alert.

With the help of DSC call to

1. All stations
2. Stations in a Geographical area.
3. An individual station 
4. A group of station

NOTE:

• DSC channels have been allocated in 2 MHz, 4MHz. on MF/HF bands.
• CH-70 on VHF.
• All DSC calls automatically includes phasing signals, error-checking signals and identity (MMSI) of the calling stations.

NBDP procedures during distress working :-

Silence Mayday :-

Controlling station may impose silence on interfering stations by sending the radio telex message.