Digital Selective Calling (DSC)

DSC alerts are classified according to their priority and they are indicated below in decreasing order of priority:

• DISTRESS – indicates that a person or a ship, is in grave and imminent danger and requires immediate assistance (the highest priority usually broadcast to all stations)
• URGENCY – indicates an urgent call concerning the safety of a person or vehicle (can be broadcast to all ships, a geographical area or to an individual station)
• SAFETY – indicates a call concerning MSI (Maritime Safety Information), for example, meteorological forecast/broadcast or navigational warning (can be broadcast to all ships, a geographical area or to an individual station)
• ROUTINE – indicates a call at the lowest priority concerning routine communication (normally broadcast to all ships or to an individual station)

A VHF DSC radio allows you to transmit a message in seconds, which can include the nature of your distress if you select it. If the set is connected to a GPS, this message will also include the GPS position of your boat.

Presuming another station is within range, they become aware of your predicament, without you spending time transmitting your Mayday voice message. The DSC alert maximises the possible audience for your Mayday call by making sure any station within range, with DSC equipment, that acknowledges your digital alert, is listening to the channel you are going to transmit on.

The Maritime Identification Digits (MID) for the United Kingdom are 232, 233, 234, and 235. The MID is the first three non-zero digits of a Maritime Mobile Service Identity (MMSI) number, which is a unique nine-digit number that identifies a vessel's radio station.

• Maritime Mobile Service Identity (MMSI) numbers are issued in a few ways, depending on the country and the situation. In the UK: MMSI numbers are assigned when a ship's radio license is applied for or amended. You can apply for a new MMSI online through the Ofcom website. If you already have a license, you can amend it to add the relevant equipment.
• Format of MMSI: First three digits - The Maritime Identification Digits (MID) that identify the country of registration for a ship or the country of location for a coast station. Last six digits - The Mobile Station Identifier (MSI) that uniquely identifies the radio or vessel
• The purpose of a Maritime Mobile Service Identity (MMSI) is to uniquely identify a ship or coast radio station

Maritime Mobile Service Identity (MMSI) numbers are unique nine-digit numbers that identify ship stations, coast stations, and other non-shipborne stations. There are three types of MMSI numbers:

• Ship Station Identity: Starts with a non-zero digit, such as 232000000
• Group Ship Station Identity: Starts with a single zero, such as 023200000
• Coast Station Identity: Starts with a double zero, such as 002320000 

The first three digits of an MMSI number are the Maritime Identification Digits (MID), which identify the region that issued the number. The last six digits are the Mobile Station Identifier (MSI), which is unique to a vessel or radio.

Maritime Mobile Service Identity (MMSI) numbers

• A Maritime Mobile Service Identity (MMSI) number uniquely identifies a vessel and is only issued to vessels fitted with Digital Selective Calling (DSC) and/or (Satellite) Ship Earth Station (SES) equipment. The UK administration does not normally issue MMSI numbers to EPIRBs/PLBs.
• In order to use SES equipment, arrangements must be made with an Inmarsat Point of Service Activation Entity. An MMSI number is required before the equipment can be commissioned.
• If an MMSI has already been issued for use with DSC equipment and subsequently a satellite earth station terminal is installed, if a new MMSI is required, the DSC equipment may need to be re-programmed. MMSIs issued (as described above) are free of charge as part of a Ship Radio Licence.
• Ofcom notifies the ITU of all vessel MMSI numbers issued by the UK, however it does not notify Portable MMSI numbers. MMSIs are also made available to HM Coastguard to assist in Search and Rescue operations.
• With the advent of VHF/DSC portable radios in  Ofcom has set aside a unique block of MMSI numbers which will be allocated only to this equipment. This enables HM Coastguard to differentiate between alerts sent from fixed and portable VHF/DSC radio equipment. However, such equipment is no longer being produced in the UK and consequently Ofcom only envisages licensing equipment that is only currently on the market.
• Portable VHF/DSC radios must on no account be programmed with the vessel MMSI numbers. HM Coastguard needs to be able to differentiate between the types of equipment sending a DSC alert, as it is possible for the DSC alert from a transportable radio to come from any vessel. The resulting search on the database for vessel details may provide incorrect information that could jeopardise a Search and Rescue operation. The first four digits of a ‘portable’ MMSI will generally be . 

Text © Ofcom