Digital Selective Calling
Digital Selective Calling (DSC) is a service that allows to communicate with a vessel or shore station by making an “individual” call. With DSC it is no longer required to listen to all radio messages to see if there is a call or message for you. DSC is also used to automatically send distress messages.
All modern VHF, MF and HF maritime radios are equipped with Digital Selective Calling (DSC). All DSC equipment is programmed with a unique nine-digit identification number, known as a Maritime Mobile Service Identity or MMSI. This MMSI number is the unique DSC contact number for a vessel. The MMSI is sent automatically with each and every DSC transmission made. A vessel or shore station can use the MMSI number to alert another vessel that they wish to communicate. DSC equipment can be coupled with a GPS receiver to transmit position information.
Digital Selective Calling (DSC) is part of the requirements for GMDSS.
DSC supports 4 different call categories. These are, in order of priority:
The most eminent category of calls are distress calls. A DSC equipped maritime radio is able to instantly send an automatically formatted distress alert to the coast guard or another rescue authority anywhere in the world, with only one push of a (normally red) button. A DSC distress message is not just a message to indicate that the sending vessel is in distress. The message can include the identification of the ship in distress, the position of the ship in distress and the nature of the distress. Distress alerts are automatically addressed to all stations.
Digital selective calling can also be used to initiate or receive calls over the maritime radio. These calls can be urgency, safety or routine radiotelephone calls to or from any similarly equipped vessel or shore station. Urgency, safety and routine calls can be addressed to all stations, an individual station, or a group of stations
DSC is basically a paging system. DSC works with a designated radio channel (e.g VHF channel 70) which is used to transmit short messages indicating for which MMSI number there is a call, the priority of the call and which radio channel to use for the subsequent radio conversation. The DSC equipped maritime radio listens to this radio channel. It will give a special tone to alert to a incoming call, just like a telephone rings to alert to a incoming call.