On the vessel that is being called, however, the DSC controller will recognise its unique identification number, and sound an alarm to draw attention to the fact that it is being called. At the same time, it will display the identity of the calling vessel on its screen.
Assuming the operator decides to accept the call, he can acknowledge it — usually by pressing a button on his own DSC controller. This switches his radio to the appropriate working channel, and sends another burst of digital data back to the original caller, signalling that the call has been acknowledged, and switching the radio to the chosen working channel, ready for normal voice traffic.
Classes of DSC Controller
There are several classes of DSC controller, with different capabilities to suit the needs of different users.
Class A controllers offer all the facilities available from DSC, including the ability to control HF and MF radios, to acknowledge or relay distress messages, and to make calls to vessels within a chosen geographical area. They are intended primarily for commercial vessels, and are seldom installed in small craft.
Class B controllers are also intended primarily for commercial vessels, but offer a reduced DSC capability. They comply with the minimum requirements for vessels which are required by law to carry MF and VHF radio, but cannot control HF radios, or make geographical area calls.
Class D controllers are intended primarily for small craft. They offer a reduced DSC capability, for VHF only, but do not conform to all the requirements for vessels which are required by law to carry VHF radio (other than small commercial craft operating under MCA Codes of Practice).
Class E controllers are also intended primarily for small craft, and offer a reduced DSC capability, for MF and HF only, and do not conform to all the requirements for vessels which are required by law to carry VHF radio.
MMSIs
DSC cannot handle boat names, any more than an automatic telephone exchange can handle names or addresses. Instead, every vessel with a DSC controller is issued with a unique nine- digit number, known as its Maritime Mobile Service Identity (MMSI).
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