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DSC - Digital Selective Calling
DSC is primarily intended to initiate ship-to-ship, ship-to-shore and shore-to-ship radiotelephone over VHF. It has also been adopted by the GMDSS standards for transmitting distress messages from vessels.

A DSC capable unit must have a second VHF receiver which is always listening on channel 70. If you listen to channel 70 you can hear small data messages being transmitted. Channel 70 is dedicated for DSC worldwide.

Note! This page is only intended as an introduction to DSC and is in no way a description of the complete DSC standards.

Shore - Ship
When a shore stations is calling a vessel.

  1. A DSC message is transmitted on channel 70 from the shore station. This message contains as source MMSI, the MMSI of the shore station. It contains a proposed channel to use for communication, this is 64 in this example. The radio on the vessel will see that the destination MMSI number in the message matches the vessels MMSI, the radio will then be ringing (like a telephone).
  2. When the onboard personell lifs the handset an acknowledgement message will be transmitted from the vessel to the shore station and the radio will automatically switch to channel 64.
  3. When the shore station receives the acknowledgement message, the shore radio station will change to 64 and the shore operator and the vessel can talk.

Ship - Shore
According to the standards, only shore stations are allowed to propose a specific duplex channel. A vessel may request a duplex conversation to a shore station, but only the shore station should select the channel.

  1. The DSC message is transmitted from the vessel with source MMSI set to the ship and destination is MMSI of the shore station. The Type field is set to duplex and channel field is empty.
  2. The shore operator selects a channel, 64, and sends an acknowledgement message containing this channel
  3. When the ship station receives the acknowledgement message, the radio will change to 64 and the shore operator and the vessel can talk.

Ship - Ship
Ship - ship communications is only possible on simplex channels.

  1. The initiating vessel proposes channel 6. DSC message is sent on ch 70.
  2. The receiving vessel lifts handset, the radio automatically changes to ch 6, and an acknowledgement is sent.
  3. When the initiating ship station receives the acknowledgement message, the radio will change to 6 and the two vessels can talk.

Ship - Telephone
Ship - ship communications is only possible on simplex channels.

  1. Vessel is sending a DSC msg with a phone number.
  2. System automatically tries to call the number.
  3. Phone user answers the call from the system.
  4. System sends an acknowledge message to ship containing a duplex VHF channel.
  5. Voice communication is established between ship and basestation via VHF and then to Telephony Gateway via Voice over IP and then to telephone user via normal telephone network.

Distress
Distress messages via DSC is part of the GMDSS standards. It is very important that a GPS source is connected to the DSC radio. This way, in case of an emergency, the onboard personell may only press Distress button on radio and a DSC message containing the GPS position of the vessel is sent. This message has an open destination MMSI field, meaning it will be received by all DSC radios withing VHF range.

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