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Digital Selective Calling System

I. Read and translate the following text

Digital Selective Calling System is an integral part of the GMDSS and is used for transmitting distress alerts from ships and for transmitting the associated acknowledgements from coast stations. The DSC system is very comprehensive and caters for setting up of all types of calls using internationally agreed codes. It is also used by ships and coast stations for relaying distress alerts and for other urgency and safety calls. DSC encoders/decoders, in some circumstances, can be interfaced with existing equipment. A dedicated DSC watch receiver is required to keep continuous watch on the distress frequency; this can be a simple single channel receiver.

DSC is basically a calling system. Each call consists of a packet of digitized information of one oа four priorities: Distress, Urgency, Safety or Routine. Messages can be routed to “All Stations”, to an individual station or to a group of stations, using their Maritime Mobile Selective-call Identity Code (MMSI). Distress messages are automatically broadcast to “All Stations”. In addition to the MMSI of source and destination stations, information can be conveyed in the DSC message. For example, to aid the Rescue Services, the DSC Distress alert message is configured to contain the following:

1. Ship’s identity (the nine digit MMSI);
2. Time;
3. Nature of distress (in the form of a standard code);
4. Ship’s position.

Where fitted, ship’s officers should ensure they are fully conversant with the procedure for entering the necessary information into the DSC equipment. At sea, unless provided automatically form a navigation system, ship’s position should be updated at least once during each watch period.

The content of a DSC call includes the numerical address of the station (or stations) to which the call is transmitted, the self-identification of the transmitting station and a message, which contains several fields of information indicating the purpose of the call.

Various types of DSC calls are available, being broadly either distress and safety related calls or “commercial” calls (to indicate that a commercial communication, e.g. a telephony or telegraphy call etc., is required). In the case of VHF, automatic connection to the public network can also be established through suitably equipped coast stations.

The receipt of DSC call by a receiving stations is accompanied by a suitable display or printout of the address, the self-identification of the transmitting station and the content of the DSC message, together with an audible or visual alarm or both for certain categories of calls (e.g. for distress and safety related calls).

In order to increase the probability of a DSC distress call or a DSC distress relay being received it is repeated several times to form a distress call attempt.

Summarizing all above information, one may say that Digital Selective Calling is a form of data transmission, and calls sent by this technique can be printed out or shown on a visual display unit. The system is selective and calls can be sent to a specific station, groups of stations or to all stations. However, the primary purpose of a digital selective call is to establish communication between stations so that information can be sent either by radiotelephony (RT) or narrow band direct printing (NBDP).

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