DSC (Digital Selective Calling) Operation
What this domain tests
DSC Operation tests your command of digital calling: formats, MMSI addressing, the distress alert sequence, acknowledgments, and the discipline that keeps Ch 70 and the DSC distress frequencies clean.
Formats, categories and addressing
DSC formats are distress, all ships, individual, group and geographic area; categories are distress, urgency, safety and routine. An MMSI is nine digits: a three-digit MID plus six digits for a ship; group calls begin with 0, coast stations with 00, and special devices use reserved prefixes (970 AIS-SART, 972 man-overboard device, 974 AIS-EPIRB). DSC runs on Ch 70 (156.525 MHz), 2187.5 kHz and the HF pair 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz. Ch 70 is DSC only - voice there is a violation.
Distress alert
The alert carries the MMSI, position and UTC time, nature of distress (for example 101 fire or explosion, 102 flooding, 103 collision, 104 grounding, 106 sinking, 107 disabled and adrift, 109 piracy, 110 man overboard), the type of subsequent communication and, on MF/HF, the working frequency. Without GPS input, position and time must be entered manually and refreshed at least every 4 hours.
On VHF: send the alert on Ch 70, then follow on Ch 16 with the voice distress call and message. On MF/HF: send the alert, then listen on the paired DSC frequency for acknowledgment for up to five minutes; if none comes, repeat or try other frequencies, then alert by radiotelephony, NBDP or satellite. Acknowledgments come from coast stations and RCCs; a ship that receives another ship's alert keeps watch and offers assistance rather than sending a DSC acknowledgment.
Routine and test calls
An individual routine call proposes a working channel; the other station acknowledges by DSC and both move there. Test calls use the test category and a non-distress working frequency for the follow-up.
Worked example
You want a routine call to a coast station for a telephone link. Reasoning: address that coast station's MMSI individually, category routine, propose a working channel, transmit on Ch 70, wait for the DSC acknowledgment, then switch to the proposed channel and identify by voice. No voice is transmitted on Ch 70 at any point.
Traps
Sending a distress alert addressed to one ship instead of all ships; leaving a false alert uncancelled (send a DSC distress cancellation to all ships, then announce it on Ch 16 or 2182 kHz); making test calls with the distress category; skipping the 4-hour position update; failing to follow a Ch 70 alert with the Ch 16 voice message.