DIGITAL SELECTIVE CALLING (DSC)
One of the keys to a successful GMDSS system is the use of DSC or Digital Selective Calling. We have come into the computer age on the High Seas! By using DSC we can call other ships and change their frequency to an operating frequency by one call. We can call a ship or all ships to indicate special messages to each. To do this we had to set up a new series of call signs for each ship. It seems like we have a lot of call signs for our ship, Radio call (WABX), SECAL call (11122), Ships Name (SS Neversail), and now we have another call sign. I guess it's a world of call signs.
This new call sign is called a Maritime Mobile Service Identity number (MMSI). It is a nine digit number and the first three digits to denote the ships state of registery. The United States is assigned 366 and 367, while Norway uses 257 and 258. The rest of the digits (six) are the individual ships identity. Example: 367112233.
The number system also needs to have a group station identity and we place a zero in front of the country identifier and the remaining five digits denote a group of ships, such as all OILCO Tankers. Example: 036711223.
Coastal Station have their own special identity and it has two zeros prior to the country identifier. Example: 003671122. A Coastal Group would have the following group of digits: 002673344 (the 3344 would denote a group of coastal stations).
We have shown the ship/station call numbers in decimal format, DSC uses a sequence of seven unit binary combinations to transmit the number. It is only by the use of binary codes that true digital communications and the reliability is achieved. While DSC calls are important for distress alerting and acknowledgment, the system is valuable for handling other routine communications.
This digital system is used on MF, HF and VHF communications. It is used for automated position reporting as in the US Coast Guard Vessel Traffic Service areas.
The DSC system will permit the establishment of communications without manual listening watch requirements. By this means we will no longer have to have a manual listing watch on VHF channel 16 nor on MF 2182 KHz .
There has been frequencies set aside for guarding these DSC calls so that the equipment will be able to respond to distress calls without an operator at the radio console. We can scan these frequencies automatically without operator intervention. The GMDSS system bring into the network the NBDP so that a printed message can be used for distress and this is automatically printed. Some of the frequencies use in DSC and NBDP are:
VHF DSC Channel 70 (156.525 Mhz)
MF DSC 2187.5 KHz
HF DSC 4207.5; 6312; 8414.5; 12,577 and 16,504.5 KHz
MF NBDP 2174.5 KHz
HF NBDP 4177.5; 6268; 8376.5; 12,520 and 16,695 KHz
It should be noted that channel 70 (156.525 KHz) was a intership frequency but in 1986 this frequency was set aside for Digital signaling only and is not to be used for voice conversations .
The message sent via Digital Signaling Calling includes as basic information, the MMSI number of the station sending, time of call, location of ship and one or several messages concerning the nature of the distress . This information could be "Sinking, Grounding, Fire, Taking on Water, etc . When this message arrives at the receiver it will sound an alarm and print out the message on a printer (if the printer is on line). The GMDSS Radio Operator can then silence the alarm and take what actions is necessary.
One of the nice features of the DSC method is that the "follow on" communications, the channel to "work" the alert, can be programmed by the calling person and when the alert communications is established, the mode and frequency will automatically be set-up on the receiving ships radio.
