Digital Selective Calling

Digital Selective Calling (DSC) is a technique which allows to connect two or more radio stations over a common communication channel in an automated way. The connection between the parties that want to communicate with each other is established by digital signalling on a dedicated channel without using the traditional distress and calling channels on MF (2182 Khz) or VHF (Ch 16 / 156.80 MHz). Digital Selective Calling was conceived by an international committee in the early 1970's. Its purpose was to expedite the handling of traffic in the maritime service by facilitating more efficient calling and to provide a more automated and thus more reliable distress and safety system. More in particular, DSC should help to relieve some of the congestion on the traditional distress and calling channels (e.g. Ch 16 on VHF) as it uses a dedicated channel (e.g. Ch 70 on VHF) for the calling procedures.

DSC is primarily intended to initiate selective ship-to-ship and ship-to-shore radio communication. DSC calls can be made to individual stations, groups of stations, or "all stations" in one's reach. DSC distress alerts, which consist of a pre-formatted distress message, are used to initiate emergency communications with ships and rescue coordination centres.

Starting in 1992, the IMO introduced Digital Selective Calling on MF, HF and VHF maritime radios as part of the GMDSS system. Since 1999 GMDSS regulations, which include DSC radio equipment, are compulsory for all commercial vessels over 300 grt, registered fishing vessels and craft vessels carrying more than 12 passengers.

As a result of the introduction of DSC radio equipment, the watch of the distress, safety and calling channels (Ch 16 VHF and 2182 kHz MF) by coast stations and commercial vessels has been discontinued. This has also consequences for pleasure yachts, for which the GMDSS regulations are not compulsory. In order to be able to utilize all the safety features of the world wide global maritime distress and safety system, also "non-compulsory fit vessels" will now have to be equipped with DSC radio.

Basic Principles of DSC Radio Systems

Digital Selective Call can be compared with telephone switching. The radio equipment is principally the same as for non-DSC operation. The only difference is that the calling procedure is now controlled by a digital signal processor (the DSC Controller), which uses a dedicated channel for the procedures of calling the other station(s) and the switching to the working channel. This way no communication over the standard calling channel (e.g. Ch 16 for VHF) is necessary and the required voice communication is automatically directed to a working channel (e.g. Ch 72 for ship-to-ship communication).
One of the basic advantages of the digitally controlled calling procedure is that calls can be performed in a selective way and that incoming distress, safety and routine calls can be stored in the local DSC controller. This way a continuous watch on the calling channel(s) is not necessary any more. Especially for pleasure yachts with a small crew this considerably simplifies the required radio watches.

Maritime Mobile Service Identity

In order to participate in the selective calling ability of the DSC system, each DSC-equipped station is assigned a unique 9-digit identification number. This number, which is comparable to an international telephone number, is called Maritime Mobile Service Identity (MMSI). Each MMSI contains a 3-digit country code (or Maritime Identification Digit - MID) and an individual station number. Some countries have more than one MID or even a series of MID codes. All MID codes start with a number in the range 2 to 7 which can roughly be associated with a certain world region.

There are four kinds of maritime mobile service identities:

 00 MID xxxx

 MID xxx xx0

 0 MID xxxxx

 00 MID xxxx

 Country         MID              MMSI example    Station 

 Great Britain   232 - 235        00 232 0011     Solent coast guard shore station
 Denmark         219              219 xxx xx0     danish ship station     
 Italy           247              0 247 xxxxx     italian group MMSI
 USA             366              00 366 9995     Chesapeake coast guard shore station  
 Australia       503              503 xxx xx0     australian ship station
 Germany         211, 218         00 211 3200     group of all german coast stations 
                                                  at the baltic sea coast

For DSC compliant radio stations on board of pleasure yachts, owners can obtain an MMSI assignment from the telecommunications authorities or ship registry, usually by obtaining or amending the ship station license.

Technical Details

Through the unique MMSI number, each DSC radio station can be selectively called. This selective calling procedure requires a signalling process which will involve all reachable radio stations in the vicinity of the "calling" station. This signalling process is conducted on a dedicated "DSC call channel" and cannot be "heard" directly with the standard DSC radio receiver. The digital signalling prefix is generated and transmitted by the calling station and received and decoded by all other station. But only the addressed station will further process the transmitted message. For distress and emergency calls, DSC also features a broadcasting ("to all ships") or a group call service.

For receiving and decoding incoming signalling prefixes, a DSC radio station will have at least one "watch receiver", which continuously evaluates the (digital) signalling information sent over the DSC call channel. The evaluation basically consists of restoring signal integrity and filtering the information addressed to the associated radio station. The ITU has published details of the DSC signalling in the "specification of the digital selective-calling system for use in the maritime mobile service".

The signalling information is generated and analysed by the controlling units of the DSC radio station. The digital information is transmitted over a DSC call channel using an FSK modulation scheme with a 1700 Hz sub-carrier and a data rate of 1200 baud in the VHF band and 100 baud in the MF/HF bands. The frequency shift (frequency difference between "mark" and "space") for the digital modulation is 170 Hz on MF and HF channels and 800 Hz on the VHF channel.

The DSC call channels and frequencies

The ITU has allocated the following DSC distress and safety channel in the MF, HF and the VHF marine radio bands:

  MF/HF  DSC
  DISTRESS AND SAFETY CHANNELS  

   2187.5 kHz
   4207.5 kHz
   6312.0 kHz
   8414.5 kHz
  12577.0 kHz
  16804.5 kHz

  VHF  DSC
  DISTRESS AND SAFETY CHANNEL  

  156.525 MHz (Channel 70)

Note that voice transmissions are absolutely PROHIBITED on the DSC channels. The listed MF/HF DSC channels are restricted to distress, urgency and safety traffic only (no calling procedures!) because of the relatively low speeds of transmission of 100 baud. If too many calls were permitted on the MF/HF DSC channels, the channels would quickly become overloaded to the point where distress calls may be blocked. For routine DSC calls other channels are available (e.g. 2177.0 KHz and 8189.5 KHz in the 2-MHz band).
VHF DSC operates at 12 times the speed of MF/HF - accordingly, all priorities of calls are allowed on the single VHF DSC channel.

Legal Issues

To participate in the DSC system a type approved DSC station is required. The owner must apply for a MMSI calling number from the national telecommunications authorities or ship registry and also obtain a ship station license. The MMSI will be programmed into the equipment by the manufacturer before installation.

A special operation permit is required for DSC equipment.