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Digital Selective Calling is the fundamental reason why
the GMDSS system is different from what has been
available until now. Prior to DSC, most radio calls were
initiated by making a voice call to the desired station,
and relying on somebody at that station hearing the call
and then responding to it. All too often the sheer volume
of calls made this an impossible task. On a summer
afternoon in popular sailing areas it was not unusual for
there to be over 400 calls per hour on VHF channel 16. At
best, this made it very tedious, and often impossible, to
hear calls meant for you, or more importantly, for
anybody to hear a distress call. Clearly, something had
to be done.Initially a partial answer was found by shifting some of the calling traffic off channel 16. In some areas Coast Stations stopped monitoring channel 16 and listened for calls only on their working channels. Similarly, calls to marinas were stopped on 16 and moved to other designated channels. That relieved some of the pressure, but now there was no single channel that everybody was listening to, so vessels missed calls meant for them and distress calls sometimes went unheeded. On board commercial ships, the problem was even worse. They were not only expected to monitor channel 16, but also 13 for bridge-to-bridge calls, 2182 MHz on MF, and if on the high seas, one or more HF distress frequencies. What a headache - literally! The salvation lay in DSC. The DSC control unit monitors all the required frequencies, and lets the operator know when there is a call for his station, or when there is a distress or other urgent call. Best of all, it does all this in total silence. No longer are we subjected to the continuous cacophony on channel 16 nor the static crashes and bangs on 2182 MHz. Instead, we wait for the phone to ring, as it were. |
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How DSC Works |
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How is this miracle achieved? The DSC control unit is somewhat like a pager which alerts the receiving station that there is traffic waiting for them. The name, Digital Selective Calling, goes a long way to explaining how it works. DSC uses a Digital signal to send a message, via the sending station's radio, to alert the receiver that there is traffic for them, and to tell them where and how to receive the traffic. This DSC message is termed a DSC Call, or sometimes a DSC Alert. The digital message includes several pieces of information which will be displayed on the receiving station's DSC control unit: 1. The Maritime Mobile Service Identity of the sending station. The MMSI is a nine digit number, and it is the DSC equivalent to the call sign, or the 'phone number' of a station. It is issued as part of the GMDSS station license. 2. The MMSI of the station being called. This can be an individual vessel or Coast Station, a specific group of vessels, or in the case of distress, urgency or safety traffic, to all stations. 3. The priority of the call - distress, urgency, safety or routine. DSC can be used for setting up any type of call, from a Mayday to making a phone call home. 4. For distress calls, the DSC alert can include the nature of the distress, e.g. fire, sinking, explosion, pirate attack etc. 5. For distress calls the position of the vessel is normally included. If the control unit is interfaced to a Global Positioning System receiver, then the position will be added automatically. If there is no GPS receiver connected to it, then the position, and the time that the position was valid, must be keyed in. 6. The requested working frequency and mode of transmission for the traffic. The DSC is only used to set up the call; the actual communication is conducted on another radio channel - distress or working channel as the case may be - and it can be conducted in either voice or telex mode, depending on circumstances. DSC calls are Selective, because as we have seen above, they can be directed to a selected station or stations. Previously virtually all calls were received by all stations who were in range of the transmission. However, the DSC control unit looks at each call that it receives and determines if the call is addressed to it specifically, to it as part of a group, to all stations, or to vessels in the area where the receiving station is located. When such a call is received then the operator is alerted, usually by a buzzer and an indication on the screen. The information as to where and how the traffic will be sent is displayed on the screen. All other calls are simply ignored. The last part of the name sums up what DSC is about. It is used for calling another station to advise them that there is traffic for them, and to what channel or frequency they should tune their radio to receive this traffic. When a DSC call is transmitted by VHF, it is sent in F1B modulation, at a speed of 1,200 baud. At this speed, all the data in a DSC Alert can be transmitted in about half a second. On MF/HF things happen just a little slower. The modulation is J2B and the speed of transmission is 100 baud. This results in a DSC Alert on MF or HF taking between 6 and 7 seconds to be transmitted. The DSC control unit transmits the DSC calls through the ship's VHF and/or MF/HF radio. Although installations and equipment obviously vary, the control unit is normally also connected to a dedicated receiver which monitors the DSC frequencies for incoming DSC calls, regardless of to which frequencies the ship's radios are tuned. |
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