DSC stands for "Digital Selective Calling". In a nutshell, it's a way for VHF radios to hail each other automatically, without having to do it by voice.

Courtesy ICOM

Why should you care?

Consider this typical VHF scenario: You call out on the radio for another boat, maybe many times, on an agreed-upon channel (in the San Diego area, the usual channel for private boaters is 72). After the other boat acknowledges, you transmit your messages and sign off. If you want a little privacy, you might switch up or down to a different channel to continue the conversation.

• You can get "stepped on" by other people who are also trying to call on the same channel
  
  
• Everyone knows who you are calling. Switching channels up or down requires you to establish a secret offset with the other person ("When I say switch up 2, I really mean down 5").
  
  
• If the other person is not answering, you have to repeatedly call until they answer. In an emergency, you may not be able to do that.

So what's the solution?
DSC, of course....sort of. Here's how DSC works:

Courtesy Standard Horizon

• Every fixed mount marine VHF radio made in the US since June 17, 1999 must have DSC. DSC is part of an international treaty to improve "Safety Of Life At Sea" (SOLAS), and will eventually replace the aural (voice) hailing process, even for emergency communications.
  
  
• Upon request, the FCC (or its authorized representatives) assigns a 9 digit unique "Marine Mobile Service Identity" or MMSI to the vessel that the radio is installed in.
  
  
• The operator (that's you!) keys the MMSI into the radio and saves it.
  
  
• When you want to call someone using DSC, you switch your radio to the station you want to use.
  
  
• You then enter the MMSI of the vessel that you want to call into your radio (most fixed mount VHFs now have a "phone book" feature for frequently called numbers), then you press a "Hail" or "Send" button.
  
  
• Your radio will automatically start hailing the other vessel, using the MMSI you selected, on channel 70. This is done DIGITALly (no voice involved), and only a radio with the desired MMSI will answer (the "SELECTIVE" part). When the other party's radio receives and acknowledges the CALL, both radios beep or set off an alarm, and switch to the desired station.
  
  
• You then hold a regular VHF radio conversation with the other vessel like you did before.

Note that the radio-to-radio digital hailing process goes on behind the scenes, with no intervention from the operator, until the two radios connect with each other. As part of the DSC standard, if you have your GPS hooked up the radio, your position is also transmitted to the other vessel.

All DSC radios have an emergency call button. If you hit the emergency call button, the radio changes to channel 16, and sends out an emergency hail using DSC on channel 70. Any VHF with DSC enabled will answer the emergency hail and switch to 16. No need for you to repeatedly make Mayday calls, although you can continue to do that if you want. And for the other vessel, there is no need to monitor channel 16 for emergency transmissions.

Common Misconceptions about DSC

• DSC is secured communication over VHF:
  This is a common misconception. Here's why DSC is not secure:
  • There is nothing in the technology to prevent someone from plugging someone else's MMSI into their radio, and pretending they are the other vessel.
    
    
  • While the hailing process is done without voice, and radios only answer when their MMSI is requested, the subsequent communications are done using regular voice channels. These communications are not scrambled in any way by DSC (although you can get scramblers for VHF).
• 
  Courtesy RayMarine
  Registering a DSC radio costs money: This used to be true, but not anymore. For recreational boaters, Boat U.S. and Sea Tow do it for free. See these websites for an application:
  http://www.boatus.com/mmsi/ or
  http://www.seatow.com/boatingsafety/mmsiinfo.htm
  
  
• The USCG does not monitor DSC, so why bother with it?
  The long term goal of DSC is to eliminate the need for voice hailing, and its implementation is a requirement under law.
• As a result, the USCG has been running a modernization project called "Rescue 21" which will implement a DSC monitoring system along the entire US coastline, including Hawaii and Alaska.
  
  
• Rescue 21 is a big project which will take years to complete (the current schedule calls for completion by 2010). Because it has taken so long, there have been rumors that the system will never become operational. This is not true.
  
  
• As of this writing, the USCG monitors DSC over most of the East Coast, Gulf Coast, and the Pacific Northwest. Of particular interest to BTB readers, the DSC monitoring stations for Los Angeles and San Diego will become operational in October, 2007 (as in NEXT MONTH).

Why Isn't DSC Used Very Much?

Between delays on Rescue 21, and confusion over features and functionality, DSC has gotten a bad rap. It has been criticized by some as being a useless feature that the government has forced on people for no reason; that it increases the cost of a radio without adding any value. Hopefully, you can see from the above that there is value in using DSC.

In our view, however, VHF radio manufacturers need to find a more user-friendly interface for entering and retrieving MMSIs; perhaps something more like a cellular phone. That will come in time, we're sure.

Why You Should Use DSC...

• In an emergency, the automated features of DSC calling could be a life saver. If you are injured, the radio will continually send out a hail until it receives an answer. If you have your GPS hooked up to the radio, it will automatically transmit your coordinates as well.
  
  
• If you have a select number of people you want to call while on the water, DSC is a convenient way to call someone without alerting the world - perfect if you want to share that secret WFO bite . You just have to make sure you have their MMSI number, and it is stored in your radio. Then chose a channel that is not in common use before you ring them up.
  
  
• Commercial vessels in international waters are no longer required to monitor Channel 16. So it you are about to be run over by a freighter while fishing offshore, hailing them on Channel 16 may not work.

A BTBFF Proposal....

If you have an MMSI number, why don't you post it on the DSC thread on our discussion forum. Add other folk's numbers to your radio, then give it a try when on the water...

More resources:

The USCG Navigation Center: http://www.navcen.uscg.gov/marcomms/gmdss/dsc.htm

The USCG Rescue 21 Project: http://www.uscg.mil/rescue21/home/index.htm

The US Power Squandron VHF/DSC Tutorial: http://www.usps.org/e_stuff/seminars/vhf_sem.htm