DSC is Digital Selective Calling
Also Icom 502 Great radio. Great price.
This link should provide you with some info about DSC and MMSI.
Boat US MMSI
Also here is some reading,
I believe Sea tow is also issuing MMSI numbers from there website.
How's zit work? Good question.
DSC for VHF (there is a spearate DSC protocall for high frequency radio as well) works like this; your radio, which will be connected to a location device, can put out a broadcast which will have attached to it some information about you. The signal will be broadcast (by broadcast I mean it in the true sense of the word, where there is no particular intended receipient) with some attached information. The first and most important part sent out will be your identy, hence the MMSI, and if a suitable device is connected, your location. This information will be sent out digitally. Also, depending on the type of call being made it may send out a second frequency to communicate on). This broadcast will be sent out on low power and the channel that will be used is 70. That's on your end. If your broadcast is received, and this is just for emergency calls, the receiving radio will give an answering response and when it does that both your radio and the other one will automatically be switched to channel 16. From then on there is no real difference between DSC and any other radio.
That's one way it may be used. A second way is to use it to contact someone for other than emergency use. In this case you need to know the MMSI for the radio you are calling. Here you will also use channel 70, as is the case for all DSC contact calls, but you will not be broadcasting to the world in general, you will be sending your signal to the radio that has the MMSI you are calling only, and only that radio will respond to your call. Before you make that personal call you will have designated some other channel, any one other than 16, as the communication channel. If you are able to make contact with the other radio, the one for which you entered the other fellow's MMSI, his radio will, if he has set it up to do so, automatically contacty your radio and both of them will automatically go to the preselected channel, and then you may have your conversation.
A third way the things can be used is in their "All Ships" mode. In this mode you broadcast to the world on 70, first having set a preselected channel for the actual communications, and if anyone bothers to respond to your call both radios will go to that channel and you go on with your business.
So that's basically it, contact is made on 70, calls are sent digitally and your radio will be dientified in the call as well as your location if you have a device connected to give that information. After that depending on the type of call made you will either flip over to channel 16 or to a channel of your choice (preselected before the channel 70 initiating call was made) and you may continue. In no case will you actually talk to anyone on channel 70 - and that's the part that I think confuses a lot of folks.
As it is right now if you are in international waters all ships are required to monitor channel 70, but they are no longer required to monitor channel 16. The Cost Guard still monitors 16 and in many places (but not all) they are now also monitorning 70....