14 September 2012

VHF DSC group MMSI numbers demystified

My sailing club is organizing internal training sessions on how to operate DSC VHF equipment. During the development of the training material it became apparent that even the instructors had a warped view of the capabilities and operation of DSC groups. I'm going to use this post to dispel some of the common misconceptions of DSC groups, and explain how they can be used in practice.

Misconception #1

A DSC group is like a mailing list, in other words a group call works by sending out the MMSI of the parties that you want to speak to. Not true, the DSC system can only send out a single MMSI target ID.

Misconception #2

A DSC group is decided by the sender. Not true, it is up to the receiving party to decide which MMSI group they are part of. Depending on the VHF equipment it could theoretically be part of more than one group, but in practice you can set only one group ID.

Misconception #3

Group calls are private. Not true, like all VHF traffic anyone can listen in on the voice part of the communication.

How DSC calls DO work

Let's start with a small recap on DSC operation and MMSI numbers, as these lay the ground work for MMSI group numbers.

VHF traffic uses a number of distinct channels. Most of these channels are used for voice communications, but in the last two years three channels have been appropriated for digital radio to radio traffic.

Channel 87b and 88b are used for AIS.

Channel 70 is used for DSC, or Digital Selective Calling. As the name implies DSC is about calling other radios, with a certain selectivity. This can range from all radios, for instance if you declare an emergency, to a single radio when you call up an individual radio.

All stations call

Almost everyone that owns a DSC VHF transceiver should be familiar with the "all stations" calls. One reason for this is that red emergency button. The other is the obnoxious wailing coming from your VHF if someone else uses that button, or if the local coast guard broadcasts an all ships weather or safety bulletin.

Here's some ASCII art that shows what happens when a maritime safety warning is broadcasted by the coast guard (assuming that Gizmo, Merrimac and Rainbow Warrior are the only vessels in range of the sender:

From
MMSI          To               Activates 

                          /--> 367412350 = Gizmo
Coast Guard   ALL         |
002442000 --> 000000000 --+--> 244163000 = Rainbow Warrior
                          |
                          \--> 244060807 = Merrimac

In other words, all VHFs know that the call is for them and that they should 'handle' it by doing something because they decide that the 'target' includes it.

Individual call

Most people know that you can call a single DSC VHF by entering their MMSI number. If you have an AIS receiver you know the MMSI of all vessels whose AIS transmissions you are able to receive. It's then a question of entering that MMSI and placing the call.

In ASCII art this looks like this when the Coast Guard calls Gizmo:


From
MMSI          To               Activates

                          /--> 367412350 = Gizmo
Coast Guard               |
002442000 --> 367412350 --+    244163000 = Rainbow Warrior
                          
                               244060807 = Merrimac


Group call

And now you can probably already guess how a group call works. Like all DSC calls it is received by all VHF stations that are within range, but only those stations that have programmed it will show it on their display and warn the operator that they are requested to go to a particular channel.

If Gizmo and Merrimac have programmed 024400001 as their Group MMSI and that number is called by the coast guard the diagram looks like this:


From
MMSI          To               Activates

                          /--> 367412350 = Gizmo
Coast Guard               |
002442000 --> 024400001 --+    244163000 = Rainbow Warrior
                          |
                          \--> 244060807 = Merrimac

The MID in a MMSI 

All MMSI contain a three digit MID (Maritime Identification Digits) that show which country the MMSI originates from. The MIDs are assigned by the ITU. Countries that have large numbers of MMSI vessels use more than one MID. For instance the Cook Islands use 518 whereas the USA uses 366, 367, 368 and 369, and my own MID is from the Netherlands which uses 244, 245 or 246. Thus Gizmo can be seen to be a US flagged vessel whereas Rainbow Warrior and Merrimac are from the Netherlands.

MMSI formats

MMSI have several formats:
<MID><xxxxxxx> = Normal ship MMSI
0<MID><xxxxxx> = Group MMSI
00<MID><xxxxx> = Base Station MMSI

How do you obtain a Group MMSI?

As you can see from the MMSI formats above a Group MMSI contains a MID. It is up to the MMSI assigning office how they go about the assignment. As with more things the way this is done is somewhat culturally defined.

In Australia, the assigning office is the AMSA and they do not register group MMSI. Instead they recommend the following practice: take an existing individual ship MMSI, lop off the final digit (which is usually zero in the USA) and add a zero at the start.

In the USA, where the FCC doesn't even really want to bother with assigning individual MMSI it seems that they also advocate self-assignment using the lop-off-last-digit method. I cannot find a 'definitive' online source for this, but it is suggested in various forums etc.

In the UK, the Ofcom registers group MMSI but they do not publish a list. It appears they have issued a few hundred group MMSI to institutions like the RNLI, various sailing clubs and regatta committees.

In the Netherlands, the Agentschap Telecom issues MMSI. At the moment of writing they have issued two group MMSI numbers, requested by the author.

If you know of the practice in some other country please let me know.

What can you do with a group MMSI?

A group MMSI is useful for any group of boaters where it can happen that someone wants to contact the others in the group, whether for safety or leisure purposes. 

This is especially true if you are in an area where the local authorities have issued a directive that you should continually listen on a particular working channel, and your VHF does not support dual watch or only allows a fixed channel list on dual watch. [If you are from the USA you may find this weird, but in some European countries it is illegal for VHFs to support scanning and/or free channel dual watch.] In such a case a group MMSI can effectively be used to call everyone onto a channel that can be used for routine traffic.

1 comment:

  1. That was very helpful. Thanks so much for the write up!

    ReplyDelete