twistedtree
Guru
This is indeed why I changed my VHF to a non-AIS model. Here are the details, and it's another example of how marine electronics vendors are doing a very poor job implementing N2K and creating a bad name for it.
I have an AIS transponder on my boat made by Simrad (NAIS400). When selecting VHFs, I figured I'd pick a model with integrated AIS so I would have a backup receiver if the main transponder ever failed. This is consistent with pretty much every other critical part of my nav system - everything has a backup so I can keep going if any critical component fails. This did not work at all with AIS, and surfaced a number of bugs and/or design oversights in multiple products. Here's what I found:
1) N2K has a mandatory feature called a Device Instance. It's just a number that you assign to devices to distinguish them when they are otherwise sending the same info. If, for example, you have two GPSs, they should be assigned different Device Instance numbers, say 1 and 2. Then, you can tell your chart plotter to use GPS #1, and it knows which one to listen to. If your GPSs are in sufficiently different locations on your boat, they will be reporting different positions. A chart plotter that can't tell them apart is going to get confused because depending on which it hears at any given time, your boat will seem to be jumping around. Plus, if GPS #1 fails, you can tell the plotter to switch to GPS #2. It's a really useful and important aspect of N2K, but most vendors manage to screw it up in one way or another. After this pre-amble of Device Instance info, the NAIS400 does not let you change the device instance. Even though it's a NMEA 2000 Certified device, it violates the spec in this fundamental way. Step one for having multiple AIS receivers on N2K is to be able to distinguish them from one another, and so far we are not off to a good start.
2) I'll skip over my attempt to use Simrad RS35 VHFs, three of which were returned to the manufacturer stone dead after less than 24hrs of operation.
3) The Simrad VHFs were replaced by ICOM M506 AIS models. These were unable to properly discover the available GPSs on the N2K network, let alone select which to use for position for DSC. A call with ICOM support, who were very good, by the way, revealed that I needed V1.002 firmware, so I got the VHFs updated. Now they were able to discover and select a GPS on the network. So that problem was solved.
4) Now, with 3 AIS receivers on N2K, I needed a way to tell my chart plotter and radar which one to use. The usual way to do that is to assign Device Instance numbers to the three AIS devices, then go to the listening devices and tell each which AIS to use. Really clever listening devices can be given a priority list of sources, and will always pick the highest priority device that's available, and automatically work down the lists in the event of failures. Now recall that the Device Instance can't be changed in the NAIS400, but being the clever sort that I am, I left it at zero and changed the VHFs to 1 and 2. But now I discovered that none of the listening devices have the ability to select which AIS to use. And I can't tell whether they are listening to all of them and processing three copies of every position report, or if they are somehow picking one of the three to listen to. It's anyone's guess. The good news is that nothing seemed to blow up, so I figured I'd just let it go. Wrong.
5) With AIS there is the whole issue of your own ship's position report, and how to filter it out so you don't always appear to be crashing into yourself. As best I can tell there is no standard or convention for who does the filtering. It appears that the NAIS400 transceiver filters and does not issue your own ships report on N2K, but the VHF AIS receivers don't filter it and instead report your own position on N2K. Coastal Explorer can't filter my own AIS reports because it doesn't know my MMSI number, so it starts complaining that I'm about to crash into myself any time one of the VHFs is turned on. The Simrad chart plotter appears to filter the reports sent by the VHFs because it does not report imminent death, and it does know my MMSI number. Undaunted by this, I figured I'd just disable the AIS reports in the VHFs. No-go. There is no such feature.
6) Because there is no means to turn off AIS reports from the various receivers, ship's position reports come out in duplicate or triplicate, resulting in a lot of excess traffic on the bus. And AIS reports are long and frequent, so they do tend to gum up the works.
It would appear that in specifying how AIS is carried over N2K, the standards folks failed to consider more than a single device, and failed to be clear on what filtering of own-ships reports is required and where. I think the crap is really going to hit the fan as these AIS-equipped VHFs start finding there way into everybody's boat, and pretty soon you have more than one whether you want it or not.
I have reported all this to NMEA, ICOM, and Simrad. I would urge anyone buying product to check with your preferred vendor and ask them whether the product supports Device Instancing (they probably won't know what you are talking about), and whether the AIS repots on N2K can be disabled. If you get the wrong answers, tell the vendor why you are not buying their product.
In the mean time, I'd suggest only one AIS receiver on N2K.
