Hi Paul, 

I think there may be some terminology mixed up in your first sentence.

There are actually two classes of AIS transponders - class A and class B.  Class A is usually used by commercial vessels, costs a lot more, and usually updates more frequently. Most recreational boaters use Class B AIS.

For VHF radios, there are classes of DSC (digital selective calling) from A to E - you can read more about the levels at https://www.navcen.uscg.gov/?pageName=DSCClasses . Most recreational boaters use Class D DSC VHF radios.

The standalone Garmin VHF 215 AIS will only allow you to receive AIS, not transmit your location/AIS details. It uses the VHF antenna that you connect it to and listens using an AIS receiver, grabs the various AIS data from the airwaves, and converts it into NMEA 2000/0183 so you can see ships on other devices on your boat, including your chart plotter. 

A Garmin VHF + the Garmin AIS 800 would allow both transmit and receive, plus normal VHF radio capabilities. If you already have a VHF radio, you could just install the Garmin AIS 800 and it would both transmit and receive AIS data onto your NMEA 2000 network.

It's good to note that AIS transponders are not as finicky as many other pieces of marine electronics. You can use other vendors AIS transponders on NMEA 2000 networks with very little difference in setup challenges. In fact, many other manufacturers have some additional options such as WiFi, screens, etc. 

Thank you Steve, for a clear explanation! 

Hi Paul, I agree with all that Steve wrote, but there is a nuance of note. There are now two distinct Class B AIS technologies -- Carrier Sense (CS) and SOTDMA (SO) -- and the Garmin AIS 800 you're considering is one of the latter. That's why it's quite new and maybe not quite available yet, and why it's a little more expensive than many older CS type Class B transceivers. (While Garmin's older AIS 600 cost about the same, it includes an active antenna splitter that will cost about $200 as a separate item).

I tried to explain the differences between CS and SO here...

https://panbo.com/sotdma-class-b-ais-the-new-middle-way/

... but essentially the faster SO transmit rate doesn't apply if you don't regularly cruise over 14 knots, though SO transmissions may also get sent more frequently in heavily AIS congested areas regardless of boat speed because the SO protocol is less "polite" about demanding transmission slots.

Ben -  the new AIS800 does have a splitter built in to the chassis, just as the AIS600 did/does.   I have one sitting on my desk right in front of me.  Garmin has only recently started shipping them and the backorder is loooong.   It is also about half the size of the 600.  And while many sailboats won't get over that 14kt transition speed, the 5w power output of the SOTDMA AIS is great.  Unless I am forced to use a CS Class B for some reason (ie., insurance will only pay for replacement of an identical unit if available) I don't see myself ever installing a non SO Class B unit again.  

I agree with Jonathan about the transmit power. I wrote about the AMEC SOTDMA transponder here https://sailbits.com/amec-widelink-sotdma-ais/  and one of the main benefits to me on the sailboat was that transmit power. 

I sail in the Puget Sound area near Seattle, specifically Elliott Bay. There are tons of commercial and recreational vessels, and often times I would find my AIS track not as reliable as I would like when reviewing logs on MarineTraffic later on. The AMEC with its higher transmit power, and SOTDMA timing made that far more reliable. 

I've been particularly interested in em-trak's new SOTDMA line, specifically the B360 which not only has the 5W transmit, SOTDMA, but also an integrated GPS antenna and WiFi. Not having to run a new cable for GPS is super convenient, and I'm glad to see em-trak, Vesper, Digital Yacht and others start offering that option.