If I remember correctly it all depends if you are connection with NEMA 0183 or 2000 which cable you use. Most simply use 0183 so they do not need a converter or backbone.
Just ask at WM or Defender for NMEA
I might add there are known issues with the C120 and AIS
make sure you have the latest SW update. Some messages are not processed correctly even with the update but that is just a nuisance you will be seen and they will see you.
"Please note that while the C-Series Widescreen MFDs are compatible with a number of NMEA 2000
devices (most notably for engine
integration), they are not NMEA 2000
Certified. The complete list of NMEA
2000 Certified Raymarine
products may be found here. Should you be experiencing the reported symptoms, then it would be recommended that the AIS/VHF Radio be interfaced via NMEA 0183 communications
protocol to the MFD."