Differences between Class A and Class B Transceivers

Describing the differences between messages from Class A and Class B AIS transceivers.


There are two classes of AIS transceivers onboard ships: Class A and Class B. On a high level, the ways in which they differ include: 

  • Transmitted AIS Message Types
  • AIS Channel Access Methods 
  • AIS Channel Access Priority (based on channel access method)

Transmitted AIS Message Types

Read our Learning about Spire's available AIS messages article for more about the Message Types Spire collects and which classes report them.

AIS Channel Access Methods

Class A transceivers reserve their time slots for AIS transmission via Self Organized Time Division Multiple Access (SOTDMA). They first perform a scan of the area to ascertain which slots have been taken by other vessels and reserve an empty slot. Any transmissions in the newly reserved slot will contain information notifying other nearby AIS devices that the transceiver intends to continue using this slot.

Class B transceivers are permitted to transmit via Carrier Sense Time Division Multiple Access (CSTDMA). Unlike SOTDMA, slots are not reserved. They instead simply scan for available space and transmit when a free one is determined to be available.

AIS Channel Access Priority

Transmission priority is given to Class A transceivers which use SOTDMA since they reserve time slots. The timing of Class B transmissions via CSTDMA must work around the time slots reserved by Class A transceivers. If a Class B transceiver is unable to find an empty space, their transmissions are delayed.