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Class A And Class B Automatic Identification System (AIS)

AIS information is divided into two classes – classes A and B – depending on the AIS transponder transmitting the AIS information. These classes are of great importance to the capabilities of your AIS. There is a great difference between the two classes, both in terms of extent, complexity and price.

AIS information from a class A transponder will always be prioritised and, thus, be shown to other ships in the area, whereas AIS information from a class B transponder will not be shown until or if there is room on the AIS channel. AIS of class A In order to avoid that the ships’ AIS systems all speak at the same time, large ships use an AIS system of class A, which is called SOTDMA (Self-Organized TDMA).

An algorithm ensures that the AIS transmitter of a ship first notices how other ships transmit their messages and, subsequently, adjusts its own transmission pattern to that of the others. In case there are more ships fitted with AIS of class A in an area than the capacity of the bandwidth, the system will automatically limit the area of coverage so that the remotest ships in the area are discarded. AIS of class B Small vessels fitted with AIS, such as recreational craft, can use a less expensive AIS station of class B, which transmits less frequently. This system is called CSTDMA (Carrier Sense TDMA). A class B station will listen for a couple of milliseconds to hear whether a large ship is transmitting before it transmits its own message.

Some rather old AIS stations of class A can see only the position, but not the identity of these class B stations – and large ships can choose not to show AIS stations of class B on their displays if their displays become confused because of too many yachtsmen. Thus, the class B information will be shown to other ships in the area only if or when there is room on the AIS channel. On an AIS transponder of class B you can see whether the information transmitted is prioritised and thus displayed on other ships.

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