the core of AIS is the concept of Self Organizing Time Division Multiple Access (SOTDMA) which is used for Class A de-vices. Each frame (one minute long) is divided into 2250 slots
which gives 4500 slots as AIS signals are transmitted on two frequencies, AIS1 (161.975 MHz) and AIS2 (162.025 MHz). Based on previous traffic history, each station when transmitting re-
serves a future slot for its next message. Messages may be longer than one slot. To avoid overlap stations continuously synchronize to each other (see fig. 4).
Base stations use another ac-cess method, FTDMA (Fixed TDMA) with fixed slots. Class B equipment use CSTDMA (CarrierSense), an adaptation of the Ethernet collision sense access