TDMA

Time-Division Multiple Access. AIS uses TDMA across two dedicated VHF frequencies – AIS 1 (161.975 MHz) and AIS 2 (162.025 MHz).

Although Class A and Class B both use TDMA, Class A units use a Self-Organised TDMA (SOTDMA; Self-Organizing amongst Class A's) whereas Class B units use CSTDMA (Carrier-Sense TDMA, polite to Class A's). Class B's therefore determine free slots in the VHF wavelengths and decide when to transmit a report, enabling a ‘politeness’ factor, so as not to interfere or cause degradation of Class A signals.

The transmission of signals and information varies from every 2 seconds to every 3 minutes depending on the speed of the vessel; more signals at higher speeds.