An AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. AIS transponders use two different frequencies, VHF maritime channels 87B (161.975 MHz) and 88B (162.025 MHz), and use 9.6 kbit/s Gaussian minimum shift keying (GMSK) modulation over 25 or 12.5 kHz channels using the High-level Data Link Control (HDLC) packet protocol.

Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.

AIS data is transmitted using GMSK modulation, filtered to fit within the confines of a 25kHz channel mask (see image below). Data sent out side of this mask is liable to be lost and anot be received by other AIS devices operating within the AIS system. Transmitting data within this mask, ensures the integrity of the AIS system and enasures all data is received.

In figure 1, the data isd sent within the mask (the lines that surround the radio wave. all of this data will be received by other AIS devices.

In figure 2, some of the data is sent outside the mask, which will not be received by other AIS devices in range.

For 'Class B' operation, the channel mask is specified for a slotted transmission burst, and so includes, and accounts for, the effects of transients at the beginning and end of the data burst.

Figure 1

Figure 2

There are 27 different types of top level messages defined in ITU 1371-4 (out of a possibility of 64) that can be sent by AIS transceivers.

A full list can be downloaded from the download section on the right.

AIS messages 6, 8, 25, and 26 provide "Application Specific Messages" (ASM), that allow "competent authorities" to define additional AIS message sub-types. There are addressed (ABM) and broadcast (BBM) variants of the message.

Addressed messages, while containing a destination MMSI, are not private and may be decoded by any receiver. One of the first uses of ASM's was the Saint Lawrence Seaway use of AIS binary messages (message type 8) to provide information about water levels, lock orders, and weather. The Panama Canal uses AIS type 8 messages to provide information about rain along the canal and wind in the locks. In 2010, International Maritime Organization issued Circular 289 that defines the next iteration of ASM's for type 6 and 8 messages.