ITDMA is used in specific situations to pre-announce the transmission slots for non-periodic messages. ITDMA is defined in ITU-R M.1371-4 Annex 2, §3.3.4.1.

The key elements of ITDMA operation are as follows:

• All stations share a common time reference (derived from GPS time) ensuring they can all accurately determine the start time of each TDMA slot.
• The station transmitting using ITDMA uses its internal ‘slot map’ to randomly select a slot that is not currently in use by another station for its own future use. It uses ITDMA transmission to announce use of this slot.
• ITDMA is used when a station needs to announce a temporary change in the reporting interval of a periodic message, to pre-announce a non-periodic message (such as a safety related message) or during network entry.
• ITDMA is required to support SOTDMA operation; however it is not used as a standalone access scheme.

The essential hardware requirements to support ITDMA are:

• VHF transmitter capable of operating on any AIS channel in the marine VHF band
• Two VHF receivers capable of operating on any AIS channel in the marine VHF band
• Full time decoding of all received messages in order to populate an internal slot map
• GPS receiver to provide a time reference for TDMA timing
• Sufficient memory (RAM) to store a slot map for at least five minutes of TMDA slot allocations (the allocation status for 22500 TDMA slots)