Available downloads

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, §

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)