CSTDMA is defined for Class B AIS stations and permits development of a low cost transceiver that is fully interoperable with SOTDMA transmissions whilst ensuring priority is always given to SOTDMA transmissions.

CSTDMA is defined in ITU-R M.1371-4 Annex 7.

The key elements of CSTDMA operation are as follows:

• TDMA slot timing is determined from the timing of AIS Class A or AIS base station transmissions within receiver range. GPS based timing is not required.
• Stations using CSTDMA continuously monitor the AIS radio channels background noise level. This background level is used as a reference for a received signal strength measurement at the start of each TDMA slot.
• When a transmission is required a TDMA slot is randomly selected and the signal strength at the start of the slot measured. If the signal strength is significantly above the background level the slot is assumed to be in use and the transmission is deferred. If the signal strength at the start of the slot is close to the background level the slot is assumed to be unused and the transmission is made.
• The ‘listen before transmit’ or ‘carrier sense’ scheme works on a slot by slot basis; this limits CSTDMA transmissions to a single TDMA slot. Multiple consecutive slots cannot be allocated using this technique.

The essential hardware requirements to support CSTDMA 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 perform carrier sense measurements.

CSTDMA is used by Class B transceivers to transmit data. It transmits data as follows:

• AIS device scans for available space in the AIS slot map
• Transmits into an available slot
• If no available space is found, the transmission is delayed until space is available
• Repeat

FATDMA is a manually managed TDMA access scheme where AIS devices are pre-configured to use specific TDMA slots for all transmissions. FATDMA is used only for AIS base stations and AIS AtoN stations.  FATDMA is defined in ITU-R M.1371-4 Annex 2, §3.3.4.3.

The key elements of FATDMA 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.
• Stations are configured at installation to transmit in a specific TDMA slot or slots.
• Stations configured for FATDMA operation transmit a Data Link Management message which advises other stations of the FATDMA slot allocations. This blocks these slots from use by any other station in range. For this reason the use of FATDMA is minimised in order to minimise impact on the dynamic behaviour of the AIS network.
• Stations configured for FATDMA operation transmit only into the pre-defined slots.

The essential hardware requirements to support FATDMA are:

• VHF transmitter capable of operating on any AIS channel in the marine VHF band
• GPS receiver to provide a time reference for TDMA timing

Note that no receiver capability is required to support the FATDMA access scheme.

FATDMA is used by AIS Base Stations and AIS Aids to Navigation to transmit their data. It is a managed system and the slots are configured manually when the equipment is installed. The slots allocated to FATDMA transmissions are blocked for use by other AIS equipment.

• An AIS base station broadcasts messates to reserve slots in the AIS slot map for its own transmissions or for a nearby AIS Aid to Navigation
• The Base Station or AIS Aid to Navigation transmits data into the reserved slots periodically

RATDMA is a simple TDMA access scheme available for certain types of data transmission and AIS device types. RATDMA is defined in ITU-R M.1371-4 Annex 2, §3.3.4.2.

RATDMA is used when a station needs to allocate a slot, which has not been pre-announced. This is generally done for the first transmission slot during data link network entry, or for messages of a non-repeatable character (such as transmission of text messages).

The key elements of RATDMA 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 RATDMA uses its internal ‘slot map’ to randomly select a slot that is not currently in use by another station. It does not announce use of this slot, or subsequent slots used for non-periodic transmissions.
• RATDMA is not suitable for periodic transmissions as slots allocated using this technique can not be known by other AIS devices. Use of RATDMA for periodic transmissions by many devices would result in significant data collisions and compromise the integrity of the system.
• RATDMA is used by Class A AIS stations for ‘network entry’. This occurs when a Class A device is first switched on and has not previously announced its own slot allocation using SOTDMA. An initial RATDMA transmission is used to solve this problem.

The essential hardware requirements to support RATDMA 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)

AtoNs using RATDMA behave like Class B transceivers, and transmit and receive data as follows:

• AtoN scans for available slots in the AIS slot map
• Data transmited into available slot
• Receives data from the AIS slot map
• Depending on configration, scans for available slot in the map
• Transmits data into the AIS slot map
• Repeat