Modified SOTDMA (or Pre-Announced TDMA) is a simple TDMA access scheme defined for use in transmit only devices. It has specific application in emergency beacons such as AIS Search and Rescue transceivers (SART).

Whilst sharing the ‘SOTDMA’ name with the SOTDMA access scheme this technique has little else in common with SOTDMA. Modified SOTDMA is defined in ITU-R M.1371-4 Annex 9 and described for use in “devices that have limited range and operate in a low volume”. Further definition of the access scheme is given in the AIS SART equipment standard IEC61097-14.

The key elements of modified SOTDMA 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.
• A station randomly selects a slot for transmission. In the first transmission it announces its intention to use this slot for the following 8 minute period. At the start of the next period a new slot is randomly selected.
• Transmissions are made in ‘bursts’ of 8 messages once per minute. This is intended to ensure successful transmission when the device is operating near the surface of the sea and may be blocked from reception by periodic swell.
• The system is referred to as ‘modified’ SOTDMA as it uses the same pre-announcement system for future transmissions as the complete SOTDMA scheme.
• As modified SOTDMA randomly selects transmission slots without knowledge of their use by other stations it is likely to create data collisions. This has been deemed acceptable for use in emergency beacons where the cost benefits of the modified SOTDMA system outweigh the impact of data collisions (which are small as emergency beacons do not operate regularly or in high densities).
• Due to the likelihood of random transmissions colliding with transmissions from other AIS transceivers the modified SOTDMA technique is not suited to use in systems installed in large volume, or where large numbers of systems are used in a small area.

The essential hardware requirements to support modified SOTDMA 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 modified SOTDMA access scheme.

PATDMA is designed for Search and Rescue Transponders (SARTs) and transmits data as follows:

• SART transmits data eight times in one minute
• SART transmits safety message every four minutes

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

SOTDMA is the most complex TDMA access scheme defined for AIS and also provides the backbone for autonomous operation of the network offshore.  The complete technical definition of SOTDMA can be found in ITU-R M.1371-4 Annex 2.

The key elements of SOTDMA 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.
• Each data transmission includes indication of the TDMA slot that will be used by the transmitting station for subsequent transmissions. This allows receiving stations to build up a ‘map’ of which slots are in use by which station.
• Each station avoids slots known to be in use by other stations for its own transmissions. This prevents two stations in range of one another using the same slot.
• As mobile stations move from one area to another they encounter new stations with different slot allocations. This may cause the station to modify its own slot allocation leading to a dynamic and self organising system over time and space.

SOTDMA also provides for dynamic and autonomous management of capacity in busy areas. Should a situation occur where all TDMA slots are occupied ‘slot re-use’ rules are applied. This allows the slots occupied by the stations most distant to a particular mobile station to be re-used for its own transmissions. This effectively reduces the size of an AIS ‘cell’ and ensures that position reports from the nearest vessels (which are most relevant to safety of navigation) are not affected.

The essential hardware requirements to support SOTDMA 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)

Designed for Class A devices, SOTDMA transmits data as follows:

• AIS device scans for available slot in the AIS slot map
• Reserves available slot
• Transmits data into the reserved slot and is notifies other AIS equipment of its intention to use this slot for the next transmission