Understanding AIS Transmission Modes: FATDMA vs RATDMA vs SOTDMA

In many marine navigation projects, AIS equipment is selected based on communication range and price.
However, one key technical factor is often overlooked: the transmission mode.

Different AIS transmission modes determine how time slots are assigned and how reliably a device can broadcast its messages.
This directly affects system performance, especially in remote or infrastructure-limited environments.

FATDMA operates based on pre-assigned time slots.
The transmission schedule is typically managed by a shore station or a central configuration system.

From a system perspective, this means:

• Transmission depends on external coordination

• Operation is stable when shore infrastructure is available

• Autonomous broadcasting is limited when shore systems are unavailable

FATDMA is suitable for controlled environments such as ports and coastal areas with established AIS base stations.

RATDMA allows a device to randomly select available time slots for transmission.
It is mainly used during initial access or in low-density communication environments.

From an application perspective:

• It supports temporary or low-traffic operation

• It does not guarantee long-term stable slot occupation

• Collision risk increases as traffic density grows

RATDMA is best considered as a supplementary or transitional transmission method rather than a primary operating mode.

SOTDMA enables a device to autonomously organize its own time slots by analyzing channel activity.
This allows stable and continuous broadcasting without relying on shore-based slot management.

From a project standpoint, SOTDMA provides:

• Autonomous operation without shore infrastructure

• Higher reliability in offshore and remote deployments

• Reduced risk of data loss when network conditions are unstable

This mode is particularly valuable for navigation buoys, offshore markers and monitoring platforms deployed outside dense coastal networks.

Two AIS devices may appear similar in range, appearance and price, yet behave very differently in real applications depending on their transmission mode.

If a device only supports FATDMA, its operation relies heavily on shore-based systems.
If a device supports SOTDMA, it can continue broadcasting independently even when shore stations are unavailable.

In practical terms, this affects:

• System reliability

• Infrastructure dependency

• Maintenance strategy

• Risk of message loss

• Long-term operational stability

Therefore, price alone does not reflect true system capability.
Understanding transmission modes allows project teams to evaluate AIS equipment based on functional performance, not just on cost.

AIS is not only a hardware product, but part of a communication system.
Transmission mode defines how that system behaves in real environments.

For navigation and offshore projects, selecting the appropriate transmission mode is essential to ensuring reliable operation over the entire service life of the equipment.

A clear understanding of FATDMA, RATDMA and SOTDMA helps project owners and system integrators make informed decisions and avoid under-specification during procurement.