AIS Class B - SOTDMA or CSTDMA, what's the difference?

In this blog post we explain the differences between SOTDMA and CSTDMA.

In 2018, IMO launched a new standard for AIS Class B transceivers. Since then, it is possible for an AIS class B to use the same system for access to transport channels as commercial shipping SOTDMA instead of the former CSTDMA.

SOTDMA stands for "Self Organized Time Division Multiple Access" and CSTDMA stands for "C arrier Sense Time Division Multiple Access ".

Forör to firstå differences, you need to know how an AIS works in general. AIS stands for "Automatic Identification System" and was developed for the continuous exchange of position data between ships. AIS uses the UTC time stamp with an accuracy of; a millionth of a second to ensure that each ship has the same timer. Within the AIS system, a minute is a frame and is divided into 2250 time slots. Each position report is sent within such a time slot. This ensures that every single transmission will be seen and not overwritten.

Using the previous CSTDMA, the AIS Class B transceiver must first listen to; The AIS traffic has to find a free slot before the position report is sent. If there is no available slot, CSTDMA must wait for next transmission interval.

With SOTDMA, time slots for next transmissions will be reserved in advance to ensure that the position report will sent out.

A big advantage with AIS Class B with SOTDMA It has 5 Watts of transmitter power, which increases the transmission area where your AIS data will be received.

By reserving future time slots, the next big advantage that class B SOTDMA has been given higher priority within the AIS system than class B CSTDMA.

There is everythingå there are several advantages to choosing an AIS with SOTDMA and it can be especially important if you are moving far from the coast, have a fast-moving boat or are in heavily trafficked areas; the.