Posted by Yachtbits Blogger 30/12/2018 0 Comment(s)

Class B AIS now has two formats SOTDMA and CSTDMA, here follows a basic comparison


The two formats stand for Self Organised Time Division Multiple Access and Carrier Sense Time Division Multiple Access


In simple terms, the SOTDMA device claims a transmission slot and reseves it for itself regardless of any other vessels, this is the same as Class A AIS found on large commercial vessels. 


The CSTDMA device claims its transmission slot but is not guaranteed it. If a Class A transceiver comes along and attempts to claim that particular slot, the CSTDMA device will lose priority and has to find another slot, which may mean a missed transmission and upto 1 minute and 30 seconds before it transmits again.


That is the fundamental difference between the technologies, there are further technical differences which are listed in the tables below. 


In reality, new CSTDMA transceivers are phasing out. 


Transmit Power5W2W
Transmit RateUp to every 5 secsEvery 30 secs
Guaranteed Time Slot AllocationYESNO



Ship’s Dynamic ConditionsSOTDMACSTDMA
Ship at Anchor or Moored3 mins3 mins
SOG 0-2 knots3 mins3 mins
SOG 2-14 knots30 secs30 secs
SOG 2-14 knots and changing course30 secs30 secs
SOG 14-23 knots15 secs30 secs
SOG 14-23 knots and changing course15 secs30 secs
SOG > 23 knots5 secs30 secs
Ship Static Information6 mins6 mins

Note on Transmitted power, Watts do not relate proportionally to range, lots of factors affect transmission range. Typically you would assume 8NM for 2W and 12-14NM for 5W.