SOTDMA vs CSTDMA

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.

Function	SOTDMA	CSTDMA
Transmit Power	5W	2W
Transmit Rate	Up to every 5 secs	Every 30 secs
Technology	SOTDMA	CSTDMA
Guaranteed Time Slot Allocation	YES	NO

Ship’s Dynamic Conditions	SOTDMA	CSTDMA
Ship at Anchor or Moored	3 mins	3 mins
SOG 0-2 knots	3 mins	3 mins
SOG 2-14 knots	30 secs	30 secs
SOG 2-14 knots and changing course	30 secs	30 secs
SOG 14-23 knots	15 secs	30 secs
SOG 14-23 knots and changing course	15 secs	30 secs
SOG > 23 knots	5 secs	30 secs
Ship Static Information	6 mins	6 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.