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AIS CSDTMA – what is it ?

AIS Class B, as specified within IEC-62287-1, in fact operates slightly differently in the way transmissions are controlled it does not use SOTMA it uses CSDTMA .

Bob Lee, Technical Sales Consultant at Software Radio Technology plc – says

…”Class B vessels using CSTDMA (Carrier Sense Time Division Multiple Access) mainly send fixed length telegrams using time slots not used by Class A vessels. Class A vessels reserve a particular time slot (SOTDMA) and negotiate the use of that time slot with other Class A vessels within radio range.

Class B vessels use any time slot as and when it is available. If no slot is available and a Class B vessel wants to transmit it simply misses out that ‘go’ and tries again after a pre-set time.

Its ability to provide a service therefore degrades gracefully and the user is informed if three potential transmission slots are missed by a warning signal. As soon as time slots become available the Class B unit picks up the reporting pace and goes back into the standard reporting timing.

So the environment is dynamic and depends on such issues as:
– How many Class A and Class B vessels are within radio range of each other
– What the Class A vessels are doing (moored, under way, etc)
– What the mix is of Class A and Class B vessels at that location …”


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