Detailed information about how Class B AIS works

The July 2006 issue of Yachting Monthly featured an article on the Class B AIS system which broadcasts a yacht’s position and other information.

An area of concern for yachtsmen is the issues of preference and whether Class B units might get filtered out in congested areas – i.e., not be seen by ships using the Class A system. So we put the below questions to Bob Lee, Technical Sales Consultant at Software Radio Technology plc – the company who have developed the first low cost AIS class B unit.

What happens in crowded AIS areas? Does bandwidth congest reduce? Is there a priority system? And what would be the max number of vessels in vicinity, given this situation?

Class A vessels use SOTDMA (Self Organising Time Division Multiple Access) to send variable length telegrams, short ones on a regular basis, longer ones (static information like cargo, destination, etc.,) less often. 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 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

Now the maximum number of AIS time slots is 2250. However all this tells you is that, starting at slot 1, in theory 2250 vessels can send messages in sequence before slot 1 comes round again. A Class B vessel can ‘borrow’ a slot when not being used. In addition individual vessel antenna mounting will determine that vessels radio range, which throws another variable into the pot!
However, in general:
– 100 vessels within radio range of each other will be fine.
– 500 vessels within radio range of each other will be fine.
-1000 vessels in radio range of each other may show the occasional degradation to individual Class B unit performance.
– 2000 vessels in radio range will show a greater degradation to individual Class B unit performance.
In all cases Class A performance is protected as far as is practicable and the AIS system is designed to degrade gracefully, not fail catastrophically.
But to re-iterate: the actual AIS performance in a real-life scenario at a particular location all depends on the variables listed above, and it will be dynamic from minute to minute.