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Author Topic:   AIS Class-A and Class-B Differences
jimh posted 02-28-2009 11:58 AM ET (US)   Profile for jimh   Send Email to jimh  
In some technical literature published by CML, a micro-circuit chip manufacturer in the United Kingdom, additional details about the differences between Class-A transponders and Class-B transponders of the Automatic Identification System (AIS) are clearly presented for readers with an interest in the protocol. The CMP white paper on AIS is available on-line and as a PDF document:

In the CML white paper I found a mention for the first time of the unusual nature of the Class-B radio transmission protocol, which differs from the protocol used in Class-A transponders. The Class-A protocol uses precision timing data from global navigation satellite systems to frame the time windows of the self-organizing time division multiple access (SoTDMA) protocol used to avoid data transmission collisions. In Class-B transponders, a carrier-sensed time division multiple access protocol is used, and timing can be derived from the framing of Class-A signals. Class-B transponders always listen before transmitting so as to minimize their interference with Class-A signals.

I found this information interesting, and I wish to pass it along to other readers who may share a desire for deeper understanding of the underlying technology of AIS devices.

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