The operation of Class B ‘CS’ (Carrier Sense) transceivers is defined by the equipment and test standard IEC62287-1 Edition 2.
Class B units operate a different system to Class A transceivers, Carrier Sense Time Division Multiple Access (CSTDMA). The unit transmits to find a free time slot and then sends its information in the available space. CSTDMA does not reserve a future time slot, but relies on finding free slots while scanning the slot map. As Class A units take priority over Class B ones, they can reserve a slot resulting in a potential loss of Class B data. As well as transmitting data, CSTDMA devices also receive when to transmit data and information from other ships, AtoNs or base stations. Unlike Class A units using SOTDMA, Class B units don’t reserve future time slots.
CSTDMA type devices are regarded as 'polite' devices as they scan for available slots and send their data into free space. They use both spare time slots and, to utilise the AIS slot map as efficiently as possible, use any available space in slots reserved by class A devices.
Class B units contain the same amount of receivers and transmitters as Class A, but they are used slightly differently. The VHF receivers are designed for CSTDMA; one of the two must be multiplexed with the VHF DCS receiver. The unit also includes an active GPS antenna as standard. As the units are not interfaced with a compass, heading information is rarely transmitted. The main difference between Class A and Class B units is the power output, Class B transmits at 2W, giving a range of between 8-10 miles, whilst Class A transmits at 12.5W giving a much larger range.
The operation of Class B ‘SO’ (Self Organising) AIS transceivers is defined by the equipment and test standard IEC62287-2. This standard has recently been developed by the IEC technical working group and is currently in final draft format. Publication is anticipated during Q4 2012.
Class B ‘ SO’ transceivers use the same access schemes as defined for Class A transceivers in the transmission types section.