which operates using SOTDMA. Targeted at large commercial vessels, SOTDMA requires a transceiver to maintain a constantly updated slot map in its memory such that it has prior knowledge of slots which are available for it to transmit. SOTDMA transceivers will then pre-announce their transmission, effectively reserving their transmit slot. SOTDMA transmissions are therefore prioritised within the AIS system. This is achieved through 2 receivers in continuous operation. Class A's must have an integrated display, transmit at 12.5 W, interface capability with multiple ship systems, and offer a sophisticated selection of features and functions. Default transmit rate is every few seconds. AIS Class A type compliant devices receive all types of AIS messages. Class B There are now two separate IMO specifications for Class B transceivers (aimed at lighter commercial and leisure markets): a carrier-sense time-division multiple-access (CSTDMA) system, and a system that uses SOTDMA (as in Class A). In the original CSTDMA-based system, defined in ITU M.1371-0 and now called Class B "CS" (or unofficially as Class B/CS), transceivers listen to the slot map immediately prior to transmitting and seek a slot where the 'noise' in the slot is the same (or similar) to background noise, thereby indicating that the slot is not being used by another AIS device. Class B "CS" transmits at 2 W and is not required to have an integrated display: Class B "CS" units can be connected to most display systems where the received messages will be displayed in lists or overlaid on charts. Default transmit rate is normally every thirty seconds, but this can be varied according to vessel speed or instructions from base stations. The Class B "CS" standard requires integrated GPS and certain LED indicators. Class B "CS" equipment receives all types of AIS messages. The newer SOTDMA Class B "SO" system, sometimes referred to as Class B/SO or Class B+, leverages the same time slot finding algorithm as Class A, and has the same transmission priority as Class A transmitters, helping to guarantee that it will always be able to transmit. The Class B "SO" technology will also change its rate of transmission depending on the speed the vessel is going, up to every five seconds over 23 knots, instead of the constant rate of every thirty seconds in Class B "CS". Finally Class B "SO" will also broadcast at a power of 5W instead of the previous 2W of