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This message will be variable in length, based on the amount of binary data. The length should vary between 1 and 5 slots. Since the data content of this binary message is defined by the application, Message 8 is an Application Specific Message (click on this link for a registry of recognized Application Specific Messages)
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Parameter
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Number of bits
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Description
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Message ID
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6
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Identifier for Message 8; always 8
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Repeat indicator
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2
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Used by the repeater to indicate how many times a message has been repeated. 0-3; default = 0; 3 = do not repeat any more
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Source ID
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30
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MMSI number of source station
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Spare
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2
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Not used. Should be set to zero. Reserved for future use
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Binary data
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Maximum 968
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Application identifier
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16 bits
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Bit
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Description
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15-6
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Designated area code (DAC). This code is based on the maritime identification digits (MID). Exceptions are 0 (test) and 1 (international). Although the length is 10 bits, the DAC codes equal to or above 1 000 are reserved for future use
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5-0
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Function identifier (FI). The meaning should be determined by the authority which is responsible for the area given in the designated area code
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Application data
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Maximum 952 bits
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Application specific data
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Maximum number of bits
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Maximum 1 008
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Occupies up to 3 slots, or up to 5 slots when able to use FATDMA reservations.
For Class B “SO” mobile AIS stations the length of the message should not exceed 3 slots
For Class B “CS” mobile AIS stations should not transmit
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The table below gives the number of binary data bytes (including application ID and application data), so that the whole message fits into a given number of slots. It is recommended that any application minimizes the use of slots by limiting the number of binary data bytes to the numbers given, if possible:
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Number of slots
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Maximum binary data bytes
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1
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12
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2
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40
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3
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68
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4
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96
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5
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121
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(Source: Rec. ITU-R M.1371-5)
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