The addressed safety related message could be variable in length, based on the amount of safety related text. The length should vary between 1 and 5 slots.
|Parameter||Number of bits||Description|
|Message ID||6||Identifier for Message 12; always 12|
|Repeat indicator||2||Used by the repeater to indicate how many times a message has been repeated. See § 4.6.1, Annex 2; 0-3; 0 = default; 3 = do not repeat any more|
|Source ID||30||MMSI number of station which is the source of the message.|
|Sequence number||2||0-3; see § 5.3.1, Annex 2|
|Destination ID||30||MMSI number of station which is the destination of the message|
|Retransmit flag||1||Retransmit flag should be set upon retransmission: 0 = no retransmission = default; 1 = retransmitted|
|Spare||1||Not used. Should be zero. Reserved for future use|
|Safety related text||Maximum 936||6-bit ASCII as defined in Table 47|
|Maximum number of bits||Maximum 1 008||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 the length of the message should not exceed 1 slot
Additional bit stuffing will be required for this message type. For details refer to transport layer, § 5.2.1, Annex 2.
Table 62 gives the number of 6-bit-ASCII characters, 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 characters to the numbers given, if possible:
|Number of slots||Maximum 6-bit ASCII characters|
These numbers also take bit stuffing into account.