Addressed safety related message

msg_ID: 
12
Description: 
Safety related data for addressed communication
Priority: 
2
Access scheme: 
FATDMA
RATDMA
ITDMA
Communication state: 
N/A
Transmitted by: 
Mobile station
Base station

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.

 

TABLE 61
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
1 10
2 48
3 85
4 122
5 156
 

 
These numbers also take bit stuffing into account.