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AIS ADDRESSED SAFETY RELATED MESSAGE (MESSAGE 12)

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. 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

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

 

 

The table below 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.  The probability that a message will be successfully sent and received decreases with each additional slot required:

 

 

Number of slots

Maximum 6-bit ASCII characters

1

10

2

48

3

85

4

122

5

156

 

 

(Source: Rec. ITU-R M.1371-5)