Text using 6-bit ASCII

Introduction/Additional information: 

This IFM should be used by a ship or base station to send 6-bit ASCII text telegram to other AIS stations. The text telegram can be sent with either binary message 6, 8, 25 or 26. The acknowledge required flag should not be set when using the broadcast message 8.

Registrant: 
ITU-R.M.1371-5
Message number: 
25
DAC: 
1
FI: 
0
State: 
in force
Number of slots (max): 
5
Permitted as from: 
01/02/2014
Details: 
TABLE 27
International function message 0 using Message 25, broadcast or addressed binary message
Parameter Number of bits Description
Message ID 6 Identifier for Message 25; always 25
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 source station
Destination indicator 1 0 = Broadcast (no Destination ID field used)
1 = Addressed (Destination ID uses 30 data bits for MMSI)
Binary data flag 1 Always 0
Destination ID 0/30 Destination ID if used. If Destination indicator = 0 (Broadcast), no data bits are needed for Destination ID.
If Destination indicator = 1, 30 bits are used for Destination ID and spare bits for byte alignment.
Spare 0/2 Spare (if Destination ID used)
DAC 10 International DAC = 110 = 00000000012
FI 6 Function identifier = 010 = 0000002
Text sequence number 11 Sequence number to be incremented by the application.
All zeros indicates that sequence numbers are not being used.
Text string 6-66/6-96 6-bit ASCII as defined in Table 47, Annex 8. When using this IFM, the number of slots used for transmission should be 1 taking into account Table 29.
For Message 25 the maximum is 66 for Addressed or 96 for Broadcast.
Spare bits Max 7 Not used for data and should be set to zero. The number of bits should be either 1, 3, 5 or 7 to maintain byte boundaries.
NOTE 1 – When a 7-bit spare is needed to satisfy the 8-bit byte boundary rule, the 6-bit spare will be interpreted as a valid 6‑bit character (all zeros is the “@” character). This is the case when the number of characters is: 1, 5, 9 and 13.
Total number of application data bits 112-168/
80-168
112-168 bits for Addressed, or 80-168 bits for Broadcast.
 

 
 

 
TABLE 29
Estimated number
of slots
Maximum number of 6-bit ASCII characters
based upon typical bit stuffing
Addressed binary
Message 6
Broadcast binary
Message 8
Message 25 Message 26
Addressed
binary
Broadcast
binary
Addressed
binary
Broadcast
binary
1 6 11 6 11 2 7
2 43 48 40 45
3 80 86 77 82
4 118 123 114 120
5 151 156 150 163
NOTE 1 – The 5-slot value accounts for the worst case bit stuffing condition.