Text using 6-bit ASCII

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 Message 6	Broadcast binary Message 8	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.