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Parametric sentences
(See IEC 61162-1 clause 5 Data format protocol)
The sentences beginning with the “\$” (HEX 24) delimiter are the primary means of communication as outlined by IEC 61162-1 and this PAS For further information, please consult clauses 5 and 6.
The maximum number of characters in a sentence shall be 82, consisting of a maximum of 79 characters between the starting delimiter “$” and the terminating delimiter .
A sentence must contain at least one field, with the initial field serving as an address that identifies the speaker and the sentence formatter This formatter indicates the number of data fields, their types, and the sequence in which they are transmitted The rest of the sentence can include either no data fields or multiple data fields.
The number of fields permitted in a single sentence is constrained solely by the maximum sentence length, and all fields should be utilized regardless of whether data for any specific field is absent.
The basic rules for parametric sentence structures are:
- The sentence begins with the “$” delimiter.
- Only approved sentence formatters are allowed Formatters used by special-purpose encapsulation sentences cannot be re-used (See IEC 61162-1, clause 6.2 (table 5).)
- Only valid characters are allowed (See IEC 61162-1, clause 6.1 (tables 1 and 2).)
- Only approved field types are allowed (See IEC 61162-1, clause 6.2 (table 6).)
- Data fields (parameters) are individually delimited, and their content is identified and often described in detail by this standard.
- Encapsulated non-delimited data fields are NOT ALLOWED.
Encapsulation sentences
(New definition not currently in IEC 61162-1)
! The special-purpose sentence structure, marked by the “!” delimiter, serves to convey information when specific data content is unknown ! This approach allows for greater information bandwidth, akin to a modem that transmits data without prior knowledge of how it will be decoded or interpreted.
The basic rules for encapsulation sentence structures are:
- The sentence begins with the “!” delimiter.
- Only approved sentence formatters are allowed Formatters used by conventional parametric sentences can not be re-used (See clauses 5 and 6, and IEC 61162-1, clause
- Only valid characters are allowed (See IEC 61162-1, clause 6.1 (tables 1 and 2).)
- Only approved field types are allowed (See annex B.5 and IEC 61162-1, clause 6.2
- Only Six bit coding may be used to create encapsulated data fields (See annex B.5.)
- Encapsulated data fields may consist of any number of parameters, and their content is not identified or described by this standard.
- The sentence must be defined with one encapsulated data field and any number of parametric data fields separated by the “,” data field delimiter The encapsulated data
The data field must consistently be positioned as the second to last in the sentence, excluding the checksum field, in accordance with IEC 61162-1, clause 5.2.2.
- The sentence contains a “Total Number Of Sentences” field (See annex A.)
- The sentence contains a “Sentence Number” field (See annex A.)
- The sentence contains a “Sequential Message Identifier” field (See annex A.)
- The sentence contains a “Fill Bits” field immediately following the encapsulated data field.
The Fill Bits field shall always be the last data field in the sentence, not counting the checksum field (See annex A.)
This information delivery method should be employed only when essential, specifically under one or both of two conditions, and only when no alternatives are available.
Devices must convey information without knowing the data parameters For instance, the ABM and BBM sentences exemplify this condition, as their content remains unknown to the Automatic Identification System.
Condition 2: When information requires a significantly higher data rate than can be achieved by the IEC61162-1
(4,800baud) and IEC61162-2 (38,400baud) standards utilising parametric sentences.
Encapsulating extensive information reduces the number of overhead characters, such as field delimiters, leading to improved data transfer rates This condition is rarely met; however, an AIS transponder exemplifies this by achieving a data rate of 4,500 messages per minute, thereby generating VDM and VDO sentences.
4 Data requirements of the AIS
A portion of the information broadcast by an AIS unit is obtained from sensors using existing
IEC 61162-1 sentence formatters The sensor data and the existing sentence formatters recognised by the AIS unit are listed in IEC 61993-2 ( See IEC 61993-2 clauses : 6.10.1.1;
7.6.2.3, table 9 – preferred IEC 61162-1 sensor sentences; and 7.6.3.3.)
The new sentence formatters outlined in clauses 6 and 7 address the expanded data requirements that the current IEC 61162-1 formatters do not meet These updated sensor input sentence formatters include ABM, ACA, AIR, BBM, LRF, LRI, SSD, and VSD.
5 Existing IEC 61162-1 sentences for the AIS
Listing of approved sentences as given in IEC 61162-1 that apply Only the sentence header and description to be given here Refer to IEC 61162-1 clause 6.3
GBS GNSS satellite fault detection
GLL Geographic position, latitude/longitude
RMC Recommended minimum specific GNSS data
VBW Dual ground/water speed
VTG Course over ground and ground speed
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6 Additional IEC 61162-1 parametric sentences for the AIS
Listing of the new approved sentences, including structure and notes
ABK - AIS addressed and binary broadcast acknowledgement
The ABK-sentence is generated when a transaction, initiated by reception of an ABM, AIR, or
BBM sentence, is completed or terminated This sentence provides information about the success or failure of a requested ABM broadcast of either ITU-R M.1371 messages 6 or 12.
The ABK process utilises the information received in ITU-R M.1371 messages 7 and 13.
When a VHF Data-link message 7 or 13 is received, or if messages 6 or 12 fail, the AIS unit transmits the ABK sentence to the external application This sentence is also utilized to inform the external application about the AIS unit's processing of the AIR (ITU-R M.1371) message.
The AIS unit interacts with external applications through specific sentences, including AIR (ITU-R M.1371 messages 15) and BBM (ITU-R M.1371 messages 8, 14, 19, and 21) sentences An external application can initiate an interrogation using the AIR sentence or broadcast using the BBM sentence, prompting the AIS unit to respond accordingly Upon processing the request, the AIS unit generates an ABK sentence to report the outcome of the AIR or BBM broadcast process, providing a clear indication of the result.
$ ABK,xxxxxxxxx,x,x.x,x,x*hh
Type of acknowledgement 5 Message sequence number 4 ITU-R M.1371Message ID 3 AIS channel of reception 2 MMSI of the addressed AIS unit 1
The NOTE 1 specifies the distant AIS unit's MMSI involved in the acknowledgment In cases where multiple MMSIs are addressed (as per ITU-R M.1371 messages 15 and 16), the MMSI of the first distant AIS unit mentioned in the message is reported This field remains null for ITU-R M.1371 message types 8 or 14.
NOTE 2 Indication of the VHF Data Link channel upon which a message type 7 or 13 acknowledgement was received An “A” indicates reception on channel A A “B” indicates reception on channel B.
NOTE 3 This indicates to the external application the type of ITU-R M.1371 message that this ABK sentence is addressing Also see the Message IDs listed in Note 4.
The Message sequence number, along with the Message ID and MMSI of the addressed AIS unit, uniquely identifies previously received ABM, AIR, or BBM sentences The generation of an ABK sentence allows for the re-use of a sequence message identifier, while the Message ID specifies the source of the Message sequence number.
ITU-R M.1371 Message ID Message Sequence Number source
6 sequential message identifier from ABM-sentence, (See clause 5, ABM sentence)
7 addressed AIS unit’s message 7, sequence number, ITU-R M.1371-1
8 sequential message identifier from BBM-sentence, (See clause 5, BBM sentence)
12 sequencial message identifier from ABM-sentence, (See clause 5, ABM sentence)
13 addressed AIS unit’s message 13, sequence number, ITU-R M.1371-1
14 sequential message identifier from BBM-sentence, (See clause 5, BBM sentence)
15 no source, the Message sequence number shall be null
0 = message (6 or 12) successfully received by the addressed AIS unit,
1 =message (6 or 12) was broadcast, but no acknowledgement by the addressed AIS unit,
2 =message could not be broadcast (i.e quantity of encapsulated data exceeds five slots)
3 =requested broadcast of message (8, 14 or 15) has been successfully completed,
4 =late reception of a message 7 or 13 acknowledgement that was addressed to this AIS unit (own-ship) and referenced as a valid transaction
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ACA – AIS Channel assignment message
An AIS device can receive regional channel management information in four ways: ITU-R
The M.1371-1 message 22 indicates that a DSC telecommand has been received on channel 70, requiring manual operator input along with an ACA sentence The AIS unit is capable of storing channel management information for future reference, which is utilized based on the actual location of the AIS device When the AIS unit employs this channel management information, it effectively manages the operation of its VHF receiver and/or transmitter.
The ACA sentence is utilized for both entering and retrieving channel management information When transmitted to an AIS unit, it delivers regional data that the unit utilizes for managing its internal VHF radio Conversely, when sent from an AIS unit, the ACA sentence conveys the current channel management information stored within the unit This information parallels that found in an ITU-R M.1371-1 message 22 and is directly associated with the initialization process.
Phase and Dual Channel Operation and Channel Management functions of the AIS unit as described in ITU-R M 1371.
$ ACA,x,llll.ll,a,yyyyy.yy,a,llll.ll,a,yyyyy.yy,a,x,xxxx,x,xxxx,x,x,x,a,x,hhmmss.ss*hh
Time of “inuse” change 9 In-Use Flag 8
Information source 7 Power level control 6 Tx/Rx mode control 5 Channel B bandwidth 4 Channel B 2
Transition Zone Size 2 Region southwest corner longitude - E/W Region southwest corner latitude - N/S
Region northeast corner longitude - E/W Region northeast corner latitude - N/S
The ACA and ACS sentences are interconnected through a binding note The ACS sentence, supplied by the AIS unit, must directly follow the corresponding ACA sentence, sharing the same sequence number Each time the AIS unit generates these sentences, it will increment the sequence number accordingly.
The ACA/ACS pair is established, and the process restarts from 0 after reaching 9 The information in the ACS sentence is unrelated to that in the ACA sentence if their sequence numbers differ When an AIS unit is queried for an ACA sentence, it should respond with the corresponding ACA/ACS sentence pair Additionally, when an external device transmits an ACA sentence to the AIS unit, the sequence number may be null if no ACS sentence accompanies it.
NOTE 2 Range of 1 to 8 nautical miles.
NOTE 3 VHF channel number, see ITU-R M.1084, Annex 4
NOTE 4 Value of 0, bandwidth is specified by channel number, see ITU-R M.1084, Annex 4
Value of 1, bandwidth is 12.5 kHz.
NOTE 5 Value of 0, transmit on channels A and B, receive on channels A and B
Value of 1, transmit on channel A, receive on channels A and B
Value of 2, transmit on channel B, receive on channels A and B
Value of 3, do not transmit, receive on channels A and B
Value of 4, do not transmit, receive on channel A
Value of 5, do not transmit, receive on channel B
NOTE 6 Value of 0, high power
A = ITU-R M.1371 message 22: Channel Management addressed message,
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B = ITU-R M.1371 message 22: Channel Management broadcast geographical area message,
C = IEC 61162-1 AIS Channel Assignment sentence,
This field should be null when the sentence is sent to an AIS device.
The value in NOTE 8 signifies whether the parameters in the sentence are actively utilized by an AIS unit when the sentence is transmitted A value of “0” denotes that the parameters are not in use, while a value of “1” indicates that they are in use This field should be left null when the sentence is sent.
Message
Message: A message consists of 2 or more sentences with the same sentence formatter.
Messages are utilized when conveying related information requires two or more sentences that exceed the maximum length of a single sentence This is applicable only to sentence formatters that are configured with key fields to support multi-sentence messages.
Sequential Message Identifier
This field is essential for recognizing groups of two or more sentences that form a multi-sentence message It increments each time a new multi-sentence message is created using the same sentence formatter and resets to zero when it exceeds the defined maximum value The maximum value, size, and format of this field are specified by the relevant sentence definition in clause 7 This is one of three crucial fields that enable the multi-sentence message functionality.
Multi-sentence Messages
Multi-sentence messages can be sent when a data message exceeds the character limit of a single sentence It is essential to always include key fields that support this capability, which are the total number of sentences, the sentence number, and the sequential message identifier fields Only sentence definitions that contain these required fields are permissible for message formation.
VDM sentences are good examples of how a sentence is defined to provide these capabilities.
Listeners must recognize that a multi-sentence message can be interrupted by a higher priority alert, such as an alarm, leading to the original message being deemed incomplete and requiring re-transmission It is essential for the Listener to ensure that the multi-sentence messages are contiguous.
If an error is detected in any part of a multi-sentence message, the listener must disregard the entire message and be ready to receive it again during the next transmission.
Proprietary Sentences
Proprietary sentences allow manufacturers to utilize specific sentence structures defined by the standard to transmit data that is not covered by approved sentences This typically occurs for two main reasons: first, the data is intended for a specific device from the same manufacturer and is not relevant to the general user; second, the data is utilized for testing purposes before the implementation of approved sentences.
This document is licensed to MECON Limited for internal use at the Ranchi and Bangalore locations, and it has been supplied by the Book Supply Bureau The data provided does not possess the type or general usefulness that would warrant the formulation of an approved sentence.
A proprietary sentence contains, in the order shown, the following elements:
“$” or “!” Hex 24 or Hex 21- Start of sentence
Hex 0D 0A - End of sentence
Proprietary sentences shall include checksums and conform to requirements limiting overall sentence length Manufacturer’s data fields shall contain only valid-character but may include
The use of “^” and “,” is designated for delimiting or representing manufacturer’s data While proprietary data field details are not required for approval submission, it is essential that these sentences are included in the manufacturer’s manuals for reference.
Future additions to Approved sentences
Future revisions of this Standard may enhance or expand existing sentences by inserting new data fields before the checksum delimiter character “*” and the checksum field Listeners should identify the end of a sentence using the and “*” markers instead of counting field delimiters The checksum value must be calculated based on all characters received between “$” or “!” and “*”, regardless of whether the Listener recognizes all fields.
Changes to the Reserved Character List
Changes to the reserved character list are as follows:
! 21 33 Start of Encapsulation sentence delimiter
7F 127 Reserved for future use
Changes to Character Symbol Table
S South; Statute miles; Statute miles/hour; Shaft; Salinity in parts per thousand s Seconds; Six bit number
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Additions to field type summary
Three new data field types have been added.
Variable HEX field h—h Variable length HEX numbers only, MSB on the left.
Fixed Six-bit field ss _ Fixed length Six bit coded characters only.
See table C-1 and figures C-1 and C-2 for field conversions
Variable Six -bit field s—s Variable length Six bit coded characters only.
See table C-1 and figures C-1 and C-2 for field conversions
In addition a new note has been added describing the representation of fixed field types in sentence definitions.
Fixed length field definitions specify the exact number of characters required For instance, a field designated with a fixed length of 5 HEX characters is denoted as hhhhh within delimiters in a sentence definition.
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Six bit binary field conversion
Valid Characters (see IEC 61162-1 Table 2)
Binary Field, Most Significant Bit on the left The two MSB’s of the Valid Characters are not used.
Table C-1 -.Six-bit binary field conversion table :
The six bit binary field conversion can be done mathematically as well as with table C-1.
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The algorithm for converting a 6-bit binary field into a valid 8-bit IEC 61162-1 character field is illustrated in figure C-1 Additionally, figure C-2 demonstrates the algorithm for converting valid IEC 61162-1 characters back into 6-bit binary values.
Figure C-1 - 6-bit binary code converted to valid IEC 61162-1 character
000001 is less than 101000, therefore add 00110000
000010 is less than 101000, therefore add 00110000
111010 is not less than 101000, therefore add 00111000
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Figure C-2 - Valid IEC 61162-1 character converted to 6-bit binary code
00110001 + 101000 = 01011001 which is not greater than 10000000.
Therefore, add 101000 to 01011001 = 10000001 and take the six right bits.
000001 are the six binary bits represented by a “1”.
00110010 + 101000 = 01011010 which is not greater than 10000000.
Therefore, add 101000 to 01011010 = 10000010 and take the six right bits.
000010 are the six binary bits represented by a “2”.
Therefore, add 100000 to 10011010 = 10111010 and take the six right bits.
111010 are the six binary bits represented by a “r”.
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New clause 7 to IEC 61162-1
These examples are intended as samples of correctly constructed encapsulation sentences.
They are representative samples only and show part of the wide range of legal variations possible with sentences They should not necessarily be used as templates for sentences.
7.2.1 AIS VHF data-link message VDM sentence encapsulation example
This standard facilitates the transport of encapsulated binary coded data, emphasizing that accurate decoding and interpretation necessitate information from external sources It outlines the coding, decoding, and structuring of the data, while the specific meanings of the binary data are derived from referenced standards.
What follows is a practical example of how encapsulated binary coded data might be translated into meaningful information The example is drawn from the operation of universal
Automatic Identification System (AIS) equipment built to the ITU-R M.1371 recommendations.
The sample sentence that will be used in this example is:
Number of "fill-bits" added to complete the last six-bit character 5 (0 to 5)
Contents of the ITU-R M.1371 radio message using the 6-bit field type 4
!AIVDM,1,1, ,A,1P000Oh1IT1svTP2r:43grwb05q4,0*01
AIS Channel 3 , (A or B) Sequential message identifier to link multiple sentence messages 2 , (0 to 9) Sentence number 1 , (1 to 9)
Total number of sentences needed to transfer the message 1 , (1 to 9)
NOTES 1-5 See VDM sentence notes.
Also included with this example are:
- A worksheet for decoding and interpreting and encapsulated field,
- Annex E, a copy of Table 15 from ITU-R M.1371-1:2000.
Background Discussion - encapsulation coding
To comprehend the decoding process, it is essential to first identify the origin of the binary bits within the string AIS consists of a series of radio broadcasts utilized in the marine industry.
The VHF band allows AIS units to broadcast multiple messages, with detailed descriptions of these messages provided in documented tables.
ITU-R M.1371 international standard for AIS Annex E is a sample from ITU-R M.1371-
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1:2000 This table identifies all of the information needed to convert the encapsulated binary bits into information The table identifies the bits, gives them parametric names, and units.
The Message Data portion of a larger binary packet, as outlined in ITU-R M.1371, Table 15, is generated and transmitted by an AIS unit Each AIS unit that successfully receives a broadcast produces a VDM-sentence, exemplifying the output of the AIS system Figure 5 illustrates the message data segments of the "radio packet" broadcasted by an AIS unit, highlighting that only the message data bits specified in the relevant tables, like ITU-R M.1371, Table 15, are included in the VDM-sentence string.
Message Data (maximum of 168 bits for one-slot, maximum of 1008 bits for five-slot)
Assume, as an example, that the first 12 bits of the Message Data in Figure 5 (bits 1 to 12) are: 000001100000 These would be the first 12 bits coded into the VDM encapsulate string.
The VDM-sentence utilizes the "Six-bit" Field Type to represent data, where each of the 64 combinations of binary digits (ones and zeros) corresponds to a unique valid character For detailed assignments, refer to "table C-1 - Six-bit binary field conversion table" in annex C.
For example, the first 12 bits would be divided into six bit strings, that is: 000001 and 100000.
Using table C-1, the binary string 000001 can be represented by a "1", and the binary string
The number 100000 is denoted by the letter "P", resulting in the initial two characters of the VDM-sentence encapsulated string being "1P" It is crucial to pay attention to the distinction between upper and lower case letters when referring to table C-1.
The maximum number of Message Data bits that can be contained in an AIS radio message is
A total of 1008 bits translates to 168 Six-bit symbols, exceeding the capacity of a single standard sentence The encapsulation sentence structure enables the division of an encapsulation field into smaller strings, which can be transmitted across multiple sentences It is crucial to note that the encapsulation fields from these multiple sentences, identified by the sequence number and ordered by sentence number, can be recombined into a single continuous encapsulation string.
The string in this example can be conveyed in a single sentence or divided into two sentences It is not necessary to split it at a specific point The following pairs of sentences are equivalent and effectively transfer the same encapsulated string.
!AIVDM,2,1,7,A,1P000Oh1IT1svT,0*58
!AIVDM,2,1,9,A,1P000Oh1IT1svTP2r:43,0*7B
The complete encapsulated Message Data string remains unchanged in both pairs, while the "checksum" for the sentences varies Regardless of the VDM encapsulation pair used, the encapsulated string consistently reads: 1P000Oh1IT1svTP2r:43grwb05q4.
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Figure D-1 illustrates the Message Data as a horizontal bit table Alternatively, as depicted in the left table of Figure D-2, the Message Data bits can be organized into a table with six columns, accommodating as many rows as necessary to display all the bits Each position in the table is numbered to indicate the corresponding position of the Message Data bit.
AIS unit's broadcast Organising the bits in this manner allows easy use of the conversion information shown in table C-1 (see annex C).
The following discussion will use "table lookup" methods to describe the decoding process.
The reader should also be aware that this standard also contains binary mathematical methods that a computer would use to accomplish the same results.
Decoding the Encapsulated String
The Background Discussion, above, described how the AIS unit codes the received binary
Message Data bits into the characters of an encapsulation string It explained that the AIS unit:
- Organises the binary bits of the Message Data into 6-bit strings,
- Converts the 6-bit strings into their representative valid characters - see table C-1,
- Assembles the valid characters into an encapsulation string, and
- Transfers the encapsulation string using the VDM sentence formatter.
Again, the sample sentence that will be used in this decoding and interpretation example is:
!AIVDM,1,1,,A,1P000Oh1IT1svTP2r:43grwb05q4,0*01
A calculation shows that the checksum, 71 HEX , is correct This permits the interpretation of the sentence contents to continue Based upon the definition of a "VDM" sentence, this is a
"single sentence encapsulation of an AIS VHF data link message" This message was produced by an AIS unit The binary data, that has been encapsulated, was received on the
AIS unit's "A" channel Also, no bits were added to the binary string when it was encapsulated The remainder of this example will focus on the proper interpretation of string:
The process of decoding and interpreting the contents of the encapsulated string is a three step process:
1) The string symbols are converted back into the binary strings that they represent.
2) The binary strings are organised or parsed using the rules contained in the referenced document, in this case ITU-R M.1371-1:2000, Table 15.
3) The referenced document rules are used to convert the binary strings into the relevant information.
Conversion from symbols to binary bits
Figure D-2 serves as a visual aid for understanding the process related to the example string The left side of the figure features a table of VDM bit positions, which helps identify the exact bit position of corresponding binary bits shown on the right side, represented by encapsulation symbols This "reference grid" will be further clarified as the example is elaborated upon.
In figure D-2, a column illustrates the entry of the example string from top to bottom, while the arrows indicate the logical flow of the decoding process The decoding of the VDM encapsulated string initiates with the first element.
The binary representation of the symbol "1" is "000001," which is derived from table C-1 This six-bit string is placed in the grid adjacent to the "1," occupying bit positions 1 through 6 The leftmost "0" is located in position 1, while the rightmost "1" is in position 6, aligning with the reference diagram shown in figure D-2.
The second symbol in the string, "P", is processed next The "P" represents the binary string
The binary string "100000" is entered into VDM bit positions 7 to 12 in the right grid This process is repeated for each symbol in the encapsulated string, concluding with the symbol "4," which corresponds to the binary string "000100." This final binary string is placed in the last row of the right grid, specifically in VDM bit positions 163 to 168.
Loading the appropriate grid with binary strings is a mechanical task unrelated to the information content of the binary data This process is essentially the reverse of what the AIS unit performed to generate the encapsulation string while creating the VDM-sentence.
Organizing the Binary Message Data
The worksheet has been completed to decode an "AIS Message 1," featuring two grids in figure D-2 with various shaded grey blocks This design aids in easily identifying the specific bits that constitute the message 1 parameters within the decoded binary bit array.
The fact is, these blocks could not be filled in until the message type (message number) of
The identification of an AIS message begins with the first six bits of the binary Message Data, where the message number is determined by its decimal equivalent; for instance, 000001 corresponds to message 1 Once the message number is established, the subsequent blocks of the message can be interpreted using the guidelines provided in Annex E.
The parameters listed in ITU-R M.1371-1:2000, Table 15 are transmitted over the radio link as
The "Number of bits" column from ITU-R M.1371-1:2000, Table 15, is utilized to determine the applicable bits for each parameter This consistent ordering of bits will be maintained across all messages.
1" That is, until the reference table itself is changed.
Each referenced AIS message table should follow the same ordering process For instance, if the decoding process results in bits 1-6 being 000101, the identified VDM message corresponds to message 5, as 000101 in binary equals 5 in decimal This pertains to the "Ship Static and
Voyage related data" message - Table 17 of ITU-R M.1371-1:2000.
The process or organising the decoded binary Message Data requires:
1) Identification of the message number, and
2) Organising or parsing the binary bits following the appropriate message table(s).