This ASM message is used to broadcast data to all stations, and utilizes MITDMA communication state. Multiple messages, or periodically broadcasted messages may be chained together using the MITDMA communication state as described in § A3-4.5.7.2. Scheduled broadcast message is defined in Table 27.
Table 27
Scheduled broadcast message
|
Parameter |
Number of bits |
Description |
|
|---|---|---|---|
|
Message ID |
4 |
1 – Broadcast message with MITDMA communication state. |
|
|
Retransmit flag |
1 |
0 – Indicates there will be no retransmission of data. 1 – Indicates there will be retransmission of data. |
|
|
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. |
|
|
Session ID |
6 |
The Session ID associates the VDL transmission with a specific PI transaction. |
|
|
Source ID |
32 |
The Unique Identifier of the transmitting station as described in § A1-2.4. |
|
|
Data count |
11 |
Size of actual data in Application Data field in bits, excluding padding bits. range: from 1 – Max data count. |
|
|
Application data |
1 slot: 256 / 160 2 slot: 768 / 544 3 slot: 1280 / 928
|
ASM Identifier |
The first 16 bits of the Application data are the ASM Identifier, which is described in § A3-6.2. When chaining multiple blocks using MITDMA, the ASM Identifier is only contained in the first transmission. |
|
Application data as specified by the ASM Identifier. |
The available length of the binary data is specified by the Link ID. Unused payload data is zero-filled. |
||
|
Communication state |
38 |
MITDMA communication state as described in § A3-6.4. |
|
|
Spare bits |
2 |
Spare bits – reserved for the future. |
|
A3-6.2 Application specific message identifier
Addressed and broadcast binary messages should contain a 16-bit application identifier (Table 22).
Table 22
Application specific message identifier parameters
|
Bit |
Description |
|---|---|
|
15-6 |
Designated area code (DAC). This code is based on the maritime identification digits (MID). Exceptions are 0 (test) and 1 (international). Although the length is 10 bits, the DAC codes equal to or above 1 000 are reserved for future use. |
|
5-0 |
Function identifier. The meaning should be determined by the authority which is responsible for the area given in the designated area code. |
Whereas the application identifier allows for regional applications, the application identifier should have the special values for international compatibility, as defined in Recommendation ITU-R M.1371, Annex 5 “Application specific messages”. (this is a wrong reference from the 2092-2 should be Annex 4-1.2 of ITU-R M.1371)
A4-1.2 Definition of application identifiers
The application identifier uniquely identifies the message and its contents. The application identifier is a 16-bit number used to identify the meaning of the bits making up the data content. The use of application identifiers is defined in § A4-2.
The DAC is a 10-bit number. DAC assignments are:
– international (DAC = 1-9), maintained by international agreement for global use;
– regional (10 ≤ DAC ≤ 999), maintained by the regional authorities affected;
– future use (1000 ≤ DAC ≤ 1023), reserved for future use;
– test (DAC = 0), used for test purposes.
It is recommended that DAC 2-9 be used to identify subsequent versions of international specific messages and that the administrator of application specific messages base the DAC selection on the MID of the administrator’s country or region. It is the intention that any application specific message can be utilized worldwide. The choice of the DAC does not limit the area where the message can be used.
The function identifier (FI) is a 6-bit number assigned to uniquely identify the data content structure within an application under a DAC assignment. Each DAC can support up to 64 applications.
– The definition of the technical characteristics, as defined in Annexes 2 and 3, of any AIS station covers layers 1 to 4 of the open systems interconnection (OSI) model, only (see § A2‑1).
– The layers 5 (session layer), 6 (presentation layer) and 7 (application layer, that includes the human‑machine-interface) should be in accordance with the definitions and guidelines given in this Annex in order to avoid application conflicts.
A3-6.4 Multiple incremental time division multiple access
When the length of the data requires more than three consecutive slots, then the data should be divided up into sub-groups of three slot packets and MITDMA should be used to chain the transmissions together. A total of 15 MITDMA transmissions may be chained together. If the data provided by the PI exceeds this limit, a negative acknowledgment should be provided on the PI.
If data transmissions are repetitive in nature, and have a transmit interval less than two frames (4 500 slots), then MITDMA should be used to maintain the link.
If multiple messages are queued for transmission, then MITDMA should be used to allocate slots for the additional messages.
When using MITDMA for addressed messages, the MITDMA will provide the return slot for the message acknowledgment in the last three transmissions as specified in example in § A3-6.4.1.
A3-6.4.1 Multiple incremental time division multiple access example
An MITDMA access example is shown in Fig. 21. The first transmission (Tx 1) of a MITDMA chain is always a single slot transmission.
Determine the candidate slots for the Tx 1. Apply the RATDMA algorithm until the transmit criteria is meet.
Before transmitting at Tx 1, determine the candidate slots for up to three additional transmissions. Randomly select the transmit slots from the candidate slot lists. Calculate the offsets for these future transmissions. This information is provided in the MITDMA communication state. Slot Increment 1 reserves Tx 2, Slot Increment 2 reserves Tx 3, and Slot Increment 3 reserves Tx 4.
Before transmitting at Tx 2, determine the candidate slots for the next transmission, e.g. Tx 5. Randomly select a transmit slot from the candidate slot list. This information is provided in the MITDMA communication state. Slot Increment 1 reserves Tx 3, Slot Increment 2 reserves Tx 4, and Slot Increment 3 reserves Tx 5.
If this is a broadcast message, then starting at Tx n-2, the unused Slot Increments are set to 0. If this is an addressed message, then the following process happens.
At Tx n-2, determine the candidate slots for the acknowledgment message. Randomly select the acknowledgment slot from the candidate slot list. Calculate the offset for the acknowledgement (ACK) slot. This information is provided in the MITDMA communication state. Slot Increment 1 reserves the Tx n-1, Slot Increment 2 reserves the Tx n, and Slot Increment 3 reserves the ACK slot.
At Tx n-1 a new offset is calculated for the ACK slot. This information is provided in the MITDMA communication state. Slot Increment 1 reserves the Tx n, Slot Increment 2 reserves the ACK slot, and Slot Increment 3 is set to 0.
At Tx n a new offset is calculated for the ACK slot. This information is provided in the MITDMA communication state. Slot Increment 1 reserves the ACK slot, Slot Increment 2 and 3 are set to 0.
At the ACK slot, the receiving station transmits the acknowledgment message with the ACK/NACK Mask field set to indicate the success or failure of the MITDMA transmission chain. If one or more of the blocks failed during the transmission of the chain, then the transmitting station should re‑transmit the failed block(s). When starting the re-transmission of the failed MITDMA block(s), Block 1 (the single slot RATDMA transmission) is always sent first.
Figure 21
Multiple incremental time division multiple access example
To use MITDMA to transmit a periodic broadcast, the “Transmit Block Counter” field of the MITDMA communication state is set to 0. The “Number Slot 1” field of the MITDMA communication state is set to 0 to allow “Slot Increment 1” and “Slot Increment 2” fields to be combined into a 16-bit value. A slot increment can now be set to a value which has a maximum range of 360 seconds (6 minutes).