This ASM message is used to broadcast data to a group of stations as defined by the specified geographical area. The broadcast message does not contain a communication state. These broadcast messages are used for non-periodic transmission of data, and access the link using RATDMA. Geographical multicast message is defined in Table 32.
Table 32
Geographical multicast message
|
Parameter |
Number of bits |
Description |
|
|---|---|---|---|
|
Message ID |
4 |
6 – Geographical addressed message with no communication state. |
|
|
Retransmit flag |
1 |
Indicates that this is a 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. |
|
|
Longitude 1 |
18 |
Longitude of area to which the group assignment applies; upper right corner (north-east); in 1/10 min |
|
|
Latitude 1 |
17 |
Latitude of area to which the group assignment applies; upper right corner (north‑east); in 1/10 min |
|
|
Longitude 2 |
18 |
Longitude of area to which the group assignment applies; lower left corner (south-west); in 1/10 min |
|
|
Latitude 2 |
17 |
Latitude of area to which the group assignment applies; lower left corner (south‑west); in 1/10 min |
|
|
Data Count |
11 |
Size of actual data in Application Data field in bits, excluding padding bits. range: from 1 – Max data count. |
|
|
Spare bits |
2 |
Spare bits – reserved for the future. |
|
|
Application data |
1 slot: 224 / 128 2 slot: 736 / 512 3 slot: 1248 / 896 |
ASM Identifier |
The first 16 bits of the Application data are the ASM Identifier, which is described in § A3-6.2. |
|
Application data as specified by the ASM Identifier. |
The available length of the binary data is specified by the Link ID. |
||
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.