This message should be used by base station(s) to pre-announce the fixed allocation schedule (FATDMA) for one or more base station(s) and it should be repeated as often as required. This way the system can provide a high level of integrity for base station(s). This is especially important in regions where several base stations are located adjacent to each other and mobile station(s) move between these different regions. These reserved slots cannot be autonomously allocated by mobile stations.
The mobile station, within 120 nautical miles should then reserve the slots for transmission by the base station(s) until time‑out occurs. The base station should refresh the time-out value with each transmission of Message 20 in order to allow mobile stations to terminate their reservation for the use of the slots by the base stations (refer to § 188.8.131.52, Annex 2).
The parameters: offset number, number of slots, time-out, and increment should be treated as a unit, meaning that if one parameter is defined all other parameters should be defined within that unit. The parameter offset number should denote the offset from the slot in which Message 20 was received to the first slot to be reserved. The parameter number of slots should denote the number of consecutive slots to be reserved starting with the first reserved slot. This defines a reservation block.
This reservation block should not exceed 5 slots. The parameter increment should denote the number of slots between the starting slot of each reservation block. An increment of zero indicates one reservation block per frame. The values recommended for increment are as follows: 2, 3, 5, 6, 9, 10, 15, 18, 25, 30, 45, 50, 75, 90, 125, 150, 225, 250, 375, 450, 750, or 1125. Use of one of these values guarantees symmetric slot reservations throughout each frame. This message applies only to the frequency channel in which it is transmitted.
If interrogated and no data link management information available, only offset number 1, number of slots 1, time-out 1, and increment 1 should be sent. These fields should all be set to zero.
|Parameter||Number of bits||Description|
|Message ID||6||Identifier for Message 20; always 20|
|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 station ID||30||MMSI number of base station|
|Spare||2||Not used. Should be set to zero. Reserved for future use|
|Offset number 1||12||Reserved offset number; 0 = not available(1)|
|Number of slots 1||4||Number of reserved consecutive slots: 1-15;
0 = not available(1)
|Time-out 1||3||Time-out value in minutes; 0 = not available(1)|
|Increment 1||11||Increment to repeat reservation block 1;
0 = one reservation block per frame(1)
|Offset number 2||12||Reserved offset number (optional)|
|Number of slots 2||4||Number of reserved consecutive slots: 1-15; optional|
|Time-out 2||3||Time-out value in minutes (optional)|
|Increment 2||11||Increment to repeat reservation block 2 (optional)|
|Offset number 3||12||Reserved offset number (optional)|
|Number of slots 3||4||Number of reserved consecutive slots: 1-15; optional|
|Time-out 3||3||Time-out value in minutes (optional)|
|Increment 3||11||Increment to repeat reservation block 3 (optional)|
|Offset number 4||12||Reserved offset number (optional)|
|Number of slots 4||4||Number of reserved consecutive slots: 1-15; optional|
|Time-out 4||3||Time-out value in minutes (optional)|
|Increment 4||11||Increment to repeat reservation block 4 (optional)|
|Spare||Maximum 6||Not used. Should be set to zero. The number of spare bits which may be 0, 2, 4 or 6 should be adjusted in order to observe byte boundaries. Reserved for future use|
|Number of bits||72-160|