RTCM 12301.21 Draft Annex 1

RTCM Paper 189-2012-SC123-098

RTCM 12301.21 Draft Annex 1

1. Background and Purpose

Currently EAIS operations are designed and implemented to co-exist with the Class A AIS operations as defined in: “Technical Characteristics for USCG Encrypted Automatic Identification System (EAIS) Very High Frequency Data Link (VDL) Standard (v4.0).” The purpose of this Annex is to provide a means for implementing EAIS on an additional non-AIS channel, assuming that the current requirements for EAIS on AIS channels remain unchanged.

2. General Requirements

The follow general requirements are considered for implementing EAIS on a 3rd channel (not AIS 1 or AIS 2).

· The 3rd channel operation shall be subject to the AIS operations such that the AIS operations are not impaired or delayed (shall not interfere with the normal AIS operations).

· Encrypted message shall not transmit any vessel specific information in the clear.

· Slots in the previous frame which have an RSL (Received Signal Level) value 10dB[1] above the Noise Floor shall be considered unavailable for slot re-use.

· Before transmitting on a slot, the RSL shall be evaluated 2ms before the slot boundary to ensure that the transmit channel is not currently occupied. If the RSL value is greater than 10dB above the Noise Floor, then the transmission shall be aborted.

· If a message is aborted because the channel is occupied, it should be re-scheduled with the next position report. In this case, EAIS timing requirements must be subject to non-interference with calls in progress whenever the 3rd EAIS channel is shared with other services.

· When a message is addressed to a specific EAIS station, the destination station’s transmit schedule shall be considered to avoid sending a message to the destination station while it is transmitting.

· Transmitter switching time shall be within 1 slot.

· All message transmissions shall be announced using the next position report.

· All AIS traffic and 3rd EAIS channel traffic shall be available except during transmission.

· The EAIS shall support a list of candidate channels for EAIS operations.

· The PI (Presentation Interface) shall support a legacy mode which will convert the new EAIS message payload to the legacy AIS VDL message payload.

· The 3rd channel shall support all legacy ASM and STEDS messages.

3. Slot Selection

Slots in the previous frame which has a RSL value 10dB above the Noise Floor shall not be considered as available slots for slot re-use. When doing slot selection, the traffic on the other channels will have to be considered. When a message is addressed to a specific unit, the destination unit’s transmit schedule shall be consider to avoid sending a message to the destination unit while it is transmitting. The transmitter is a shared resource for all channels and can only transmit on 1 channel at a time. A minimum of 1 slot shall be allowed for switching frequencies.

Slot States:

Free – Slot which is not currently occupied

Available – Slot allocated by another station but available for slot re-use. Mobile Station (SOTDMA or ITDMA), or Base Station reserved slot (FATDMA or Message 4) beyond 120 NM.

Unavailable – Slot allocated by another station but not available for slot re-use. Base Station reserved slot FATDMA or Message 4) within 120 NM, or a Mobile Station reporting without position information, the destination unit’s transmit slot, or slots in the previous frame which has a RSSI value 15dB above the Noise Floor.

Slot Selection Rules for transmitting on the 3rd EAIS channel:

Rule / 3rd EAIS Channel / AIS Channel A / AIS Channel B
0 / Free / Free / Free
1 / Free / Available / Available
2 / Free / Unavailable / Unavailable
3 / Available / Any State / Any State

4. Transmission Rate for Position Reports

While operation on the 3rd EAIS channel the transmission rates will be a same as defined in ITU-R M. 1371 for Class A Reporting Rates with the one exception that there is no 3 minute reporting rate.

The reporting rates shall be based on SOG and maneuvering with the follow values:

· 0 – 3 knots 30 seconds

· 3 – 14 knots 10 seconds

· 0 – 14 knots changing course 3 1/3 seconds

· 14-23 knots 6 seconds

· 13-23 knots changing course 2 seconds

· >23 knots 2 seconds

· >23 knots changing course 2 seconds

5. Channel Selection

It shall be possible to set the 3rd EAIS channel from the PI or to enable a search mode from a list of candidate channels. In the search mode, the EAIS unit shall start with the default channel (the first candidate channel on the list) and monitor that channel for EAIS traffic for one minute. If no EAIS traffic is found, the EAIS unit shall increment to the next candidate channel on the list and continue until either EAIS traffic is found or the entire list is completed unsuccessfully. If no EAIS traffic from the search mode, the EAIS unit shall prompt the user to set the 3rd EAIS channel from the PI.

It shall also be possible to assign the third channel using an ASM from a base station, and this shall override all other channel selection means. The rules which govern the region size, region boundaries and the life cycle of the region shall be in accordance with ITU-R M 1371. The EAIS unit shall continuously monitor the AIS VDL and set its 3rd EAIS channel to begin with transmission of a position report in the next interval following receipt of a Message 26 with the following format:

Parameter / Number of bits / Description
Message ID / 6 / Identifier for Message 26
Repeat indicator / 2 / Used by the repeater to indicate how many times a message has been repeated.
0 = default; 3 = do not repeat any more
Source ID / 30 / MMSI for the source station
Destination Indicator / 1 / 0 = broadcast
Binary data flag / 1 / 1 = binary data coded as defined by using the 16-bit Application identifier
DAC / 10 / 366 – USA regional message
FI / 6 / TDB – EAIS Channel Assignment for 3rd channel
Longitude 1 / 18 / Longitude of area to which the assignment applies; upper right corner
(North-East); in 1/10 min, or 18 MSBs of addressed station ID 1
(±180º, East = positive, West = negative)
181 = not available
Latitude 1 / 17 / Latitude of area to which the assignment applies; upper right corner
(North-East); in 1/10 min, or 12 LSBs of addressed station ID 1,
followed by 5 zero bits
(±90º, North = positive, South = negative)
91° = not available
Longitude 2 / 18 / Longitude of area to which the assignment applies; lower left corner
(South-West); in 1/10 min, or 18 MSBs of addressed station ID 2
(±180º, East = positive, West = negative)
Latitude 2 / 17 / Latitude of area to which the assignment applies; lower left corner
(South-West); in 1/10 min, or 12 LSBs of addressed station ID 2,
followed by 5 zero bits (±90º, North = positive, South = negative)
3rd EAIS Channel / 12 / The channel used for the 3rd channel EAIS operations
Spare / 2 / Reserved for future use
Communication state selector flag / 1 / 0 = SOTDMA communication state follows 1 = ITDMA communication state follows
Communication state / 19 / SOTDMA communication state (see § 3.3.7.2.1, Annex 2), if communication state selector flag is set to 0, or ITDMA communication state (§ 3.3.7.3.2, Annex 2), if communication state
Total bits / 160 / 2 Slot message

6. Channel Access

The channel access scheme uses RTCM 12301.1 in conjunction with SOTDMA where communication states are used to announce the desired transmit slots and the RSL is evaluated 2ms prior to each transmission to avoid interference with any call in progress. The frame definition and slot allocation will be in accordance with ITU-R M 1371.

7. New (optional) EAIS Messages

For operation on the AIS VDL (AIS 1 and AIS 2), STEDS 4.0 messages were composed such One of the undesirables of the current EAIS messages is that the MMSI is transmitted in the clear. For operation on the 3rd EAIS channel, it is desirable to have the option It would be more desirable to encrypt all the message specific information. To accomplish this, the following new additional VDL messages are being constructed providedwhich are designed specifically for encrypted messaging.

7.1 Message 29: Single Slot Encrypted Payload, General format

This new message type is designed to carry an encrypted payload which will fit into a single slot and also contains comm. state information. Note that the function ID defines that content of the encrypted payload. This message assumes that the encryption key size and pattern have already been selected. Successful decryption of the payload is determined by the Encryption CRC which in contained in the encrypted payload.

7.2 Message 30: Multi Slot Encrypted Payload, General format

This new message type is designed to carry an encrypted payload which will fit into multiple slots and also contains comm. state information. Note that the function ID defines that content of the encrypted payload. This message assumes that the encryption key size and pattern have already been selected. Successful decryption of the payload is determined by the Encryption CRC which in contained in the encrypted payload.

Parameter / Number of bits / Description / Encryption Status
Message ID / 6 / Identifier for Message 30 / Unencrypted
Repeat indicator / 2 / Used by the repeater to indicate how many times a message has been repeated. 0 = default; 3 = do not repeat any more / Unencrypted
Function ID / 8 / Function ID for Payload / Unencrypted
Binary Data / 1024 / AES Encrypted payload in 128 bit block, up to 8 blocks / AES Encrypted
Spare / 4 / Reserved for future use, set to 0 / Unencrypted
Communication state selector flag / 1 / 0 = SOTDMA communication state follows 1 = ITDMA communication state follows / Unencrypted
Communication state / 19 / SOTDMA communication state (see § 3.3.7.2.1, Annex 2), if communication state selector flag is set to 0, or ITDMA communication state (§ 3.3.7.3.2, Annex 2), if communication state / Unencrypted
Total bits / 168
392
616
840
1064 / 1 Slot message
2 Slot message
3 Slot message
4 Slot message
5 Slot message

7.3 Position Report Vessel (Message 29, FID 0)

The EAIS encrypted vessel position report. This message should be used to reserve slots for all other messages such that no message is transmitted un-announced.

Parameter / Number of bits / Description / Encryption Status
Message ID / 6 / Identifier for Message 29 / Unencrypted
Repeat indicator / 2 / Used by the repeater to indicate how many times a message has been repeated. 0 = default; 3 = do not repeat any more / Unencrypted
Function ID / 8 / 0 = EAIS Position Report / Unencrypted
Source MMSI / 30 / MMSI number for the transmitting unit / AES Encrypted
Navigational status / 4 / 0 = under way using engine, 1 = at anchor, 2 = not under command,
3 = restricted maneuverability, 4 = constrained by her draught,
5 = moored, 6 = aground, 7 = engaged in fishing, 8 = under way
sailing, 9 = reserved for future amendment of navigational status for ships carrying DG, HS, or MP, or IMO hazard or pollutant category C, high speed craft (HSC), 10 = reserved for future amendment of navigational status for ships carrying dangerous goods (DG), harmful substances (HS) or marine pollutants (MP), or IMO hazard or pollutant category A, wing in grand (WIG);
11-13 = reserved for future use,
14 = AIS-SART (active),
15 = not defined = default (also used by AIS-SART under test) / AES Encrypted
Time stamp / 6 / UTC second when the report was generated by the electronic position system (EPFS) (0-59, or 60 if time stamp is not available, which should also be the default value, or 61 if positioning system is in manual input mode, or 62 if electronic position fixing system operates in estimated (dead reckoning) mode, or 63 if the positioning system is inoperative) / AES Encrypted
Position accuracy / 1 / The position accuracy (PA) flag should be determined in accordance / AES Encrypted
Longitude / 25 / Longitude in 1/1000 min (±180º, East = positive (as per 2’s complement), West = negative (as per 2’s complement). 181= (A5B5E0h) = not available = default) / AES Encrypted
Latitude / 24 / Latitude in 1/1000 min (±90°, North = positive (as per 2’s complement), South = negative (as per 2’s complement). 91° (535020h) = not available = default) / AES Encrypted
COG / 9 / Course Over Ground in degrees (0 – 359° ), 511 indicates not available / AES Encrypted
SOG / 7 / Speed Over Ground in knots (0 – 126 knots), 127 indicates not available and 126 = 126 knots or higher / AES Encrypted
Spare / 6 / Reserved for future use, set to 0 / AES Encrypted
Encryption Checksum / 16 / Cyclic Redundancy Check (CRC) 16 bit polynomial as defined in ITU-R M. 1371. / AES Encrypted
Spare / 4 / Reserved for future use, set to 0 / Unencrypted
Communication state selector flag / 1 / 0 = SOTDMA communication state follows 1 = ITDMA communication state follows / Unencrypted
Communication state / 19 / SOTDMA communication state (see § 3.3.7.2.1, Annex 2), if communication state selector flag is set to 0, or ITDMA communication state (§ 3.3.7.3.2, Annex 2), if communication state / Unencrypted
Total bits / 168 / 1 Slot message

7.4 Addressed Message Acknowledgment (Message 29, FID 1)