Class A

The operation of Class A AIS transceivers is defined by the equipment and test standard IEC61993-2 Edition 1.

Class A transceivers use the following TDMA access schemes:

• SOTDMA is used for the majority of transmissions including all periodic transmissions (position reports).
• RATDMA and ITDMA are used during network entry (when the transceiver is first switched on) to enable initial transmissions and slot allocation
• RATDMA is used for non-periodic transmissions which can not be pre-announced. This would include transmission of text messages or safety related messages
• ITDMA is used to allocate slots for periodic transmissions when the reporting interval of a periodic message is changed (for example when the position reporting interval changes due a vessel speed change)

Using SOTDMA, units transmit to locate a vacant time slot reserve it, transmit the data and reserve the next time slot automatically. Class A units receive as well as transmit data, receiving when to transmit their own data, as well as receiving data from other ships, AtoNs or base stations and when their next reserved slot is in the AIS slot map.

Class A units have one VHF transmitter, two VHF TDMA receivers and one VHF Digital Selective Calling (DSC) receiver. Class A units must also include links to the shipboard display and sensor systems via standard marine electronic communications. The unit primarily uses GPS, LORAN, Glonas or an internal navigation system to determine its position. The unit connects to shipboard systems through data connections, eg. NMEA 0183/2000/USB, to display information received through AIS.

Class B

The operation of Class B ‘CS’ (Carrier Sense) transceivers is defined by the equipment and test standard IEC62287-1 Edition 2.

Class B units operate a different system to Class A transceivers, Carrier Sense Time Division Multiple Access (CSTDMA). The unit transmits to find a free time slot and then sends its information in the available space. CSTDMA does not reserve a future time slot, but relies on finding free slots while scanning the slot map. As Class A units take priority over Class B ones, they can reserve a slot resulting in a potential loss of Class B data. As well as transmitting data, CSTDMA devices also receive when to transmit data and information from other ships, AtoNs or base stations. Unlike Class A units using SOTDMA, Class B units don’t reserve future time slots.

CSTDMA type devices are regarded as 'polite' devices as they scan for available slots and send their data into free space. They use both spare time slots and, to utilise the AIS slot map as efficiently as possible, use any available space in slots reserved by class A devices.

Class B units contain the same amount of receivers and transmitters as Class A, but they are used slightly differently. The VHF receivers are designed for CSTDMA; one of the two must be multiplexed with the VHF DCS receiver. The unit also includes an active GPS antenna as standard. As the units are not interfaced with a compass, heading information is rarely transmitted. The main difference between Class A and Class B units is the power output, Class B transmits at 2W, giving a range of between 8-10 miles, whilst Class A transmits at 12.5W giving a much larger range.

The operation of Class B ‘SO’ (Self Organising) AIS transceivers is defined by the equipment and test standard IEC62287-2^[5]. This standard has recently been developed by the IEC technical working group and is currently in final draft format. Publication is anticipated during Q4 2012.

Class B ‘ SO’  transceivers use the same access schemes as defined for Class A transceivers in the transmission types section.

Receiver

AIS receivers pick up all Class A and Class B broadcasts, however, unlike transceivers they do not transmit any AIS data. They are designed to see, but not be seen, aimed squarely at the leisure market for mariners who want to enjoy the fun aspects of AIS, but without the full AIS benefits.

As with Class B and Class A transceivers, AIS receivers use the VHF antenna to receive data, however, there are no connections to a GPS antenna as the device does not transmit its position, so does not require a connection.

Receivers allow you to see the position and other details from AIS devices within range, but they are not able to see you. AIS receivers can have the same outputs as any other AIS device, namely NMEA 0183, NMEA 2000, USB, wifi and RS232, which allows integration too many display options.

AtoN

The operation of AIS Aids to Navigation is defined the equipment and test standard IEC62320-2 Edition 1.

AIS Aids to Navigation use either the FATDMA or RATDMA access scheme depending on their hardware configuration.

• A Type 1 AIS AtoN with transmit only capability uses the FATDMA access scheme. This requires a nearby base station to reserve the slots used by the AIS AtoN via data link management messages.
• A Type 3 AIS AtoN with both receive and transmit capability can use either the FATDMA access scheme (with base station reservations) or the RATDMA access scheme. The latter allows the AIS AtoN to autonomously allocate slots for its own transmissions as required.

An AtoN AIS unit operating in RATDMA (Random Access Time Division Multiple Access) mode uses its receiver to listen to both AIS frequencies for about one minute, and makes and stores a map of all the AIS "slots" [or message spaces] on the VHF data link [VDL]. It then looks for two free adjacent slots in which to send its [2-slot long] AtoN message 21 or meteorological and hydrological message 8.

RATDMA is ideal for many applications because the AtoN or weather/tide AIS unit can be placed at any location, and requires no reservation of slots by a base station. It can be used whether base stations exist in the area or not. Its drawback is that in order to transmit to the AIS slot map the unit must turn its receiver on for at least one minute before transmitting, and this is the main power consuming factor with an RATDMA AtoN or weather AIS unit.

An AtoN AIS unit operating in FATDMA (Fixed Access Time Division Multiple Access) mode will transmit in a pair of slots which are reserved by an AIS base station. Ships receive a message from the base station, indicating that certain slots are reserved. The ship AIS transponders store this reserved slot information in their slot maps, and do not transmit in these slots. The FATDMA AtoN is programmed to transmit in two consecutive slots of those reserved by the base station. It is possible to "re-cycle" slots by having a number of AtoN units use the same pair of slots, but use them sequentially. FATDMA allows greatly reduced power drain for an AtoN AIS unit, because no receiving period to build a slot map is needed.

SART

The operation of AIS Search and Rescue transceivers is defined in the equipment and test standard IEC61097-14 Edition 1.

AIS SARTs use only the modified-SOTDMA access scheme specifically defined for burst transmission in low volume emergency beacon applications.

SARTs use Pre Announced Time Division Multiple Access (PATDMA) as they continuously transmit but don’t receive.

Once activated, the SART sends its position and heading eight times in a minute. The device sends the information this often to ensure that the life raft is at the peak of the wave on at least one of the transmissions, maximising range. SARTS also send the SART safety message 'SART ACTIVE' every four minutes.

SARTs do not receive any form of data, but keep transmitting data. They are designed to work in an emergency situation.

Base stations

The operation of AIS base stations is defined by the equipment and test standard IEC62320-1 Edition 1.

AIS base stations use the FATDMA and RATDMA access schemes. FATDMA slot allocations are manually configured and broadcast to other AIS transceivers using data link management messages. RATDMA transmissions are typically used for non-periodic messages such as broadcast of safety related messages.

Splitter

Both AIS devices and VHF radios use VHF antennas to transmit and receive data. Most smaller vessels either do not have the capacity, or owners the need, for a second VHF antenna. This means that an antenna splitter needs to be used to ensure there is no loss of data, and the antenna is managed in the correct way.

Antenna splitters offer a cost effective way to ensure the antenna is managed efficiently.

The antenna splitter monitors which device is using the VHF antenna and ensures it has access to transmit or receive. Most antenna splitters on the market ensure there is no signal loss when power fails on the vessel, and gives priority to the VHF radio over the AIS device.