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Appendix E - AIS connection for MX512 only | 159
shipping agency information, tides and currents, notices to mariners and other
information from shore to ship, as well as ship-to-ship and ship-to-shore AIS reports.
It is also possible for the VTS to broadcast the complete harbor picture to all ships in the
area, so the masters and pilots all share the same “big picture.
The VTS center can assume control over the assignment of time slots for AIS messages
to ensure optimum data exchange within the coverage area. Special dedicated channels
may be designated for local-area AIS operations. The shipboard AIS equipment will have
the ability to shift to different channels automatically when directed by the shoreside
VTS controller.
AIS Communications Scheme
AIS messages must be updated and retransmitted every few seconds at a minimum,
since the usefulness of the data decays rapidly as a function of time. To accommodate
this high update requirement, AIS utilizes a unique self-organizing time-division multiple
access (SOTDMA) data communications scheme, which uses the precise timing data in
the GPS signals to synchronize multiple data transmissions from many users on a single
narrowband channel.
Each ship broadcasts its AIS messages and receives messages from all ships within VHF
radio range. The area in which AIS messages can be received is called the ship’s “cell”.
Each ship is in this way in the center of its own communication cell.
The practical size of the cell can be varied according to the traffic density on the AIS
channel. If the number of AIS messages begins to overload the network, the ship’s AIS
system can automatically shrink its cell by ignoring weaker stations further away in favor
of those nearby.
Under the SOTDMA protocol, each minute of time is divided into 2,250 timeslots. An AIS
report fits into one or several of these 2,250 timeslots, which are selected automatically
based on data link traffic and projections of future actions by other stations currently
on the network. When a ship first enters the cell of another ship, it takes an unoccupied
timeslot. The AIS stations continually synchronize their slot selections with each other.
Timeslots and time-out periods are selected on a randomized basis. When a station
changes its slot assignment, it announces to all other stations on the channel its new
location and time-out for that location. Each station continually updates its internal “slot
map” to reflect changes in occupied slots and time-outs. Special provisions are made for
automatic conflict resolution in the event two stations end up in the same timeslot, to
ensure that stations always choose unoccupied slots.
In situations of high traffic density it may be necessary to reduce the number of ships in
a communication cell, as described above. This enables time slots used by weak stations
far away, to be used also by a station nearby. The AIS system applies very specific rules
on how this reoccupation of timeslots is done.
The key to the SOTDMA scheme is the availability of a highly accurate standard time
reference, to which all of the stations can synchronize their time slot assignments, in
order to avoid overlap. This time reference is supplied by the precise timing signal in the
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