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Weatherdock AG Sigmundstraße 180 D-90431 Nürnberg
Tel.: +49 911 376638 30 www.weatherdock.de
As mentioned previously, the „Class A“ Transmitter operates with the
so-called SOTDMA method (self-organized time division multiple
access).
This is a „self-organized time slot allocation for telegrams, which
means that the ship listens to the radio traffic and determines the
information contained in these telegrams and on which “time slot” the
own transmitter may transmit without overriding other telegrams.
The complex SOTDMA method is replaced by a technically simpler
method CSTDMA for Class B-Transponders.
CSTDMA means „Carrier Sense Time Division Multiple Access“
At the beginning of a timeslot the Class B device
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checks whether the
channel is occupied or not and then sends its message. This method is
best described as “listen-before-talk-method”. A “Class B” Transmitter
simply listens and then transmits on a free time slot without checking
this in detail once again or coordinating with the other participants.
The following table shows the stipulated repetition rate of a „Class B“
Transceiver depending on the speed of the ship.
Dynamic information:
boats at < 2 kn:
3 minutes
boats at > 2 kn:
30 seconds
The static data such as the information regarding the journey is
dispersed every 6 minutes. (as in „Class A“)
This data is sent in turns according to the stipulated period to the one
(161.975 MHz) and subsequently to the other channel (162.025MHz).
The above-mentioned repetition rates apply to the standard operation.
The „Class B“ Transmitter does not have its own automatism to change
the repetition rate independently, except for the above-stated values.
Authorities may in exceptional cases reduce the repetition rate to 5
seconds or refuse admittance of the „Class B“ to the SOTDMA method
(e.g. in the case of heavy traffic) within their responsibility.
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