AIS Know-How: Data transfer (SOTDMA vs. CSTDMA)
Two methods are available for transmitting the AIS data telegrams.
On the one hand, the CSTDMA (Carrier Sense Time-Division Multiple Access) method, which is used primarily for pleasure boating (Class B). The CSTDMA method uses carrier-sensing to defer transmissions until no other stations are transmitting. If a free slot is found, the transmission starts. Than the transmission is stopped for a 30 second time interval before the next one starts.
On the other hand, there is the SOTDMA (Self-Organized Time-Division Multiple Access) technology, which was used only by commercial vessels (Class A) but is now also authorized for pleasure boating at Class B level. With the SOTDMA method only the first free slot for the transmission has to be found. The next five slots are always reserved automatically.
The following hierarchy results for the transmission of AIS data telegrams:
- Class A (commercial vessels) & SART (rescue transmitter)
- Class B SOTDMA (pleasure boating)
- Class B CSTDMA (pleasure boating)
The main advantages of the SOTDMA compared to the CSTDMA technology for pleasure boating (Class B) at a glace:
SOTDMA (Class B)
Self-organized Time-Division Multiple Access- + 5 watt transmission power
- Transmission interval up to every 15 sec. (depending on speed)
- Reservation of future slots
- Sending/receiving of text messages (addressing to MMSI possible)
- 2nd priority to Class A devices
CSTDMA (Class B)
Carrier Sense Time-Division Multiple Access- 2 watt transmission power
- Send interval every 30 sec. (fixed)
- no reservation of future slots
- no sending/receiving of text messages
- 3rd priority to Class A devices
