CSTDMA is defined for Class B AIS stations and permits development of a low cost transceiver that is fully interoperable with SOTDMA transmissions whilst ensuring priority is always given to SOTDMA transmissions.

CSTDMA is defined in ITU-R M.1371-4 Annex 7.

The key elements of CSTDMA operation are as follows:

• TDMA slot timing is determined from the timing of AIS Class A or AIS base station transmissions within receiver range. GPS based timing is not required.
• Stations using CSTDMA continuously monitor the AIS radio channels background noise level. This background level is used as a reference for a received signal strength measurement at the start of each TDMA slot.
• When a transmission is required a TDMA slot is randomly selected and the signal strength at the start of the slot measured. If the signal strength is significantly above the background level the slot is assumed to be in use and the transmission is deferred. If the signal strength at the start of the slot is close to the background level the slot is assumed to be unused and the transmission is made.
• The ‘listen before transmit’ or ‘carrier sense’ scheme works on a slot by slot basis; this limits CSTDMA transmissions to a single TDMA slot. Multiple consecutive slots cannot be allocated using this technique.

The essential hardware requirements to support CSTDMA are:

• VHF transmitter capable of operating on any AIS channel in the marine VHF band
• Two VHF receivers capable of operating on any AIS channel in the marine VHF band
• Full time decoding of all received messages in order perform carrier sense measurements.

CSTDMA is used by Class B transceivers to transmit data. It transmits data as follows:

• AIS device scans for available space in the AIS slot map
• Transmits into an available slot
• If no available space is found, the transmission is delayed until space is available
• Repeat