Each frame lasts exactly 1 minute and starts on each minute boundary. Each frame is divided into 2250 slots. As transmission can happen on 2 channels, there are 4500 available slots per minute. Depending on the type and status of equipment and the status of the AIS slot map, each AIS transmitter will send out messages using one of the following schemes: Incremental time division multiple access (ITDMA) Random access time division multiple access (RATDMA) Fixed access time division multiple access (FATDMA) Self-organizing time division multiple access (SOTDMA) The ITDMA access scheme allows a device to pre-announce transmission slots of non-repeatable character, ITDMA slots should be marked so that they are reserved for one additional frame. This allows a device to pre-announce its allocations for autonomous and continuous operation. ITDMA is used on three occasions: data link network entry; temporary changes and transitions in periodical reporting intervals; pre-announcement of safety related messages. RATDMA is used when a device needs to allocate a slot, which has not been pre-announced. This is generally done for the first transmission slot, or for messages of a non-repeatable character. FATDMA is used by base stations only. FATDMA allocated slots are used for repetitive messages. SOTDMA is used by mobile devices operating in autonomous and continuous mode. The purpose of the access scheme is to offer an access algorithm which quickly resolves conflicts without intervention from controlling stations. Message format An AIS slot is 26.66 ms long. The data modulation is 9600 bit/s, so each slot has a maximum capacity of 256 bits. The framing is derived from the HDLC standard, described in ISO/IEC 13239:2002. Each slot is structured as such: <8 bit ramp up><24 bit preamble><8 bit start flag><168 bit payload><16 bit CRC><8 bit stop flag><24 bit buffer> 24 bit preamble: this is a sequence of 0101... Start flag: 0x7e 168 bit payload, this is the body of an AIS message. For messages requiring more data, several slots (Maximum of 5) must be used. 16 bit CRC-16-CCITT: 16-bit polynomial to calculate the checksum. Stop flag: 0x7e 24 bit buffer used for bit stuffing, synchronization jitter and distance delay. Note that the signal on the VHF carrier is NRZI encoded and uses bit-stuffing to avoid unintentional stop-flags which may otherwise occur in the data. As such, the raw bits must first be decoded, and the stuffing bits removed, to arrive at the actual usable message format