Wavecom W61PC V7.5.0 - Manual (Page 115)

WAVECOM Decoder W61PC/LAN Manual V7.5 Transmission Modes  105
ACARS Frame
Parameter Value
Pre-key 16 characters All binary „ones„
Bit Sync 2 characters „+„ , „*„
Character Sync 2 characters SYN, SYN (16h)
Start of Heading 1 character SOH (01h)
Mode 1 character
Address 7 characters
Technical Acknowledgement 1 character
Label 2 characters
Block Identifier 1 character
Start of Text 1 character
STX (02h) - if no text ETX (03h)
Text 220 characters maximum, printable characters only
Suffix 1 character, if single or terminal block ETX, else ETB (17h)
Block Check Sequence 16 bits
BCS Suffix 1 character DEL (7fh)
Messages may be single or multi-block. The pre-key sequence and the BCS have no parity bits.
ACARS communications are divided in Category A and Category B.
Using Category A, an aircraft may broadcast its messages to all ground stations. This is denoted by an
ASCII "2" in the Mode field of the downlink message. The WAVECOM software translates this character to
"A".
Using Category B an aircraft transmits its message to a single ground station. This is denoted by an ASCII
character in the range "@" to "]" in the Mode field of the downlink message.
The ground station may use either "2" or the range "‘" to "}" in the Mode field. All ground stations support
Category A, but may uplink "‘" to "}" in the Mode field.
The WAVECOM software translates the ground station address (also called the Logical Channel Number)
into a number in the range 0...29.
A station will transmit after having monitored the HF channel for traffic, otherwise it waits until the chan-
nel is clear. If a collision occurs between the packets of two stations transmitting at the same time, they
will back-off and new transmission intervals will be set by random interval timers in the radio equipment.
At the receiving end, a block check calculation is made and compared to the calculation appended to the
packet by the transmitting station. If the downlink messages contain errors, no response will be given and
the transmitting station will retransmit the packet a number of times, until a positive acknowledgement is
received and the message can be deleted from storage, or the aircrew be alerted to its non-transmission.
If an uplink message is found in error, the airborne equipment will generate a negative acknowledgement
(NAK), which triggers an uplink retransmission. Retransmission is also triggered by timeout.
Positive acknowledgement from the aircraft consists of the transmission of the Uplink Block Identifier of
the correctly received block. Positive acknowledgement from the ground station consists of a similar
transmission of the Downlink Block Identifier.
Acknowledgements are placed in the Technical Acknowledgement field.
The general response message label is "_DEL" (5fh 7fh). Messages with this label contain no information
except acknowledgements and are used for link maintenance.
The traffic exchanged can be requests for voice communication, weather reports, access to airline com-
puter systems, reading of aircraft automatic sensors, flight plans, messages to be routed to destinations in