*
Inputs from the flight crew

*
Choice of the function that sends the data.

For uplinks, the ACARS router receives data from the VHF or SATCOM tranceiver and assigns a destination. It sends the data to the CPI protocol function for final delivery to AIMS functions. The ACARS router uses the ACARS label and optional sublabel in the uplink to set the onboard destination.
Redundancy Management
The redundancy management makes sure the preferred data communications path is available. The data communications modes are:
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*
VHF

*
SATCOM

*
Auto.

The flight crew uses a FDCF manager menu to select one of these modes. The center VHF communication transceiver is the default data radio when the DCMF first gets power. This occurs when the data communications mode is auto. This also makes the left DCMF the primary DCMF and the right DCMF, secondary.
Williamsburg Protocol
Each Williamsburg Protocol uses the Williamsburg file transfer protocol. It uses a hi-speed ARINC 429 to exchange data with the SDU. Supplement 14 to ARINC Specification 429 defines the Williamsburg file transfer protocols.
The Williamsburg protocol also supplies SATCOM path status to the ACARS router. The ACARS router uses this information to determine the appropriate path for download data.
ACARS Air/Ground Protocol
The ACARS air/ground protocol makes and maintains VHF communications. To do this, the ACARS air/ground protocol uses a database of ACARS VHF frequencies to tune the data radio. This database is airline modifiable information (AMI).
The ACARS air/ground protocol tunes the default VHF communications transceiver to one of the available VHF frequencies for two minutes. If, after that time, the ACARS air/ ground protocol can not establish a VHF link, the ACARS air/ ground protocol tunes the default radio to the next VHF frequency. This procedure continues until VHF communications are successful.
The flight deck communications function (FDCF) and RTPs receive the VHF data frequency from the ACARS air/ground protocol.
To downlink data, the ACARS air/ground protocol divides the data into smaller blocks. It sends the smaller blocks to the modem. If the GSP successfully receives the downlink data, the ACARS air/ground protocol makes a downlink message acknowledgment. The protocol then sends the acknowledgment to the original function.
For uplinks, the ACARS air/ground protocol makes sure the received data is for an onboard system. It then sends the data to the ACARS router for routing to the destination function.
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RTP Interface
Each RTP transmits the current VHF voice frequencies and new VHF voice/data configuration to the RTP interface. The RTP interface receives, stores, and transmits the voice frequencies back to the RTPs. This RTP interface keeps the active DCMF in synchronization with the RTPs. The RTP interface uses the VHF voice/data configuration command it receives from the RTP to configure the default VHF radio.
Modem
For downlink data, the modem modulates a digital bit stream into an audio signal that is compatible with the VHF communication transceiver. For uplink data, the modem demodulates the received audio signal into a digital bit stream.
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General 
The  OLAN  interface  sub-function  is  the  four  layer  stack 
responsible  for  data  communication  between  the  AIMS 

functions and the OLAN systems.
The OLAN communication protocols use the International Standards Organization (ISO) Open Systems Interconnection (OSI) reference model. OSI is a data communications model which divides different communications duties into different levels or layers. Each layer supplies services to the next layer above it. A set of layers is a stack.
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