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determines whether the connected CSP should attempt delivery of the uplink or whether to route the uplink to
the CSP that is not connected to the ATSP.
In the aircraft, the avionics determine the ATS downlink media selection and ATS/AOC prioritization levels
supported.
The following paragraphs describe the SITA, ARINC, Boeing, and Airbus routing and media selection, and
level of ATS/AOC prioritization provided.
5.3.1 SITA Internetworking Function, Routing, and Use of Media Advisories, and Prioritization
SITA determines whether or not to attempt delivery of uplinks originating from FANS ATSP’s connected
to SITA over the SITA datalink network or to route to a non-SITA datalink network based on media
advisory information. Preference is given first to VHF, then SATCOM, then, HFDL (ARINC is the only
HFDL service provider.) To further describe, preference is first given to the network whose media
advisory information indicated that VHF datalink was established. If no VHF established media advisory
information was obtained, preference is then given to the network whose media advisory information
indicated that SATCOM datalink was established. In the absence of both VHF and SATCOM established
media advisory information, but, media advisory indication of HFDL establishment, SITA will base its
FANS routing decision on static information (the default SATCOM CSP). If the uplink is not successfully
delivered via the default SATCOM CSP, then the uplink will be sent to ARINC to attempt delivery.
SITA supports the case in which the avionics interrupt a multi block AOC downlink to send an ATS
downlink. There are no other levels of AOC/ATS prioritization supported.
5.3.2 ARINC Internetworking Function, Routing, and Use of Media Advisories, and Prioritization
ARINC’s GLOBALink® Service includes three Back-End Processors (BEPs) located in Annapolis, USA;
Beijing, China; and Bangkok, Thailand. The Annapolis BEP controls SATCOM and HFDL networks as
well as AOA and VHF ACARS networks in Americas, Europe, Middle East and Africa. Beijing and
Bangkok BEPs manage VHF ACARS networks in Asia.
Internetworking functionality implemented at ARINC is applicable to all ARINC networks described above
as well as the SITA network. ARINC determines routing algorithms for uplinks originated by FANS
ATSPs connected to ARINC networks based on media advisory information. If media advisory
information is available, then preference is given first to VHF, then SATCOM, and then HFDL. If media
advisory information is not available then uplink routing decision is made based on last successful
delivery. If no messages have been delivered to/from aircraft within last twelve minutes then ARINC
makes FANS uplink routing decision based on static information, which can be configurable by
customer’s request.
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FANS-1/A Datalink Communications Environment
ARINC supports the case in which the avionics interrupt a multiblock AOC downlink to send an ATS
downlink. In addition, ARINC can preempt uplink messages for a higher priority message as discussed in
Section 3.4, Message Sequencing, of ARINC 618. ARINC only does this if the uplink in process and the
higher priority uplink have different message labels (so the avionics can tell that these are two different
messages). If the labels are the same, ARINC puts the higher priority message next in queue for
transmission to the aircraft.
5.3.3 Boeing ATS downlink Media Selection and ATS/AOC Prioritization and Differences, if any, Among
Airframe Type
Media selection for all Boeing aircraft is typically prioritized as follows (highest to lowest): VDL2, VHF,
SATCOM, HF. Media selection and control of media priority will be left to each airline operator to define
based on the requirements of the regions in which they operate their aircraft.
B737, B747, B757, and B767:
On the B737, B747, B757, and B767, the FANS-1 datalink applications reside in the Flight Management
Computer (FMC) and the ACARS Management Unit (ACARS MU) or Communications Management Unit
(CMU) manages transmission of messages to and from the airplane. Downlink message prioritization occurs
at both the FMC and the ACARS MU/CMU.
The FMC prioritizes downlink message in the following order of priority (highest to lowest):
• ATC Datalink Messages
• ADS Messages
• AFN Messages
• AOC Messages
If multiple messages are queued in the FMC, higher priority messages will be inserted ahead of lower priority
messages.
The ACARS MU/CMU is typically connected to multiple peripheral devices. ACARS MUs/CMUs which are
approved for FANS-1/A communications are capable of receiving a downlink from the FMC while a message
from another peripheral is being transmitted.
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