Data link services are being provided in most of the world's oceanic airspaces and also in some trans-continental and domestic airspaces. The majority of these services are currently accessed by FANS 1/A equipped aircraft. FANS work stations
continue to be installed in many ground facilities around the world.
Aircraft equipage depends on the aircraft type and the options selected by
an airline when purchasing the type.
All new Boeing and Airbus aircraft are delivered with the capability, although not all airlines choose to use the option, and a number of aircraft have been retrofitted. The smaller models generally do not have the necessary SATCOM equipment to access satellite datalink, but they can access the services provided over VHF datalink, which are available in a number of countries and extensively across Australia.
ground systems in the South Pacific region have been designed to be compliant
with both the ATN Baseline 1 and the FANS 1/A applications. However, as the ATN
Baseline 2 system is still in a developmental phase, there are currently no ATN -equipped aircraft
operating regularly in the Southern Hemisphere. The implementation of ATN-based ADS-C is still yet to be finalised.
The ATN and the FANS 1/A systems have physical as well as philosophical differences.
One of the physical differences is the size and the content of the CPDLC message
sets. ICAO, through the ADS and OPLINK Panels, has expanded the original FANS-1
message set to include around one hundred additional messages.
Any of these additional messages delivered to a FANS 1/A equipped
aircraft will result in an error being generated by the avionics.The ATN and FANS 1/A
systems are said to have a "backward compatibility problem".
The ATN system, officially backed by ICAO, is not fully compatible with the
FANS 1/A system, which was built to accommodate a perceived commercial need
during the years until the ATN became fully operational.
Backward compatibility remains an issue, although work is currently being
conducted to accommodate and converge ATN and FANS 1/A applications. There have also been some early "dual-stack" systems implemented to allow Air Navigation Service Providers (ANSPs) to communicate with both ATN and FANS 1/A systems, however these systems require FANS 1/A messages to be converted to ATN messages in accordance with accommodation rules. Some units and field lengths are different between the two systems, so a complete one-to-one conversion is not possible.
a result of two systems that are similar, but have enough differences to make
compatibility a major issue, the data link community also has to accommodate
two vocabularies that describe the same set of system level messages.
majority of commercial ATS data link flights at present are FANS 1/A equipped,
so this web site will refer to messages by using the FANS 1/A terminology.
the terms in the right column of the following table will replace the terms
in the left column.
terms will be explained in more depth later.
first step in the Connection Management chain is the logon. The logon is a
very important element in the data link system.
logon serves a number of purposes; it provides an ATS unit with the types
of applications supported by the avionics (CPDLC and ADS), the application
version numbers, and the ACARS addresses of those applications. The logon
also provides the flight number and the registration of the aircraft.
a logon, the ground system is unable to provide data link services to an aircraft.
FANS 1/A terminology, the logon is known as the Airways Facilities Notification
(AFN) logon. The AFN is usually abbreviated to FN. The logon message is known
as the AFN Contact message, or the FN_CON.
logon must be initiated by an aircraft, but there are two ways that this initiation
can occur; manually or automatically.
manual logon is performed in the following situations;
first logon of a departure flight,
an aircraft is approaching a data link FIR after transiting a non-data link
the pilot is required to initiate or re-initiate the AFN logon following
an unsuccessful address forwarding and/or next data authority notification,
a connection failure, or an ATS system shutdown.
logon is known as the initial logon.
initial logon is performed manually by the pilot and consists of inserting
the 4 character ICAO address of the relevant ATS unit into a template in the
Flight Management System (FMS) and downlinking the FN_CON to the ground system.
message sent from an aircraft is always known as a downlink message (DL),
whether or not the aircraft is in flight, and a message sent to an aircraft
from a ground system is always known as an uplink message (UL).
pilot triggers the initial AFN logon by sending an AFN contact message (FN_CON)
containing the 4 character ICAO code of the ATS unit. The ATS system responds
with an AFN Acknowledgment message (FN_ACK).
Note: The following text refers exclusively to the initiation, transfer, and termination of the CPDLC Connection. ADS-C Contracts are handled differently and are not transferred from unit to unit under the same process as CPDLC. There is no concept of "data authority" for ADS-C. However, the logon is generally the trigger for establishing both CPDLC and ADS-C contact between the aircraft and the ground system.
a logon, the ground system sends a system level message to the avionics. This
message is known as the Connection Request message, or the CR1.
ground systems react to a logon in different ways. Some systems automatically
send a CR1 to the aircraft, other systems require the controller to send the
CR1 manually. When the CR1 is sent manually, there may be some delay between
the pilot seeing SENT and ACCEPTED on the logon page.
the CR1 is received by the avionics, a Connection Confirm message (the CC1)
is automatically returned to the ground system. It is on receipt of the CC1
that a connection is established between the aircraft and the ground system.
the connection process for the ADS and CPDLC applications is performed as
a result of the one logon, the processes, the connections, and the applications
differences will be discussed on other pages.
Connection Request Diagram
connection between an aircraft and a ground system is initiated by the sending
of the Connection Request message by the ATS unit and is established when
the Connection Confirm message is received from the aircraft.
the following link to view a Flash Movie
of the Initial Logon.
Next Data Authority Message, or the NDA, plays an important role in transferring
a CPDLC connection from one unit to another.
NDA notifies the avionics of the address of the next unit that will send a
connection request. The NDA should be sent to the aircraft as a single message
prior to the sending of the FN_CAD (see next section). The reason for this
timing is that if the next unit sends a CR1 message (Connection Request) to
the avionics after the logon, but before the avionics has received an NDA,
the connection request will be rejected.
CR1 will also be rejected if the address of the unit sending the CR1 is not
the same as the address contained in the NDA. Once an NDA has been received,
the avionics will not accept a connection request from any other unit.
FANS 1/A aircraft will accept a new NDA message to allow for changes resulting
from alternative route clearances and weather diversions.
new NDA will replace the previous NDA in the avionics, but it will disconnect
a previously connected ATS unit, even if the new NDA message specifies the
same ATS unit as the previous message.
the current data authority terminates the connection (i.e. sends an End Service
message) after sending the NDA message, but before the unit notified as the
Next Data Authority has sent a connection request, then the NDA message is
cancelled and technically any ground station could send a connection request
to the aircraft.
purpose of the Next Data Authority (NDA) message is to advise the avionics
of the next ATS unit that will make a CPDLC connection. The sending of the
NDA message is the first step in the CPDLC transfer sequence between an aircraft
and two ATS units.
automatic logon results from the ground system sending a specific message
to the avionics. This process is known as Address Forwarding.
Address Forwarding is used to instruct the avionics to forward the aircrafts
application addresses to a particular ATS unit. Address Forwarding consists
of sending the aircraft an AFN contact advisory message (FN_CAD), which contains
the address of the ATS unit. On receipt of this address, the avionics will
automatically trigger an AFN logon with this unit.
systems send the FN_CAD message automatically, but it can also be sent manually
by the controller to cover unusual situations, such as a weather diversion
proceeding into an adjacent FIR.
FN_CAD can only be sent to an aircraft that is already connected to the ground
system, and is sent at a parameter time or distance from the FIR boundary.
receipt of the FN_CAD, the avionics automatically returns an acknowledgment
message known as the AFN Response or FN_RESP, to the currently connected ATS
the sending of the FN_RESP, the avionics automatically downlinks an FN_CON
message to the next ATS unit.
receipt of the FN_CON message, the ground system will respond automatically
with another system level acknowledgment message known as the FN_ACK.
the FN_ACK is returned to the aircraft after an automatic logon to the next
ATS unit, the current ATS unit is notified that the logon has been successful
by the AFN Complete message, or FN_COMP.
messages that make up this sequence are exchanged at a system level and are
not seen by the controller or the flight crew. In many ground systems the controller will receive notification of the non-delivery of a system-level message, but will not see the messages if the system is working as expected.
one logon is required to establish a connection with the aircraft for the
CPDLC and ADS applications.
Address Forwarding Process
address forwarding process is completely invisible to the flight crew. For
AFN logons initiated by the address forwarding process, the flight crew has
no indication that the FN_CON or FN_ACK messages have been delivered successfully.
last step in the connection management sequence is to disconnect the aircraft
from the current ATS unit and to activate the connection with the ATS unit
that has been designated as being the Next Data Authority.
normal conditions the current ATS unit initiates the CPDLC connection termination
sequence by sending an End Service uplink message. The End Service message,
or EOS, is sent as the aircraft approaches the FIR boundary. On receipt of
the End Service message, the avionics will automatically downlink a Disconnect
message to the ground system.
moment that the Disconnect message is sent, the avionics considers that the
connection has been terminated. There is no consideration whether or not the
Disconnect message ever reached the ground system.
End Service message can be sent automatically by the system, or manually by
Note: Studies have shown that ATSUs that retain datalink connections when there is no longer an operational need (especially retaining ADS-C contracts when the aircraft has passed a reasonable parameter beyond the FIR airspace) have an impact on the performance of the entire datalink network. The FANS 1/A aircraft also send ADS-C reports in the order in which the connection occurred, so if the first connected ATSU retains its ADS-C contracts once the aircraft has left its airspace then the first ADS-C report downlinked to the multiple ATSUs with contracts will be sent to the first ATSU - the one that has the least need. It is therefore imperative that all ATSUs disconnect from aircraft when there is no longer an ATC operational requirement for datalink connections.
success of the CPDLC transfer is dependent upon the next ATS unit having established
a CPDLC connection prior to the End Service message being received by the
aircraft. Failure of the next ATS unit to establish a CPDLC connection before
the End Service reaches the aircraft will leave the aircraft without CPDLC
the following link to view a Flash Movie of the
transfer process from one CPDLC ATS Unit to another. In the movie the CONTACT
[unit] [frequency] message is sent combined with the End Service message as
a single uplink, which is an alternative to the diagram above (which uses
the MONITOR option).