Connections

FANS-1/A Aircraft

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.

Backward Compatibility

Some 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.


Terminology

As 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.

The 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.

Consequently, the terms in the right column of the following table will replace the terms in the left column.

These terms will be explained in more depth later.

                                    ATN                                     FANS-1/A

                            CM-LOGON-REQ                                                              FN-CON

                            CM-LOGON-RESP                                                            FN-ACK

                            CM-CONTACT-REQ                                                          FN-CAD

                            CM-CONTACT-RESP                                                        FN-COMP

 

The Logon

The first step in the Connection Management chain is the logon. The logon is a very important element in the data link system.

The 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.

Without a logon, the ground system is unable to provide data link services to an aircraft.

In 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.

A logon must be initiated by an aircraft, but there are two ways that this initiation can occur; manually or automatically.

The manual logon is performed in the following situations;

  • the first logon of a departure flight,
  • when an aircraft is approaching a data link FIR after transiting a non-data link area, or
  • when 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.

This logon is known as the initial logon.

The 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.

A 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).

 

Diagram

The 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.

The CPDLC Connection

Following 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.

Various 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.

When 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.

Although 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 themselves differ.

These differences will be discussed on other pages.


Connection Request Diagram

The 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.

Click the following link to view a Flash Movie of the Initial Logon.

 

The NDA Message

The Next Data Authority Message, or the NDA, plays an important role in transferring a CPDLC connection from one unit to another.

The 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.

The 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.

A FANS 1/A aircraft will accept a new NDA message to allow for changes resulting from alternative route clearances and weather diversions.

The 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.

If 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.

 

NDA Diagram

The 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.

 

 

Address Forwarding

The 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 aircraft’s 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.

Some 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.

The 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.

On 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 unit.

Following the sending of the FN_RESP, the avionics automatically downlinks an FN_CON message to the next ATS unit.

On receipt of the FN_CON message, the ground system will respond automatically with another system level acknowledgment message known as the FN_ACK.

When 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.

The 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.

Only one logon is required to establish a connection with the aircraft for the CPDLC and ADS applications.

 

The Address Forwarding Process

The 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.

 

 

The Connection Transfer

The 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.

Under 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.

The 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.

The End Service message can be sent automatically by the system, or manually by the controller.

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.

 

Connection Transfer Diagram

The 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 connectivity.

Click 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).

ATC Data Link News Copyright © 1999, Craig. J. Roberts - Page last modified: November 10, 2016