Datalink Overview and Architecture

The communications chapter (ATA-23) repeatedly hands one thing to "the next chapter": the datalink application layer. This is that next chapter. One sentence fixes the relationship between the two: ATA-23 covers the pipes — how VHF 3, HF and SATCOM push bits out over the air; ATA-46 covers the content — how the ATSU-hosted CPDLC, ADS-C and AOC applications let the crew talk to ATC and the company by datalink. One is hardware, one is software, and together they close the loop.

Modern oceanic and remote operations increasingly use datalink rather than voice between crew and controller — typing a request, receiving a clearance, reporting position automatically. The umbrella term is FANS (Future Air Navigation System), built on three applications: notification (AFN), CPDLC (communication), and ADS (surveillance). This first article raises the whole-chapter skeleton: what datalink is, the FANS applications, the ATSU "router and host", the three cockpit interfaces (DCDU/MCDU/ATC MSG pb), and two life-critical disciplines (clock accuracy, revert to voice).

1. What datalink is — two application classes, two directions

The FCOM gives the two classes. Per FCOM DSC-46-10-10:

The datalink has: AOC applications [...] ATC applications [...] The ATC datalink provides communication, navigation, and surveillance for Air Traffic Management (ATM) services.

The message direction. Per FCOM DSC-46-10-10:

The datalink messages are: Uplink (from a ground facilities to the flight crew), or Downlink (from the flight crew to a ground facilities).

Exchange can be automatic (no crew action) or manual (via the DCDU/MCDU and/or RMP). The standard opening of a FANS session. Per FCOM DSC-46-10-10:

At the beginning of the flight, the flight crew sends a notification message to the ATC center, via the MCDU, notification application. Then, an air traffic controller will establish a connection between the aircraft and the ATC center. As a result, the flight crew can exchange messages with the ATC center, via the DCDU (CPDLC application).

Hold the assembly line — notify (crew, via MCDU) → connect (controller) → communicate (crew, via DCDU). It is the spine of articles 2–5.

2. FANS applications and ATS 623 applications

The full application list. Per FCOM DSC-46-10-10:

The ATC datalink provides: FANS A applications (FANS 1/A in ICAO designation) using ACARS ground network, for operations in remote areas and in oceanic areas: Notification, Controller-Pilot DataLink Communication (CPDLC), Automatic Dependent Surveillance - Contract (ADS-C). ATS 623 applications (ACARS ATS in ICAO designation): Departure Clearance (DCL), Oceanic Clearance (OCL), Digital - Automatic Terminal Information Service (D-ATIS).

FANS A versus A+. Per FCOM DSC-46-10-10:

Note: FANS A+ is an evolution of the FANS A that introduces ATS 623 applications and datalink recording capability.

One line: FANS A three (notification/CPDLC/ADS-C, articles 3–5) + ATS 623 three (DCL/OCL/D-ATIS, article 6) + AOC (company, article 7) — the chapter's entire ATC-side application set.

3. The ATSU — datalink router and application host

The ATSU (Air Traffic Service Unit) is the datalink hub. Per FCOM DSC-46-10-10:

In ACARS environment (FANS A) the ATSU controls all datalink communication and automatically selects the best available communication media: VHF, HF, SATCOM.

Its "router" face, per AMM 46-00-00:

The ATSU router provides: datalink services to remote AOC applications hosted on on-board peripherals, datalink services to the ATC/AOC applications hosted on the ATSU, management of the datalink media: Very High Frequency (VHF), Satellite Communication (SATCOM) (optional), HF (optional).

Automatic VHF-frequency selection uses a scan mask, which is mandatory. Per FCOM DSC-46-10-10:

The VHF scan mask is compulsory for correct router operation. If there is no scan mask, the ECAM displays DATALINK ATSU FAULT – ATSU INIT FAULT.

[!note]- The ATSU is the datalink "router" The ATSU is to datalink what a home router is to the internet: it hosts the applications (ATC/AOC), manages the media (automatically picking the best of VHF3/SATCOM/HF), and selects the VHF frequency by scan mask (a service-provider priority list, airline policy). No scan mask = the router does not know which provider to connect = ATSU INIT FAULT. ATA-23 covers these media as physical hardware; the ATSU is the "network layer" above them.

4. The three cockpit interfaces — DCDU / MCDU / ATC MSG pb

Datalink cockpit operation splits three ways. DCDU (Datalink Control and Display Unit) — read and answer. Per FCOM DSC-46-10-10:

The DCDU enables the flight crew to: Read and answer datalink messages, received from the ground; Display datalink messages, before sending.

MCDU — manage and edit. Per FCOM DSC-46-10-10:

The MCDU enables the fight crew to access functions to: Manage the ATC datalink connections, Send notifications, Edit datalink messages, Consult reports, Access Message Record application.

ATC MSG pb — the new-message reminder (article 8). One split: the MCDU builds/manages/edits (the typing side), the DCDU reads/answers (the send-receive side), the ATC MSG pb shouts "new message". The FMS also monitors clearances and prompts via the DCDU before a clearance condition; the FWS alerts on a received message and inhibits by flight phase; the printer can print ATC messages (check the print matches the display; the display is the reference).

5. ADS-C versus ADS-B — why ADS-B is not in this chapter

Two applications share the name ADS but belong to different chapters. Per FCOM DSC-46-10-10:

There are two different ADS applications: ADS-Contract (ADS-C) [...] ADS-Broadcast (ADS-B) [...] The ATC Datalink does not host the ADS-B.

[!warning]- ADS-C is here; ADS-B is not ADS-C (contract) sends surveillance data over the ATC datalink to connected ATC centres (oceanic/remote) — a FANS surveillance pillar, article 5. ADS-B (broadcast) broadcasts position via the Mode S transponder to all equipped users — it is not carried by the ATC datalink and belongs to ATA-34. Seeing "ADS", sort C from B first: C in 46 (contract, point-to-point, via the ATSU), B in 34 (broadcast, via the transponder).

6. Clock accuracy — ±1 s UTC, do not set the clock manually for FANS

A life-critical datalink discipline. The GPS synchronises UTC. Per FCOM DSC-46-10-10:

The GPS also enables time synchronization to the clock, in order to provide UTC date and time for datalink system. If the GPS fails, the clock continues to calculate the time by incrementing the last received pertinent information as the reference time.

Why the clock matters. Per FCOM DSC-46-10-10:

The required time precision for ATC datalink communications is +/-1 s UTC. If this constraint is not respected, a rejection of datalink message or acceptance of obsolete datalink message may occur. [...] For FANS operations, the flight crew should not manually set the clock during cockpit preparation.

[!warning]- Setting the clock manually before flight can break the datalink Intuitively "setting the clock" seems tidy; but under FANS the clock must be GPS-synchronised to within ±1 s, and a manual change can push it outside ±1 s, so an ATC uplink is judged "obsolete / out of tolerance" and rejected, or an obsolete message is wrongly accepted — both safety hazards. The rule: do not touch the clock before flight; let the GPS manage it. This underpins the message-timing system (article 4).

7. Revert to voice — voice is the primary means

However capable FANS is, voice is the floor. Per FCOM DSC-46-10-10:

Voice communication is a primary means of communication on board. The flight crew must revert from datalink communication to voice communication, if: There is an emergency situation (exchange of a critical or urgent message); There is a doubt about a datalink message, the voice should be used for clarification; An operational timer of datalink message exchange times out; A response to an ATC message was not correctly transmitted via datalink.

[!note]- Datalink is the aid, voice is the primary means Do not be misled by "modern": datalink is not instantaneous, is delayed, and can time out — so on a critical/urgent/doubtful/timed-out/failed-response case, revert to voice. These four conditions are the master rule for operations (article 10) and abnormals (article 11): datalink fails, do not panic — revert to voice. HF is switched from data to voice on the RMP (a green HF VOICE memo means the HF datalink is interrupted).

8. Configuration baseline

The configuration covered here carries CPDLC datalink (FANS A/A+ over ACARS): the FANS A applications (notification/CPDLC/ADS-C) and the ATS 623 applications (DCL/OCL/D-ATIS). The datalink media are VHF (VHF 3), SATCOM and HF — all ATA-23 hardware, managed by the ATSU. The CVR also records the datalink data exchanged between crew and controller. An Aircraft Information Network System (AINS) with an EFB is fitted (article 12); the cabin/passenger information networks are a cabin layer and are not covered here.

Self-test

[!note]- Q1. In one sentence, how do ATA-23 and ATA-46 divide the work? What are the two datalink application classes and the two message directions? ATA-23 is the pipes (physical radios), ATA-46 is the content (ATSU applications). Classes: ATC and AOC. Directions: uplink (ground→crew) and downlink (crew→ground).

[!note]- Q2. How does a FANS session start (notify → connect → communicate), and on which interface is each step? The crew notifies the ATC centre via the MCDU; a controller establishes the connection; the crew then exchange messages via the DCDU (CPDLC).

[!note]- Q3. What are the FANS A three and the ATS 623 three applications? FANS A versus A+? FANS A: notification, CPDLC, ADS-C. ATS 623: DCL, OCL, D-ATIS. A+ adds ATS 623 applications and datalink recording.

[!note]- Q4. What are the ATSU's roles, how does it select media, and what if there is no scan mask? It hosts ATC/AOC applications and routing, and automatically selects the best of VHF/HF/SATCOM. Without a scan mask the ECAM shows ATSU INIT FAULT.

[!note]- Q5. Why is ADS-B not in this chapter? ADS-B broadcasts via the Mode S transponder and is not hosted by the ATC datalink — it belongs to ATA-34. ADS-C (contract, via the ATSU) is here.

[!note]- Q6. What is the datalink time-precision requirement, and why must the clock not be set manually for FANS? ±1 s UTC. A manual change can push it out of tolerance, causing uplinks to be rejected or obsolete messages accepted; the GPS keeps it synchronised.

[!note]- Q7. When must the crew revert to voice? Emergency, doubt about a message, an operational timer times out, or a response was not correctly transmitted.

Key takeaways

References

• FCOM DSC-46-10-10 — datalink classes, uplink/downlink, FANS/ATS 623 applications, ADS-C vs ADS-B, ATSU media selection, scan mask, DCDU/MCDU, GPS clock ±1 s, revert to voice.
• AMM 46-00-00 — information-systems taxonomy; ATIMS/ATS/AIS; ATSU router and media management.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.