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What is Datalink?

Simply put, datalink refers to a system of text message transmission between the aircraft and ground. Operators use datalink to transmit Airline Operational Control messages, or AOC, and flight crew and air traffic controllers can communicate via Controller to Pilot Data Link Communications, or CPDLC. 
The technological fabric supporting datalink has been in constant evolution since the 1970s.
ACARS and the beginning of datalink

The history of datalink goes back to 1978, with the introduction of ACARS: the Aircraft Communication Addressing and Reporting System. ACARS was developed in an effort to facilitate airline communications and reduce crew workload. The system was designed to replace certain air-ground messages traditionally communicated via voice radio with data messages, which would be transferred to or from aircraft via VHF, HF, or satellite networks.


The number of VHF ground stations deployed to support ACARS steadily grew throughout the 1980s and 90s, leading to a near worldwide coverage. The growing availability of ACARS allowed airlines to gradually develop a wide range of Airline Operational Control (AOC) applications. ACARS has evolved to also support applications for Air Traffic Control (ATC).


However, the widespread use of ACARS for airline and ATC communications for applications demanding more and more bandwidth made the ACARS system a victim of its own success. The VHF networks supporting ACARS became increasingly congested, leading to significant service quality loss. The aviation community needed a faster, more reliable air-ground link to support datalink messages.

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VDL Mode 2 implementation

In the early 1990s, the VHF Data Link – Mode 2 (VDL Mode 2 or VDL2) was introduced in order to cope with the ongoing ACARS congestion and performance problems. VDL Mode 2 is faster and more efficient than traditional VHF ACARS: in fact, VDL2 offers a bit rate of 31.5 kbps, compared with the lower throughput rate of 2.4 kbps in ACARS Mode. The improved VDL Mode 2 is meant to progressively replace the ACARS system or “Mode A,” and is progressively being implemented worldwide.


The VDL Mode 2 air-ground link supports all datalink applications. This includes not only AOC applications, but also a wide range of ATC applications including Controller to Pilot Datalink Communications (CPDLC) and Automatic Dependent Surveillance – Contract (ADS-C).

VDL Mode 2 serves as an air-ground link to support the Aeronautical Telecommunications Network, or ATN, and associated ATN CPDLC applications. However, it also supports ACARS messages and ACARS applications including FANS 1/A CPDLC. This mode is referred to as ACARS over AVLC (AOA) – AVLC being the data link layer of the VDL2 protocol

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What is CPDLC?

Controller to Pilot Data Link Communications, or CPDLC, refers to air-ground data messages sent between flight crew and ground air traffic controllers (ATCOs). CPDLC allows certain non-urgent ATC messages to be communicated via text message, rather than voice. The use of CPDLC messages provides several advantages over traditional voice communications. Text-based messages reduce the margin for error due to a poor voice radio connection and they liberate space on the congested VHF channels for more urgent voice communications.


VDL Mode 2 also supports the Aeronautical Telecommunications Network (ATN) – which was designed to provide heightened performance and quality of service for ATC purposes. The ATN network is currently operational in European airspace, and its deployment is mandatory following the European Commission regulation 29/2009. The regulation and its subsequent modifications mandates that all aircraft flying over flight level 285 in European airspace must be capable of supporting ATN CPLDC by 2020. Similarly, all air navigation service providers (ANSPs) must provide ATN CPDLC services by 2018.


FANS 1, developed by Boeing, and FANS A, developed by Airbus, are collectively known as “FANS 1/A.” FANS 1/A is primarily used in oceanic airspace via satellite communications (Satcom), but its implementation in the U.S. as part of the DataComm Program is ongoing.

Certain North Atlantic tracks are reserved for aircraft equipped with FANS1/A CPDLC. In VHF, FANS CPDLC is supported by either the ACARS or VDL2 air-ground digital link, whereas ATN CPDLC requires a VDL2 air-ground link to function.

Datalink in the U.S.

In the U.S., the FAA’s DataComm program – part of the NextGen initiative – is spearheading the effort to implement CPDLC. Digital departure clearance service, or DCL, has already been made available at many airports across the United States. The next phase of the DataComm program will concentrate on making data link services available in the En Route stages of flight.

The FAA Advisory Circular 120-70C (AC 120-70C) provides airspace users conducting data link operations within the National Airspace System (NAS) with guidance on how to meet applicable FAA and ICAO standards. AC 120-70C discusses both the implementation of DCL in the U.S. and ATN CPDLC in Europe.

Satellite Communications

In addition to VHF, the satellite air-ground link is also used to support air-ground voice and data communications. Satcom-equipped aircraft are able to use the satellite link both for AOC communications and CPDLC messaging in en-route and oceanic zones.

Satellite communications networks provide steadily increasing coverage worldwide, and their use is rapidly expanding. Satcom is a major enabler of numerous airspace optimization initiatives, and it will soon play an essential role in the ATM systems since it is the only link supporting oceanic and remote continental airspace datalink coverage. The satellite air-ground link will also contribute to a future multilink environment and support future communications networks such as ATN/IPS.
The future of datalink: the Multilink Concept

The future of datalink goes well beyond VHF. In a future multilink environment, many different air-ground links would be available – providing near global coverage – and aircraft would be equipped to communicate via any type of link. The avionics systems would be able to switch between links depending on a number of factors such as signal strength and application type.

The multilink concept would include legacy VHF systems, but also integrate new systems. These would include AeroMACS, the future airport surface communication subnetwork, LDACS, a line-of-sight communications subnetwork for continental coverage, and SatCom links for oceanic purposes. The multilink concept is considered as a technological enabler for many new air traffic management concepts including trajectory-based operations.