The VHF Datalink or VHF Digital Link (VDL) is a means of sending information between aircraft and ground stations (and in the case of VDL Mode 4 other aircraft). Aeronautical VHF data links use the band 117.975 - 137 MHz assigned by the International Telecommunication Union to Aeronautical Mobile Route Services. There is an Airline Industry (AEEC) standard for an ACARS VHF data link installed on approximately 14,000 aircraft and a range of ICAO standards defined by Aeronautical Mobile Communications Panel (AMCP) in the 1990s. VDL Mode 2 is the only mode being implemented operationally to support Controller Pilot Data Link Communications CPDLC.
ICAO VDL Mode 1
The ICAO AMCP defined this Mode for validation purposes. It was the same as VDL Mode 2 except that it used the same VHF link as VHF ACARS so it could be implemented using analog radios before VHF Digital Radio implementation was completed. The ICAO AMCP completed validation of VDL Modes 1&2 in 1994, after which the Mode 1 was no longer needed and was deleted from the ICAO standards.
ICAO VDL Mode 2
The ICAO VDL Mode 2 is the main version of VDL. It has been implemented in a Eurocontrol Link 2000+ program and is specified as the primary link in the recently adopted EU Single Sky rule requiring all aircraft flying in Europe to be equipped with CPDLC.
In advance of CPDLC implementation, VDL Mode 2 has already been implemented in approximately 2000 aircraft to transport ACARS messages simplifying the addition of CPDLC. Networks of ground stations providing VDL Mode 2 service have been deployed by ARINC and SITA with varying levels of coverage.
The ICAO standard for the VDL Mode 2 specifies three layers, the subnetwork, link and physical layers. The subnetwork layer complies with the requirements of the ICAO Aeronautical Telecommunications Network (ATN) standard which specifies an end-to-end data protocol to be used over multiple air-ground and ground subnetworks including VDL.
The VDL Mode 2 Link Layer is made up of two sublayers, a data link service and a media access control (MAC) sublayer. The data link protocol is based on the ISO standards used for dial-up HDLC access to X.25 networks. It provides aircraft with a positive link establishment to a ground station and defines an addressing scheme for ground stations. The MAC protocol is a version of Carrier Sense Multiple Access (CSMA).
The VDL Mode 2 Physical Layer specifies the use in a 25 kHz wide VHF channel of a modulation scheme called Digital 8-Phase Shift Keying providing a data rate of 31.5 kilobit/second. This is the highest data rate that can be achieved in a 25 kHz channel with a maximum range of 200 nautical miles. This required the implementation of VHF digital radios.
ICAO VDL Mode 3
The ICAO standard for VDL Mode 3 defines a protocol providing aircraft with both data and digitized voice communications that was defined by the US FAA with support from Mitre. The digitized voice support made the Mode 3 protocol much more complex than VDL Mode 2. The data and digitized voice packets go in Time Division Multiple Access (TDMA) slots assigned by ground stations. The FAA implemented a prototype system around 2003 but did not manage to convince airlines to install VDL Mode 3 avionics and in 2004 abandoned its implementation.
ICAO VDL Mode 4
The ICAO standard for VDL Mode 4 specifies a protocol enabling aircraft to exchange data with ground stations and other aircraft.
VDL Mode 4 uses a protocol (Self-organized Time Division Multiple Access, STDMA, invented by Swedish Håkan Lans in 1988) that allows it to be self-organizing, meaning no master ground station is required. This made it much simpler to implement than VDL Mode 3.
In November 2001 this protocol was adopted by ICAO as a global standard. Its primary function was to provide a VHF frequency physical layer for ADS-B transmissions. However it was overtaken as the link for ADS-B by the Mode S radar link operating in the 1090 MHz band which was selected as the primary link by the ICAO Air Navigation Conference in 2003.
The VDL Mode 4 medium can also be used for air-ground exchanges. It is best used for short message transmissions between a large number of users, e.g. providing situational awareness, Digital Aeronautical Information Management (D-AIM), etc.
European Air Traffic Management modernization trials have implemented ADS-B and air-ground exchanges using VDL Mode 4 systems. However on air transport aircraft the operational implementations of ADS-B will use the Mode S link and of CPDLC will use VDL Mode 2.