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 EU Single European Sky rule adopted in January 2009 requiring all new aircraft flying in Europe after January 1, 2014 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 Telecommunication 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.
Read more about this topic: VHF Data Link
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