The technical specification for a Data Link Communications (DLC) subnetworkthat interacts between the aircraft’s VHF end-systems and corresponding ground-based stations. Although several subnetworks have been defined, only two VDL subnetworks are used: Mode 0/A and Mode 2. Both VDL Mode 0/A and Mode 2 can be used with the FANS network, but VDL Mode 2 is the only available subnetwork to interact with the ATN network.
VDL subnetworks are primarily used for continental CPDLC operations. As aircraft cross the oceanic boundary, it is expected that the equipment will switch to a satellite-based subnetwork instead (e.g., Inmarsat or Iridium).
VDL M2 is the only subnetwork available for US domestic continental CPDLC operations. In addition, the US continental CPDLC system requires that aircraft VDL M2 radio / CMU equipment have tunable multifrequency capability (i.e., compliance with TSO-C160a). Operators who intend to use CPDLC for continental US flights must ensure that their equipment is appropriately compliant, although Part 91 operators do not specifically need to possess an LOA to conduct domestic CPDLC operations. (Note that anyinternational or oceanic operation does still require an LOA.)