It is a short distance (400 km maximum) service proposed through a network of VHF ground stations (on 136.975 MHz mainly but also 136.875 MHz in Europe).
The VDL2 (for 'VHF Data Link mode 2') mode permits automatic transmissions between planes and ground stations, with an exchange of different pieces of information (and especially positions) through AVLC frames (I, UI, RR, XID.) very close to Packet frames.
