ATN SUBNETWORK
ATN ground systems connect over existing X.25 networks such as SITA etc.
ATN avionics can connect to ATN ground systems using the X.25 based services of AMSS, VDL, Mode ‘S’ and HFDL
There is no limit to message length over VDL X.25 connections
Aircraft can establish an explicit VDL a/g link using a unique address for each ground station