Communications Infrastructure
The original AFTN infrastructure consisted of landline teleprinter links between the major centers. Some long distance and international links were based on duplex radioteletype transmissions. When it upgraded to CIDIN (Common ICAO Data Interchange Network), it was upgraded to X.25 links at much higher data rates. As the AMHS comes online over the next decade, it will switch to X.400 links, with either dedicated lines or tunneled through IP.
Read more about this topic: Aeronautical Fixed Telecommunication Network