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 and leased lines.
As the Aeronautical Message Handling System (AMHS) comes online over the next decade, it will switch to X.400 links, with either dedicated lines or tunneled through IP.[when?]
The letter X is used to complete the address when an explicit identification of the department, division or process is not required.
[4] Examples for some common three-letter-groups used in AFTN addresses in order to identify an organization or service: The message format of AFTN messages is defined in ICAO Annex 10 Aeronautical Telecommunications Volume II.
The following three-letter group indicates the organization or function addressed (for instance aeronautical authority, service or aircraft operating agency).
Teleprinter communication with airline operators is sometimes maintained by having a connection to the IATA Type B messaging networks which use a 7 character address.
The older tape stations (and perhaps newer ones) also included a bell that could be rung by using a set character code.
[citation needed] Other airport required reports are also transmitted through the AFTN, on daily and hourly intervals like flight plans, NOTAMs (notices to airmen), and AIRADs (Airfield Advisories).
Via the AFTN the following message categories are submitted: Priority Indicators consist of two letters SS, DD, FF, GG and KK.