The Aeronautical Fixed Telecommunications Network (AFTN) is a worldwide system of aeronautical fixed circuits provided, as part of the Aeronautical Fixed Service, for the exchange of messages and/or digital data between aeronautical fixed stations having the same or compatible communications characteristics. AFTN comprises aviation entities including: ANS (Air Navigation Services) providers, aviation service providers, airport authorities and government agencies, to name a few. It exchanges vital information for aircraft operations such as distress messages, urgency messages, flight safety messages, meteorological messages, flight regularity messages and aeronautical administrative messages. 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 and leased lines. When it upgraded to CIDIN (Common ICAO Data Interchange Network), it was upgraded to X.25 links at much higher data rates. 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. AFTN Station address format An AFTN address is an eight-letter-group composed of a four-letter ICAO Location Indicator plus a three-letter-group identifying an organization or service addressed and an additional letter. The additional letter represents a department, division or process within the organization/function addressed. The letter X is used to complete the address when an explicit identification of the department, division or process is not required. For instance: LEBBYNYX. Location Indicator - A four-letter code group formulated in accordance with rules prescribed by ICAO and assigned to the location of an aeronautical fixed station. In the ICAO DOC7910, location indicators that are assigned to locations to which messages can not be addressed over the AFTN are identified by an asterisk(*) The four-letter Location Indicators are listed in ICAO Doc 7910 — Location Indicators. The three-letter designators are listed in ICAO Doc 8585 — Designators for Aircraft Operating Agencies, Aeronautical Authorities and Services. Every location (airport or other facility) with a connection to the AFS is assigned a unique four letter code (the aeronautical location indicator) by ICAO. The first letter or two letters indicate the country and the remaining two or three letters the specific location. For instance the letter K is the first letter of the four letter ICAO address location within the continental United States. The first letter for a Canadian aerodrome, or airport address, is C. Southern Europe codes begin with L,