Всемирная аэронавигационная сеть фиксированной связи (AFTN)

Worldwide system of aeronautical fixed circuits
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.

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. IWXXM messages are lengthy and contain characters not supported by AFTN, so cannot use AFTN equipment. IWXXM requires the use of AMHS for international exchange.

The message format of AFTN messages is defined in ICAO Annex 10 Aeronautical Telecommunications Volume II. AFTN messages consist of a Heading, the Message Text and a message Ending. The message Heading comprises a Heading Line, the Address and the Origin. The Heading Line comprises the Start of Message Signal which is the four characters ZCZC, the Transmission Identification, an Additional Service Indication (if necessary) and a Spacing Signal. The AFTN Address comprises Alignment Functions, a two letter Priority Indicator depending on the message category and an eight letter group (Addressee Indicator). The first four letters of the eight letter group is a Location Indicator indicating the place of destination. The following three letter group indicates the organization or function addressed (for instance aeronautical authority, service or aircraft operating agency). The last letter of the eight letter represents a department, division or process within the organization/function addressed. The Origin consists of message Filing Time (six digit date time group), the Originator Indicator (eight letter group) identifying the message originator, a Priority Alarm (used only in teletypewriter operation for Distress Messages) and Alignment Functions. The Message Text ends with the End of Message Signal, which is the four characters NNNN. The Ending itself comprises twelve letter shift signals which represent also a Message Separation Signal. The AFTN system is backwards compatible with older transmission technology as many member states do not upgrade their AFTN centers fast enough. The message format betrays the extensive use of radioteletype links in the past.

The Priority Indicator is used to transmit AFTN messages according to their Order of Priority. So messages with Priority Indicator SS have the highest transmission priority. Messages with Priority Indicator DD and FF have the second highest transmission priority and the remaining messages with Priority Indicator GG and KK the lowest.