Editing Aeronautical Fixed Telecommunication Network (section)

== AFTN Message Format ==
The message format of AFTN messages is defined in ICAO Annex 10 Aeronautical Telecommunications Volume II.<ref>International Civil Aviation Organization (ICAO), Annex 10 to the Convention on International Civil Aviation - Aeronautical Telecommunications Volume II, Sixth Edition, October 2001, Chapter 4</ref>

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 [[International Civil Aviation Organization airport code|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.<ref name=":0" /> A typical message would look like:

 ZCZC LAA005 12032000
 DD OPKCZQZX
 120900 OPSTZQZX
 
 MESSAGE TEXT
 
 NNNN

Explanations:
* The first three lines in the example AFTN message above represent the Heading of the message.
* ''ZCZC LAA005 12032000'' is the Heading Line in which ''ZCZC'' is the Start-of-Message Signal. The ''Z'' and ''C'' characters do not normally occur together in standard text and provide a unique character pattern for automating the identification of the beginning of a message. ''LAA005'' refers to the Transmission Identification and ''12032000'' is an Additional Service Indication.
* The second line ''DD OPKCZQZX'' is the Address of the message. ''DD'' represents the Priority Indicator for the message category(an Urgency Message in this cases). ''OPKCZQZX'' is the eight-letter group identifying the addressee. ''OPKC'' refers to Karachi/Jinnah Intl, Pakistan. ''ZQZ'' refers to a Centre in Charge of Flight Information Region or Upper Flight Information Region'. In other words, the addressee is Karachi Area Control Centre or Karachi Flight Information Centre. The last character ''X'' indicates that an explicit identification of the organization or function addressed is not required. So the ''X'' is just used to complete the originator address.
* The third line ''120900 OPSTZQZX'' represents the Origin of the message. ''120900'' refers to the date-time-group and means the twelfth day of the month at time 09 o'clock [[Coordinated Universal Time|UTC]]. ''OPSTZQZX'' is the originator of the message where ''OPSTZQZX'' refers to Area Control Centre or Flight Information Centre at Sialkot, Pakistan.
* The example does not give an explicit ''MESSAGE TEXT''. In reality the contents of the message is included here.
* The Ending of the message is indicated by ''NNNN'' which is the End-of-Message Signal that also has a character pattern that is not found in standard text.
* The Alignment Functions and letter shift signals mentioned above are not visible in AFTN messages.

The message routing is easily automated by general purpose computers. 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 whole communications system is still rooted in the 'official' nature of radioteletypes.

The older tape stations (and perhaps newer ones) also included a bell that could be rung by using a set character code. The purpose of the bell was to allow the sender to alert the receiving operator of a high priority message such as an SS message. It was also possible to insert spacing between bell rings. With care and persistence, one could compose a musical tune to play to far distant stations. Jingle bells was a favourite. A particularly clever 'author' could combine the tune with an image such as a Christmas tree. The skill for this was often learnt on those long night watches when little traffic was in the air.{{citation needed|date=May 2016}}

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).