Several AFTN communication centres, when connected to each other, form a network of fixed aeronautical telecommunication lines through which every AFTN station can be reached. Sometimes AFTN stations are connected to more than one centre or to other stations.

REV (revision message) - replaces the verbal revision in case the accepting unit does not change as a result of the modification. RRV (referrred revision proposal) - used by the transferring controller to propose non-standard revisions. Urgency messages (priority indicator DD). This category comprises messages concerning the safety of a ship, aircraft or other vehicles, or of some person on board or within sight. TIP (tactical instructions proposal) - used by the transferring unit to coordinate the transfer of a flight on an assigned heading, speed, rate of climb or descent or on a direct routing. RLS (release) - used by the transferring unit to release a flight. The message may be used as a response to a RRQ or on the initiative of the transferring controller.SDM (supplementary data message) - transmits control data and changes thereto from the transferring unit to the accepting unit. Can be used by the accepting unit to notify the transferring unit of the frequency to which the flight is to be transferred. OLDI is considered as one of the major enablers of the Single European Sky (SES). As required by Regulation 1032/2006 it shall be in use from 2009 by all ATC units providing service to general air traffic, and the reliability of every OLDI link shall be at least 99.86 %. This is equivalent to a down-time of not more than 12 hours per year (based on 24-hour availability). Benefits The AFTN uses two types of aeronautical fixed stations - AFTN communication centres and AFTN stations. The AFTN communication centre is an AFTN station whose primary function is to relay AFTN messages to or from a number of other interconnected AFTN stations. RTI (request tactical instructions) - used by the accepting unit to request the transfer of a flight on an assigned heading, speed, rate of climb or descent or on a direct routing.

The following categories are accepted for transmission by the aeronautical telecommunication service: the three-letter designator identifying the organization (as seen in ICAO Doc 8585). If no such designator is allocated, YYY is used (YXY for military organizations). For aircraft in flight, ZZZ is used.TIM (transfer initiation message) - signifies the end of the coordination pjase and the start of the transfer phase. Meteorological messages (priority indicator GG) comprise messages concerning forecasts (TAFs) and observations and reports (METAR, SPECI). The messages to be used and certain parameters thereof are defined in bilateral agreements between the particular ATC units. Related Articles Coordination (dialogue procedure). This procedure provides facilities for communication and negotiation between controllers in the coordination phase and for communication in the transfer phase. The ACT and REV messages described above may also be used for this procedure. At the majority of aerodromes where ATS is provided, there is an AFTN station. A number of such stations grouped together and around an AFTN centre form an AFTN circuit.