Flights which are being provided with an ATC service are transferred from one ATC unit to the next in a manner designed to ensure complete safety. In order to accomplish this objective, it is a standard procedure that the passage of each flight across the boundary of the areas of responsibility of the two units is co-ordinated between them beforehand and that the control of the flight is transferred when it is at, or adjacent to, the said boundary.
Where it is carried out by telephone, the passing of data on individual flights as part of the coordination process is a major support task at ATC units, particularly at Area Control Centres (ACCs). The operational use of connections between Flight Data Processing Systems (FDPSs) at ACCs for the purpose of replacing such verbal "estimates", referred to as On-Line Data Interchange (OLDI), began within Europe in the early nineteen eighties.
Common rules and message formats were elaborated and agreed by the agencies concerned and incorporated in Edition 1 of the EUROCONTROL Standard for On-Line Data Interchange; this document, Edition 4.2, has been produced to meet new evolving service requirements and to incorporate feedback from the implementation of earlier versions.
The transfer of OLDI data messages is performed with the Flight Message Transfer Protocol over TCP/IP which is a means of compliance to the Flight Message Transfer Protocol implementing rule.