# Controller-Pilot Data Link Communication Security
## 3 Controller-Pilot Data Link Communication  
The section gives an overview of controller-pilot data link communications (CPDLC). It describes the main principles, the different data link services, and the basics of CPDLC operation. CPDLC is a wireless digital message-based communication system used in parts of today's air traffic, enabling air traffic controllers and pilots to communicate via a data link. CPDLC is utilized as a secondary communication method, after the primary method of very high frequency radio (VHF-radio) [11]. VHF-radio is voice-based, half-duplex, and only works as fast as human speech allows. As the amount of air traffic steadily increases and is predicted to continue to increase in the future [12], voice communication systems become more and more congested. CPDLC provides the needed benefit of freeing up critical transmitting time for voice communication. Initial experiments demonstrated an about 84% reduction of voice space occupancy [13]. CPDLC reduces the risk of mishearing a clearance or other instructions, especially when a message contains multiple elements [1]. The workload of both air traffic controllers and pilots is reduced with the automation of event-driven reports and flight plan updating [14]. CPDLC greatly improves communication capabilities in oceanic areas, especially in situations where controllers and pilots have previously had to rely on a third-party HF communications relay. CPDLC uses the VHF Data Link Mode 2 (VDL2) with the frequency band from 118.000 to 136.975 MHz, and a data rate of 31.5 kilobits [15]. It connects through either the Aeronautical Telecommunications Network which covers mainly European, continental airspace, or the Future Air Navigation System FANS-1/A [16], a system that enables the possibility of connecting through a satellite communications network to provide the service in oceanic airspace. In order to connect and use CPDLC the aircraft crew has to perform a logon request. The logon is either an initial logon done by flight crew or a part of the forwarding procedure when the aircraft has to switch between two air traffic Air Traffic Services Units (ATSUs). An initial logon is done by the flight crew by entering the four-character identifier of the ATSU that the logon request is to be sent to. When the aircraft is to switch ATSU in flight, the system allows for a contact request to be sent, which triggers an automatic logon with the next ATSU, specified in the contact request message, initiated by the current ATSU (Figure 1). The logon request message contains the following information: aircraft identification (item 7 of the flight plan), aircraft registration and/or aircraft address (both in item 18 of the flight plan), and departure and destination aerodromes (items 13 and 16 of the flight plan). An example of a standard flight plan blanket is shown in Figure 2. The logon is done to provide the ATSU with information about what data link applications are supported by the aircraft system, identifying the aircraft and ensuring future messages will be delivered to the correct aircraft, and updates are made in the correct flight plan. After the logon request has been successful, only then can a connection request be sent by the ATSU to the aircraft. The aircraft system will then, if no other connection exists, accept the connection  
Figure 1: The controller–pilot data link communications (CPDLC) logon process. ATSU: Air Traffic Services Unit. request, establish an active connection, and send a connection confirmation message to the Current Data Authority (CDA). If the aircraft already has an active connection, and the aircraft system verifies the requesting ATSU as the Next Data Authority (NDA), it will instead accept the connection request and establish the connection as an inactive connection and send a confirmation reply to the CDA. Only the CDA can send a valid next data authority message which specifies the NDA by its four-character International Civil Aviation Organization (ICAO) identifier.