The data-link layer provides the necessary protocols to facilitate concurrent and reliable data transfer for multiple users. These protocols have mainly been developed by the University of Salzburg. The functional blocks of the LDACS data link layer architecture are organized in two sub-layers: The medium access sub-layer and the logical link control sub-layer. The logical link control sub-layer manages the radio link and offers a bearer service with different classes of service to the higher layers. It comprises the Data Link Services (DLS), and the Voice Interface (VI). The medium access sub-layer contains only the Medium Access (MAC) entity. Cross-layer management is provided by the Link Management Entity (LME). The Sub-Network Dependent Convergence Protocol (SNDCP) provides the interface to the higher layers.
The MAC entity of the medium access sub-layer manages the access to the resources of the physical layer. Prior to fully utilizing the system, an aircraft has to register at the controlling ground-station in order to get a statically assigned dedicated control channel for the exchange of control data with the ground-station. The ground-station dynamically allocates the resources for user data channels according to the current demand as signaled by the aircraft. Except for the initial cell-entry procedure all communication between the aircraft and the controlling ground-station (including procedures for requesting and allocating resources for user data transmission and retransmission timer management), is fully deterministic and managed by the ground-station. Under constant load, the system performance depends only on the number of aircraft serviced by the particular ground-station and linearly decreases with increasing number of aircraft.
The DLS provides the acknowledged and unacknowledged exchange of user data. The ground-station LME provides centralized resource management for LDACS. It assigns transmission resources, provides mobility management and link maintenance. It assigns resources taking channel occupancy limitations (e.g. limiting the aircraft duty cycle to minimize co-site interference) into account. In addition, the LME provides dynamic link maintenance services (power, frequency and time adjustments) and supports adaptive coding and modulation. The VI provides support for virtual voice circuits. The voice interface provides only the transmission and reception services, while LME performs creation and selection of voice circuits. Voice circuits may either be set-up permanently by the ground-station LME to emulate party-line voice or may be created on demand.
LDACS shall become a sub-network of the Aeronautical Telecommunications Network (ATN). The SNDCP provides the LDACS interface to the network layer and a network layer adaptation service required for transparent transfer of Network layer Protocol Data Units (N-PDUs) of possibly different network protocols (ATN/IPS and ATN/OSI). The SNDCP should also provide compression and encryption services required for improving and securing the wireless channel.
A detailed specification of the LDACS protocols can be found here.
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