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Data Link Layer The layer of the OSI reference model that manages the operations of the physical layer and may utilise special error detection or retransmission techniques to achieve acceptable error rates. This layer providing service to the network layer; concerned with the reliable transfer of data across the data link. Data Model Conceptual architecture of data in a database, which comprises entities, attributes and relationships among them. Data Processing The execution of a systematic sequence of operations performed on data. Synonymous with information processing. Data Processing Function A utility of a flight data processing system (e.g. correlation, SSR code management etc.). Data Recording It corresponds to the data acquisition and conversion functions in the SASS-C system. Data Terminal Equipment A user system accessing a network. A digital data transmitter/receiver device that includes terminals and computers. Datagram service A service providing the transmission and reception of packets of data as discrete messages. Dead Reckoning Plotting position by calculating the effect of speed, course, time and wind against last known position. Dead Time The period of time during which a SSR transponder is inhibited from receiving signals after a valid interrogation is received and a reply transmitted. The term is also used to describe the time after the normal range for returns and before the next transmission of an interrogator or from a primary radar system. Debriefer Peers Debriefer peers are colleagues stressed by a critical incident. They assist in defining the educational programme and the need for CISM after the critical incident. Debriefing A review and discussion on the outcome of a training event based on a formative assessment of that event. Briefing and debriefing are always included as an integral part of the training technique. Decision Altitude Height A specified altitude or height (A/H) in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. (ICAO).