As long as these service boundaries are preserved the

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Unformatted text preview: ng as these service boundaries are preserved, the internal operations within an individual layer are unconstrained. Consequently, an entire layer within a system may be removed and replaced as needed by user or technological requirements without destroying the integrity of the rest of the system. Further, as long as the appropriate interface protocol is satisfied, a customer (user) can conceptually interact with the system/service at any of its component layers. Layering is therefore a powerful tool for designing structured, user-responsive data systems which may easily change owing to the evolution of requirements or technology. As shown in the service model (Figure 2-2), the CCSDS TC System contains the following seven distinct layers: (1) TC Application Process layer (2) TC System Management layer Issue 6 Page 2-3 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE LAYER SERVICE PROVIDED BY LAYER APPLICATION PROCESS LAYER ALLOWS HUMAN USERS TO SUPERVISE REMOTE PROCESSES BY INTERFACING WITH SPACE TELECOMMAND SYSTEMS. COMMAND DIRECTIVE SYSTEM MGMT LAYER CONVERTS USER COMMAND DIRECTIVES INTO TRANSPORTABLE APPLICATION DATA UNITS AND SUPERVISES THEIR DELIVERY AND EXECUTION. TC APPLICATION DATA PACKETIZATION LAYER TRANSPORTS APPLICATION DATA UNITS IN AN ERROR-FREE MANNER TO THE RECEIVING END OF THE SYSTEM MANAGEMENT LAYER ON THE SPACECRAFT. TC PACKET SEGMENTATION LAYER BREAKS LONG HIGHER-LAYER TC DATA UNITS INTO SHORTER COMMUNICATIONS-ORIENTED PIECES, AND MULTIPLEXES DIFFERENT DATA UNITS TOGETHER (OPTIONAL SERVICES). TC SEGMENT TRANSFER LAYER RELIABLY TRANSFERS HIGHER LAYER TC DATA UNITS TO THE SPACECRAFT, THROUGH THE SPACE DATA CHANNEL, UNDER ERROR-CONTROLLED CONDITIONS. TC TRANSFER FRAME PROTECTS HIGHER LAYER TC DATA UNITS AGAINST ERRORS INDUCED DURING TRANSMISSION THROUGH THE PHYSICAL PATH TO SPACECRAFT. CODING LAYER CLTU PROVIDES THE PHYSICAL CONNECTION, VIA RADIO FREQUENCY SIGNALS, BETWEEN A TRANSMITTING STATION AND THE RECEIVING SPACECRAFT. PHYSICAL LAYER PHYSICAL WAVEFORM Figure 2-2: Layered Telecommand Service Model Issue 6 Page 2-4 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE (3) TC Packetization layer (4) TC Segmentation layer (5) TC Transfer layer (6) TC Coding layer (7) TC Physical layer The TC DATA MANAGEMENT SERVICE, containing the top three layers, provides command data delivery and management services for the user. (1) The TC Application Process layer provides the first-level interface between the user and the TC System, by insulating the user from the physical aspects of the command delivery processes. The Application Process layer translates the user requests into high-level command directives which are interpretable by the underlying System Management layer. (2) The T C System Management layer translates the high-level command directives into process-interpretable TC application data, plus control instructions to lower layers which establish the parameters for their transport to the spacecraft. The System Management layer draws upon services...
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