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Formulate transport status reports telecommand

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Unformatted text preview: INSTRUCTIONS. UNDEFINED INPUT: TC PACKETS. TC FILES. SEGMENTATION LAYER Figure 3-3: TC Packetization Layer Packetization layer, containing control instructions which define the system conditions that must exist before the TC application data are passed back across the interface to the TC System Management layer. At the receiving end of the TC Packetization layer, the named sets of TC application data are extracted from the data fields of the TC Packets in the sequential order in which they were given at the sending end, and passed to the layer above. Status reports may be formulated and returned to the sending end of the layer (via Telemetry Packets) if information relating to the correctness, completeness, and sequentiality of the received data is required by particular mission processes: the CCSDS presently does not define these reports. It should be noted that many of the telecommanding functions within and above the Issue 6 Page 3-5 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE Packetization layer will probably be implemented within user application processes, such as the instrument and subsystems themselves, rather than as external supporting services. 3 . 2 TC DATA ROUTING SERVICE The TC Data Routing Service provides a pivotal function within the TC System by performing the error-controlled communication of higher layer TC data through the ground-to-space data link, relying on the lower layer TC Channel Service in order to perform its task. The combination of the Data Routing and Channel Services provides users with a powerful mechanism for the flow control and guaranteed transfer of TC data between the sending and receiving ends of the TC System. The CCSDS therefore has focussed its attention on fully specifying these services so that robust, mission-independent user support systems may be rapidly developed. The Data Routing Service contains two layers: the TC Segmentation layer and the TC Transfer layer. The detailed protocols and formats for the TC Data Routing Service are specified in References [3] and [5]. 3 . 2 . 1 TC SEGMENTATION LAYER The TC Segmentation layer is optional, i.e., it may be bypassed if its services are not required. The basic service of the Segmentation layer is to prepare variable-length TC data units from the layer above (e.g., TC Packets, or other user-supplied data structures) for transfer through the space data link, using the lower TC Transfer layer service. Since the data unit of the TC Transfer layer (the TC Frame) has an upper length limit, the Segmentation layer must break the higher layer TC data units into suitable sized pieces for insertion into the data field of the frame. A second aspect of its service is the capability to multiplex together segments of data from different TC data units for the purpose of flow control: to accomplish this, the Segmentation layer provides "Multiplexer Access Points" (MAPs) to the layer above. Figure 3-4 depicts the activities and interfaces of the TC Segmentation layer. TC Packets (or ot...
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