The system management layer draws upon services

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Unformatted text preview: provided by the underlying Packetization layer in order to initiate the data transport process. (3) The TC Packetization layer formats TC application data into end-to-end transportable data units called TC Packets. A TC Packet is a basic user data unit that is transported "up" to the spacecraft by the TC System: it is virtually identical to a Telemetry Packet, which is the basic user measurement data unit that is transferred "down" to the user through the CCSDS Telemetry System. The Packetization layer utilizes the underlying services of the TC Data Routing Service to accomplish its functions. The TC DATA ROUTING SERVICE contains the intermediate two layers: the TC Segmentation layer and the TC Transfer layer. The Data Routing Service supports the errorcontrolled transmission and retransmission of standard TC Packets or TC Segments through the data link to the spacecraft, or other user data structures from non-standard higher layers. (1) Issue 6 The TC Segmentation layer provides flow control services. Recognizing that user data units from the layer above may be too long for efficient handling by lower layer processes, the Segmentation layer is available to break them into smaller communications-oriented pieces (TC Segments) for transfer through the space data channel. It also provides "Multiplexer Access Points" (MAPs) which allow different user data units to be multiplexed together for data flow control purposes. TC Segments are of proper size for placement into the data unit of the next lower layer, the TC Transfer Frame. If the user data units are already short enough to be Page 2-5 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE compatible with insertion into the TC Transfer Frame, and the MAP feature is not required, the entire Segmentation layer may be bypassed. (2) The TC Transfer layer uses its own data structure, the TC Transfer Frame, to reliably transport TC Packets, TC Segments or other higher layer user data structures through the telecommand channel to the receiving spacecraft. As the heart of the CCSDS Telecommand System, the Transfer layer offers a range of delivery service options designed to satisfy various mission needs. It contains the retransmission procedures required to reliably deliver TC Transfer Frames to the spacecraft, plus the facility to multiplex these frames together into "Virtual Channels" (VCs). The Transfer layer uses the underlying TC Channel Service to accomplish its role. The TC CHANNEL SERVICE contains the bottom two layers: the TC Coding layer and the TC Physical layer. The Channel Service supports the radiation of telecommand information through the physical uplink path to the spacecraft. (1) The TC Coding layer satisfies the TC System requirement for the error-free delivery of commands to the spacecraft, by encoding the TC user data from the layer above to protect them against noise-induced errors during transmission through the underlying ground-to-spacecraft radio frequency (rf) channel. The basic protocol...
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