As long as these service interfaces are preserved the

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Unformatted text preview: change data according to established standard rules or ‘protocols’. Each layer draws upon a well defined set of services provided by the layer below, and provides a similarly well defined set of services to the layer above. As long as these service interfaces are preserved, the internal operations within a layer are unconstrained and transparent to other layers. Therefore, an entire layer within a system may be removed and replaced as dictated 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 interact with the system/service at any of the component layers. Layering is therefore a powerful tool for designing structured systems which change due to the evolution of requirements or technology. A companion standardization technique that is conceptually simple, yet very robust, is the encapsulation of data within an envelope or ‘header’. The header contains the identifying information needed by the layer to provide its service while maintaining the integrity of the envelope contents. 2.2.2 PACKETIZATION LAYER Within TM Space Data Link Protocol, spacecraft generated application data are formatted into end-to-end transportable data units called ‘TM Source Packets’. These data are encapsulated within a primary header which contains identification, sequence control and packet length information. A TM Source Packet is the basic data unit telemetered to the user by the spacecraft and generally contains a meaningful quantity of related measurements from a particular source. 2.2.3 TRANSFER FRAME LAYER The TM Transfer Frame is used to reliably transport Source Packets (and Segments) through the telemetry channel to the receiving telecommunications network. As the heart of the CCSDS Telemetry System, the TM Transfer Frame protocols offer a range of delivery service options. An example of such a service option is the multiplexing of TM Transfer Frames into ‘Virtual Channels’ (VCs). CCSDS 130.1-G-1 Page 2-3 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE The TM Transfer Frame is a fixed length unit which was chosen to improve the ability to synchronize the frame with weak signals such as those found on space-ground links, and for compatibility with certain block oriented channel coding schemes. The (primary) header contains frame identification, channel frame count information and frame data field status information. An attached synchronization marker (ASM) signals the start of the TM Transfer Frame. The transfer frame data field may be followed by an optional trailer containing an operational control field and/or a frame error control field. The first of these fields provides a standard mechanism for incorporating a small number of real-time functions (e.g., telecommand verification or spacecraft clock calibration). The error control field provides the capability for detecting errors which may have been introduced into the frame during the data handling process. The delivery of transfer frames requires the services provided by the lower layers (e.g., carrier, modulation/detection, and coding/decoding) to accomplish its role. 2.2.4 CHANNEL CODING LAYER TM Synchronization and Channel Coding is used to protect the transfer frames against telemetry channel noise-induced errors. Reference [3] describes the CCSDS Recommended Standard for TM Synchronization and Channel Coding, including specification of a convolutional code, a Reed-Solomon block-oriented code, a concatenated coding system consisting of a convolutional inner code and a Reed-Solomon outer code, and of turbo codes. The basic data units of the CCSDS TM Synchronization and Channel Coding which interface with the physical layer below are the Channel Symbols output by the channel encoder. The RF channel physically modulates the channel symbols into RF signal patterns. Within the error detecting and correcting capability of the channel code chosen, errors which occur as a result of the physical transmission process may be detected and corrected by the receiving entity. Full advantage of all CCSDS Telemetry System services could be realized if a Project complied with all CCSDS Recommended Standards. Alternatively, Projects can interface with any layer of the Telemetry System as long as they meet the interface requirements as specified in the Recommended Standards (references [2], [3], and [5]). Figure 2-2 illustrates how the various telemetry data structures map into one another. There is presently no attempt to define the data structures of the top two layers of the telemetry system; i.e., the Application Process layer and the System Management layer. The Source Packets are placed into the data field of the Transfer Frame. An attached synchronization marker is always used, as shown in figure 2-2. CCSDS 130.1-G-1 Page 2-4 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE TRANSFER FRAME UNCODED TRANSMISSION + SYNC FRM HDR USER D ATA (TLM PACKETS) TRAI...
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This document was uploaded on 03/06/2014.

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