The basic protocol data unit of the coding layer is

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Unformatted text preview: data unit of the Coding layer is the TC Codeblock, which appends a small block of information bits with parity bits that provide error detection and (optionally) correction capability. Strings of TC Codeblocks, conveying the information bits representing one or more TC Transfer Frames in the form of parity-protected channel symbols, are encapsulated within a Command Link Transmission Unit (CLTU) before being passed to the layer below. Any errors that occur in the encoded information bits as a result of the physical transmission process may be detected or corrected at the spacecraft receiving end of the Coding layer. (2) The TC Physical layer modulates the CLTUs onto the rf carrier, and provides the procedures necessary to activate and deactivate the channel. Reflecting an evolutionary philosophy, the CCSDS has focussed its attention on the Channel Service and the Data Routing Service and, with the exception of the Packetization layer, has at present left many of the functions within the Data Management Service for future specification. (Note: this is the reason why the Part 3 Recommendation is called an "architectural definition" whereas the other two Parts are called "architectural specifications".) A more complete specification of the upper layer functions and data structures may be performed once more operational experience is gained with the lower layer services. Full advantage of the CCSDS TC System architecture would be realized if a space project organization complied with all of its fully specified layers of protocol: at present, this effectively means the Packetization, Segmentation, Transfer, Coding, and Physical layers. However, a project organization can conceptually interface at any layer of the TC System, Issue 6 Page 2-6 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE provided that all lower layers are used, as long as it conforms to the interface data structures that are defined in the three main CCSDS TC Recommendations (References [2], [3], and [4]). In such cases, only a reduced set of standard services may be made available to the project. 2 . 2 . 1 TELECOMMAND/TELEMETRY SYSTEMS RELATIONSHIP The Telecommand System is balanced by a return link Telemetry (TLM) System. Figure 2-3 shows the conceptual relationship, in an operational environment, between the TC System (which allows users to control remote instruments or subsystems) and the TLM System (which returns measurement data from those instruments or subsystems back to the user). As far as the flow of telecommands is concerned, the two systems work hand-in-hand to assure the transfer of data from the sending end to the receiving end of the TC System. CCSDS recommendations for the standardization of the Packet Telemetry and Telemetry Channel Coding data structures and procedures are contained in References [6] and [7]. It should be noted that these two TLM Recommendations were written before the CCSDS telecommand work was mature, and therefore the TLM System does not contain the same formal layering hierarchy as the TC System. Howeve...
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This document was uploaded on 03/06/2014.

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