3 tc channel service operation of the tc channel

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Unformatted text preview: the TC Channel Service begins when at least one complete TC Transfer Frame is prepared for radiation through the telecommand channel to the spacecraft. The TC Channel Service provides error-controlled transmission of the TC Frame(s) through the channel. Error control is achieved via forward error detection/correction techniques. The TC Channel Service is composed of two layers, the Coding layer and the Physical layer. The operating data structures and protocols for the Channel Service are specified in Reference [4]. 3 . 3 . 1 TC CODING LAYER The basic service of the TC Coding layer is to provide for the reliable delivery of TC information bits across the physical medium to the spacecraft. Figure 3-7 depicts the activities and interfaces of the TC Coding layer. CODING LAYER SENDING END PEER PROCESS RECEIVING END TRANSFER LAYER INPUT: BUFFER OF TC DATA BITS (REPRESENTING, E.G., TRANSFER FRAMES). FUNCTION: ENCODE TRANSFER FRAMES (ADD FILL BITS AS REQUIRED). CREATE CLTUs. OUTPUT: "CLEAN" TC DATA PLUS FILL BITS. DATA START/STOP INDICATORS. STATUS SERVICE RELIABLY TRANSMIT TRANSFER FRAMES THRU LINK TO S/C FUNCTION: SYNCHRONIZE DECODER. DECODE CODEBLOCKS. ERROR DETECT OR CORRECT. COMMAND LINK TRANSMISSION UNITS (CLTU) OUTPUT: CLTUs NONE INPUT: "DIRTY" SYMBOL STREAM. STATUS. PHYSICAL LAYER Figure 3-7: TC Coding Layer Issue 6 Page 3-12 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE Inputs to the sending end of the Coding layer are buffers of TC information bits from the layer above. Each buffer corresponds to one or more serial, back-to-back TC Transfer Frames. The information bits are encoded, piece by piece, into short fixed length TC Codeblocks, whose format and encoding technique is specified in Reference [4]. Each Codeblock contains parity bits that provide error detection or correction capabilities for the information bits. Fill bits may be added by the Coding layer to complete the last Codeblock. The sequence of TC Codeblocks (i.e., the symbol representation of one or more TC Frames plus any appended fill) is then encapsulated into a Command Link Transmission Unit (CLTU). The boundaries of the CLTU are delimited for the receiving end of the Coding layer by the Start and Tail sequences. The delimited CLTUs are passed to the layer below, the TC Physical layer, for modulation onto the space data channel. At the receiving end of the TC Coding layer, a "dirty" (potentially corrupted by channel noise) symbol stream plus control information (e.g., whether the physical channel is active or inactive) is received from the layer below. Searching for the Start sequence, the Coding layer finds the boundaries of the CLTU, synchronizes the decoder with the TC Codeblocks, and decodes them. The decoder may operate in an error-detecting-only mode, or may optionally perform error correction. If no errors are detected, or (optionally) if errors are detected and corrected, the Coding layer passes "clean" octets of decoded TC data to the layer above (including any appended fill): CLTU Start and Tail Sequences, which are not decodable codeblocks, are not transferred. Should an...
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This document was uploaded on 03/06/2014.

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