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 .
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.
END TRANSFER LAYER INPUT: BUFFER OF TC DATA
BITS (REPRESENTING, E.G.,
TRANSFER FRAMES (ADD
FILL BITS AS REQUIRED).
CREATE CLTUs. OUTPUT: "CLEAN" TC DATA PLUS
FILL BITS. DATA START/STOP
INDICATORS. STATUS SERVICE RELIABLY
LINK TO S/C FUNCTION: SYNCHRONIZE
ERROR DETECT OR
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 . 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.
- Spring '14