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Unformatted text preview: try System Orientation
Page 2-8 January 1987 * * CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE The final verification of proper command delivery and execution is, of course, provided when
the user observes the effect of the command via measurement data received from the TLM
System. 2 . 2 . 2 TELECOMMAND DATA STRUCTURES
Figure 2-4 illustrates how the various TC data structures within the Packetization,
Segmentation, Transfer, and Coding layers map into one another. As previously noted, there
is presently no attempt by the CCSDS to define the data structures of the top two layers of the
TC System, i.e., the Application Process layer and System Management layer.
TC Packets which are longer than the data field of a TC Transfer Frame are broken into
suitable-sized pieces and placed into the data field of TC Segments. A TC Segment can only
contain a portion of one TC Packet, i.e., if a string of Packets is input to the Segmentation
layer, new Segments must begin at each Packet boundary. A very long TC Packet will be
broken into "n" TC Segments, where the length of segments (1) through (n-1) will match the
maximum length of the TC Frame data field, and segment (n) will be of a length that
corresponds to the residue of data from the Packet. TC Segments are, on a one-to-one basis,
placed into the data fields of individual TC Transfer Frames and are encapsulated by the frame
header and (optionally) the trailing Frame Error Control code.
As specified in Reference , the Transfer Frame Error Control field may be
needed in order to meet certain mission-defined Transfer layer performance
A guideline which describes the recommended
encoding/decoding procedure for the Frame Error Control field is contained in
Annex B of this report. Reference  discusses the performance of this code
in more detail. Each TC Transfer Frame is piecewise-encoded into a series of short, fixed length TC
Codeblocks which provide error detection or correction capabilities. Successive blocks of
information bits from each TC Frame are placed into the data space of each codeblock, to
which computed parity bits are appended.
The resulting string of TC Codeblocks is encapsulated within a "Command Link Transmission
Unit" (CLTU) data structure. N ote that each CLTU may contain the encoded
representation of one or more TC Frames, i.e., several frames may be placed back-toback, encoded and inserted into one CLTU. Each CLTU contains a Start Sequence and ends
with a Tail Sequence. It is these delimited CLTUs which are modulated onto the rf carrier and
physically radiated to the receiving spacecraft. Issue 6 Page 2-9 January 1987 PKT HDR USER DATA • SEGMENTATION SEG HDR SEG HDR SEG HDR • FRAME
CONSTRUCTION FRM HDR FRM HDR FRM HDR K BITS K BITS Page 2-10 • CODEBLOCK
CONSTRUCTION • COMMAND LINK
CONSTRUCTION CB#1 CB#2 K BITS K BITS ••• ST SEQ K BITS CB#N TAIL SEQ
CLTU 1 NOTE: THE DATA FIELD OF EACH CLTU CONTAINS THE ENCODED
REPRESENTATION OF ONE OR MORE TRANSFER FRAMES. January 1987 Figure 2-4: Telecomm...
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