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Unformatted text preview: TRANSMISSION AS REQ'D.
OUTPUT: BUFFER OF TC DATA
BITS (REPRESENTING, E.G.,
TRANSFER FRAMES). RELIABLY
FRAMES FUNCTION: RECONSTITUTE
ORDERED SET OF TRANSFER
FRAMES. PERFORM FRAME
REPORT STATUS OF SAME VIA
EXTRACT SEGMENTS. COMMAND LINK
(CLCW) INPUT: "CLEAN" TC DATA PLUS
FILL BITS. DATA START/STOP
INDICATORS. STATUS CODING LAYER Figure 3-5: TC Transfer Layer Reporting Mechanism" (FARM) at the receiving end of the layer in order to set up the receiving
The sending end of the layer passes the assembled TC Frames to a "Frame Operation
Procedure" (FOP), which provides the control of their transmission to the spacecraft. The
FOP batches the TC Frames and delivers them to the layer below (the Coding layer) as buffers
of serial information bits (corresponding to one or more back-to-back frames) for encoding and
transmission to the spacecraft.
At the receiving end of the TC Transfer layer, the inputs from the layer below are "clean"
decoded information bits and data stop/start indicators which mark the gross boundaries of the
buffers of bits that were given to the Coding layer at the sending end: however, trailing fill bits
Issue 6 Page 3-9 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE may have been inserted by the encoding process. The Transfer layer therefore uses the length
information in each TC Frame to delimit each frame in the buffer, and to discard any trailing
The reconstructed, delimited frames are input to the receiving-end FARM, which performs
validation and acceptance checks based on sequence information and other criteria that are fully
specified in Reference . The status of frame acceptance by the FARM is telemetered back to
the FOP at the sending end via the Command Link Control Word (CLCW) in the trailers of
standard Telemetry Transfer Frames. The pair of peer-layer procedures executed by the FOP
and FARM are together known as a "Command Operation Procedure" (COP). Based on
CLCW reports from the FARM, the FOP makes appropriate retransmission decisions for
frames which were rejected or otherwise missed by the FARM, according to the rules of the
Under closed-loop retransmission control of the selected COP, and utilizing the errorprotection services of lower layers, the TC Frames are assembled at the receiving end of the
layer so that they are error-free, in sequence and with no omission or duplication. Once this
service is guaranteed, their data contents are then stripped and passed to higher layers.
Three COPs, which are recursively related to each other, are presently defined. In order of
increasing complexity they are:
COP-0 operates on the principle of sequential frame acceptance and retransmission,
without frame sequence numbering. The FOP initiates the transmission of TC
Frames whose sequence numbers are not used. The FARM only accepts frames if
they are received withou...
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