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Unformatted text preview: ional 16-bit field
(which occupies the two trailing octets of the TC Frame) is to provide frame error control
information. The Frame Error Control Field is used only for error detection, but it is
particularly useful when project requirements dictate an undetected frame error performance
that exceeds the inherent performance provided by the underlying Channel Service.
Performance information is found in Annex B and Reference . If used, the preferred error
control technique is the Cyclic Redundancy Check (CRC) described in the following
188.8.131.52.1 Encoding Procedure. The encoding procedure accepts a Transfer Frame,
excluding the Frame Error Control Field, and generates a systematic binary block code by
appending a 16-bit Frame Error Control Word (FECW) as the final 16 bits of the codeblock.
The equation for the FECW is:
FECW = [(X16 . M(X)) + (X(n-16) . L(X))] modulo G(X)
where all arithmetic is modulo 2
M(X) = the (n-16)-bit message to be encoded expressed as a polynomial
with binary coefficients L(X) = the presetting polynomial given by:
15 Xi L(X) =
i=0 (i.e., all “1” polynomial of order 15)
G(X) = the generating polynomial given by:
G(X) = X16 + X12 + X5 + 1 n = the number of bits in the encoded message The X(n-16) . L(X) term has the effect of presetting the shift register to an all
“1” state prior to encoding.
A possible implementation of an encoder is described in Reference . CCSDS 202.0-B-2 Page 4-8 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT
CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE 184.108.40.206.2
Decoding Procedure . The entire TC Frame, including the Frame Error
Control field is provided to the decoder.
The error detection syndrome, S(X), is given by
S(X) = [(X16 . C*(X)) + (Xn . L(X))] modulo G(X)
where C*(X) = the received block in polynomial form
S(X) = the syndrome polynomial, which will be zero if no error is
detected and non-zero if an error is detected. A possible implementation of a decoder is contained in Reference .
4.2.2 COMMAND LINK CONTROL WORD (CLCW) FORMAT The CLCW, which is a four-octet word that is conveyed in the Operational Control Field of a
CCSDS Telemetry Transfer Frame (Reference ) or Virtual Channel Data Unit (VCDU)
(Reference ), provides the mechanism by which the FARM repor...
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