20 x 10 6 120 x 10 8 120 x 10 10

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Unformatted text preview: 10-5 PsB 1.20 x 10-6 1.20 x 10-8 1.20 x 10-10 ________________________________________________________________ PsA or PsB, above (depending on the strategy selected), constitutes the first factor of the overall frame rejection rate. It should be noted that there is also the remote possibility that the proper start sequence may be missed because of a false (premature) start. The start pattern was chosen for its property that it would require at least 6 changed bits to declare such a premature start. D-3.3 Codeblock Factor Once CLTU Start has been recognized, the decoding process begins for the codeblocks that follow. Issue 6 Page D-7 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE The code specified in Reference [4] is a (63,56) BCH code. The codeword2 contains an information field of one of the lengths shown in Table D-2, followed by 7 parity bits. To complete the codeblock (which, to meet general formatting rules for ease of handling, must be an integral number of octets), a single final fill bit set to "zero" is always appended to the codeword. Normally, one TC codeblock length is selected for a mission, and all codeblocks are this length. When decoding in the "Triple Error Detection" (TED) mode, the code has the property that it can detect one-, two- or three-bit errors. Alternatively, when decoding in the "Single-Bit Error Correction" (SEC) mode, it can correct one bit in error, and detect two bits in error within the codeblock. Decoding is performed by the Channel Service on a continuum basis. That is, as long as each codeblock is valid (after correction, if error correction is employed) the information content of each codeblock as it is accepted continues to be passed from the Coding layer to the Transfer layer where each frame is assembled. If a codeblock failure occurs, the continuum is broken; the decoder notifies the Transfer layer and ceases transferring any further data to it. While 64 bits is the preferred codeblock length (because it allows minimum coding overhead to achieve the given performance), it is possible to shorten the codeblock while still using the same coding algorithm. Permitted lengths for shortened codeblocks are 56, 48, and 40 bits, and are made possible by simply setting both encoder and decoder to assume the leading untransmitted octets of each codeblock are always "zero". This is called "virtual fill". The permitted organizations of the codeblock, including the relationship between codeblock length, virtual fill, information bits, parity bits, and fill bits, are shown below: Table D-2: Codeblock Organization ___________________________________________________________________________ Codeblock Virtual Codeword Information Parity Fill length, fill, length, bits (k) bits bit bits bits bits (n) ___________________________________________________________________________ 64 0 63 56 7 1 56 8 55 48 7 1 48 16 47 40 7 1 40 24 39 32 7 1 ___________________________________________________________________________ 2 "Codeword" refers to the code itself; for an (n,k...
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