Unformatted text preview: n of the LAST frame for a single
CLTU containing 1, 2, 3, and 4 maximum length frames. Figures are derived only for the 64bit codeblock (n=63). Table D-6: Probability of Last Frame Rejection in a Multiple Frame CLTU
Using Maximum Length Frames and 64-Bit Codeblocks
DeProbability of rejection of last
frame, with channel BER of
1 2048 37 TED
7.24x10-8 2 4096 74 TED
1.45x10-7 3 6144 110 TED
6.07x10-1 8.92x10-2 9.30x10-3
2.88x10-3 2.89x10-5 2.89x10-7
4 D-4.3 8192 147 Multiple CLTUs While in theory any number of frames may be sent in one CLTU, very long CLTUs tend to be
counter-productive: that is, the probability of frame rejection increases as the CLTU is made
excessivey long. In such cases it may be preferable to divide a large command load into a
number of CLTUs sent sequentially. These CLTUs may either be: 1) sent as independent
entities (modulation is dropped between CLTUs and the entire resynchronization process
begins anew as in the single CLTU case), or 2) strung together in sequence while maintaining
bit sync on the channel.
Issue 6 Page D-15 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE If CLTUs are separated by dropping modulation, each CLTU will perform as an independent
entity and may be analyzed as shown in Sections D-4.1 or D-4.2. However, if the CLTUs are
strung together tail-to-start with an uninterrupted bit stream as shown in Figure D-5, some
further considerations are necessary. CLTU 1 ACQ CLTU 2 START TAIL START TAIL INFO P INFO P INFO P INFO P INFO P FRAME 1 FRAME 2 Figure D-5: Multiple Frame CLTU Organization (Contiguous CLTUs) With this organization, the CLTU Tail Sequence becomes important, since the Tail Sequence
must be relied upon to declare a CODEBLOCK REJECTION and force the decoder into
SEARCH STATE at the end of each CLTU. ONLY IF THE DECODER IS IN SEARCH
STATE WILL IT BE ABLE TO RECOGNIZE THE START SEQUENCE OF THE NEXT
The decoder enters SEARCH state when a CODEBLOCK REJECTION is encountered (in this
case caused by the tail sequence.) However, it is possible for the decoder to "miss" a tail
sequence (i.e., the intended function, CODEBLOCK REJECTION, does not occur). Such a
condition could happen if bit errors on the channel suitably change the tail sequence to make it
an apparently valid codeblock, or (in SEC mode) make it appear (wrongly) to be correctable.
This requires the received tail sequence to have at least 2 introduced bit changes in the TED
mode, or 1 introduced bit change in the SEC mode.
Upon missing a tail sequence, the decoder remains in DECODE state after the first CLTU's tai...
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