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SDM-100A Satellite Modem
Refer to Figure B-6 for the Reed-Solomon code page format. The outer Codec reads the
data in blocks of n bytes and recalculates the check bytes that were appended by the
encoder. If the recalculated data bytes do not match the check bytes received, the Codec
makes the necessary corrections to the data within the data block. The letter k represents
the total number of bytes in a given block of data out of the Codec. The letter n
represents the number of data bytes in a given block.
The term, k - n = 2t, is the total number of check bytes appended to the end of the data.
The terms k, n, and t will vary depending on the data rate being used. The Codec then
sends the corrected data to a FIFO.
Because the check bytes are not part of the real data, a synchronous FIFO is used to
buffer the data and strip the check bytes out of the blocks of data. The data then passes
through a parallel converter to be serialized.
The data is sent through a self-synchronizing serial descrambler in accordance with the
INTELSAT-308 Rev. 6B specification. The descrambler converts the data back into the
original data that the user intended to send. The synchronous descrambler is synchronized
by the detection of the unique word at the end of each Reed-Solomon page. The data is
then sent to the interface PCB for further processing.
Rev. 0
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B–23
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