Comtech EF Data SDM-100A - Manual (Page 203)

SDM-100A Satellite Modem Options
Rev. 0 B–21
Since errors from the Viterbi decoder usually occur in bursts, a block-interleaver with a
depth of 4 is used in accordance with the INTELSAT-308 Rev. 6B specification. The
interleaver has the effect of spreading out the errors across blocks of data, instead of
concentrating the errors in a single block of data. Since there are fewer errors in any
given block, there is a greater chance that the Reed-Solomon decoder can correct the
errors on the receiving end of the satellite link. To allow the decoder to synchronize to
the data, four unique words are inserted in the last two bytes of the last two pages at the
end of each page of data (Figure B-6).
Once the data passes through the interleaver, it is fed through a parallel/serial converter
and sent back to the interface PCB. After further processing by the interface PCB, the
data is sent to the modulator PCB.
k BYTES
OF DATA
'RSWORD'
n BYTES
1 RSWORD
1 RS PAGE
RS SYNCH
PERIOD
UNIQUE WORD BYTES
GO INTO THESE BYTES
2+t
CHECK BYTES
TERRESTRIAL
DATA RATE
RS CODE
(n,k,t)
1544 (1640)
2048 (2144)
6312 (6408)
8448 (8544)
225, 205, 10
219, 201, 9
194, 178, 8
194, 178, 8
Figure B-6. Reed-Solomon Code Page Format