Comtech EF Data SNM-1001L - Manual (Page 236)

SNM-1001L Satellite Modem Revision 1
Reed-Solomon Codec MN/SNM1001L.OM
15–4
Since errors from the Viterbi decoder usually occur in bursts, a block interleaver with a depth of
four 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.
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.
Figure 15-4. Reed-Solomon Code Page Format
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) 225, 205, 10
219, 201, 9