SLM-5650 Satellite Modem
Revision 1
Options
MN/SLM5650.IOM
A–4
Closed Network Modes
A 225,205 code is used in closed network mode. For a rate of 225,205 data is put into blocks
of 225 bytes, of which 205 bytes are data, and 20 bytes are FEC overhead. The code was
chosen because it is compatible with legacy Comtech EF Data modems.
For closed network Viterbi Reed Solomon, an interleaver depth of 8 is used. The increase in
coding gain is at the expense of delay. The interleaving/de-interleaving delay and the delay
through the decoder itself can be as high as 25 kbps. At low data rates, this equates to an
appreciable part of a second, when combined with the round trip delay makes it highly
unsuitable for voice applications. Additionally, the de-interleaver frame synchronization
method can add significantly to the time taken for the demodulator to declare acquisition.
Table A-2. Open Network Modes
Code Rate
Mode
219, 201
126, 112
IESS-308 Standard Lower Rates
225, 205
219, 201
194, 178
IESS-308 Standard Upper Rates
225, 205
219, 201
208, 192
194, 178
126, 112
IESS-308 Extended
219, 201
IESS-309 IBS, VSAT-IBS and Extended
219, 201
IESS-310
A characteristic of concatenated R-S coding is the very pronounced threshold effect. For any
given modem design, there will be a threshold value of Eb/No below which the demodulator
cannot stay synchronized. This may be due to the carrier-recovery circuits, or the
synchronization threshold of the primary FEC device, or both. In the SLM-5650, and Rate
1/2 operation, this threshold is around 4 dB Eb/No. Below this value, operation is not
possible, but above this value, the error performance of the concatenated R-S system
produces exceptionally low error rates for a very small increase in Eb/No.
CAUTION
Care should be taken not to operate the demodulator near its sync
threshold. Small fluctuations in Eb/No may cause total loss of the link,
with the subsequent need for the demodulator to re-acquire the signal.
