P300H
P300 Series Modem Installation and Operating Handbook
Page 28
Closed Net plus ESC [Async ESC Feature] provides variable rate
async ESC, optional synchronous scrambler above 32kbps to replace
error multiplying V.35 scrambler, optional backward alarm facility, and
optional Timeslot ID Maintenance when used with Drop/Insert, all in
minimum possible overhead down to <0.5%.
INTELSAT IBS & Eutelsat SMS [IBS/SMS Feature]. Framing to IESS
309 & IESS 310
INTELSAT IDR [IDR Option]. Framing to IESS 308 & IESS 310
Custom Framing [`Custom Features` feature]. Provides custom
framing based on IBS or IDR formats. For example the use of 16 kbps
ADPCM on the IDR audio ESC channels, providing an IDR carrier with
all normal facilities but with 32kbps of overhead spare for continuous
traffic quality BER monitoring (using built in PRBS tester with automatic
results logging or external BERT)
Poor BER performance
Deframer includes extended threshold operation, which improves
performance when used with Reed-Solomon in very poor BER
conditions (where a single uncorrectable RS codeword can contain
enough corrupt frame alignment words to knock an INTELSAT
specified deframer out of frame sync).
[INTELSAT Reed Solomon Codec & Custom Features]
Format
Concatenated Reed Solomon outer codec to IESS 308/310
Code Rate
Default n, k, t = (126, 112, 7) depth 4, automatically switching to: (225,
205, 10) depth 4 for 1544 kbps IDR mode. (219, 201, 9) depth 4 for
2048 kbps IDR mode & TCM <=1544 kbps, and (219, 201, 9) depth 8
for TCM >1544 kbps
Processing Delay
Combined Encoder & Decoder: 8 x (2n - k + 60)
Combined Interleaver & De-interleaver: 8 x n x depth (Calculate delay
time using data rate including RS overhead)
[Custom Features]
When available allows arbitrary selection of `n` & `k` to provide fully
variable code rate.
`n` = 60-255
`k` = n-2 to n-20 step 2
Interleaving depth of four or eight
The `Custom Features` Feature allows use of shorter codewords to
reduce interleaver / de-interleaver delay on low data rate circuits. For
example switching from n, k, t = 126, 112, 7 to 64, 56, 4 provides
approximately the same correction ability (7 in 126 = 5.5% and 4 in 64
= 6.25% respectively), with similar overheads (126/112=12.5%,
64/56=14.3%), but with interleaving & decoder delays reduced from
5632 to 3104 bits (156 ms to 85 ms at 32 kbps).
