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P300H P300 Series Modem Installation and Operating Handbook Page 27
Framing & Deframing
Formats Closed Network (unframed)
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).