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 f acility, and optional
Timeslot ID Maintenance when used with Drop/Insert, all in minimum
possible overhead down to <0.5%.
INTELSAT IBS & Eu te lsa t S M S [ I B S /S M S Feature]. Framing to IESS 309
& IESS 310
INTELSAT IDR [IDR Option]. Fram ing 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, pr oviding an IDR car rier with all no rmal
facilities but with 32kbps of overhead spare for continuous t raffi c quali ty
BER monit o r i n g (u s ing built i n P RBS tester with automatic results logging
or external BERT)
Poor BER performance D eframer includes extended threshold operation which improves
performance when used with Reed-Solomon in very poor BER conditions
(where a single unc orrectable RS codeword can contain enough corrupt
frame alignment words to knock an INTELSAT specified deframe r 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 switc hing 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 interleav i ng & decoder delays reduced from 5632
to 3104 bits (156 ms to 85 ms at 32 kbps).