Comtech EF Data SDM-9000 - Manual (Page 151)

SDM-9000 Satellite Modem Theory of Operation
Rev.4 5–23
5.5.3.1 Transmit Data Path
Terrestrial data is passed through the baseband loopback relay, and is translated from the
selected baseband format into TTL. If the terrestrial data is in G.703 format, the data rate
clock is recovered from the data and line coding (B3ZS, etc.) is removed.
An alarm can optionally be set for AIS or data stable (AIS/Data) conditions. Blocks of
16 kbit/s of data are examined. If alarm on AIS is selected and fewer than 16 zeros are
found in a block, the alarm is asserted. If alarm on data stable is selected and fewer than
16 zeros or fewer than 16 ones are found in a block, the alarm is asserted.
The data is dejittered to remove pattern jitter from the modulator spectrum. If a transmit
fault is detected, AIS is substituted for the transmit data. If transmit 2047 data is selected,
the transmit data is substituted with a pseudo-random 2047 pattern (using the internal
clock if no data clock is available).
If ESC overhead is selected, 96 kHz of additional data is inserted into the transmit data
stream per IESS-308, where applicable. This data includes:
•
Two ADPCM audio channels or one 64 kbit/s digital data channel (64 kbit/s
digital channel available only with the AS/5618 interface)
•
Four backward alarms
•
8 kbit/s digital data channel
If the scrambler is enabled and Reed-Solomon encoding is not used, the data is then
scrambled. The standard IDR scrambler is normally used. The following three
scramblers are selectable:
IDR IESS-308
V.35 CCITT V.35
EFD Compatible with the proprietary alternate scrambler
used by the SDM-450
Unscrambled data is passed to an installed and enabled Reed-Solomon Codec. This
plug-on module includes a scrambler, and does not have interface board scrambler
selection. Coded data is returned. The data is then sent through the interface loopback
logic to the modulator.