Comtech EF Data SDM-9000 - Manual (Page 152)

Theory of Operation SDM-9000 Satellite Modem
5–24 Rev. 4
5.5.3.2 Receive Data Path
Data from the demodulator is routed directly to an installed and enabled Reed-Solomon
Codec and decoded data is returned. The returned data is descrambled by the
Reed-Solomon onboard descrambler (if enabled).
If Reed-Solomon is not used, one of the three descramblers (or no descrambler) can be
selected. The ESC overhead is then removed from the descrambled or decoded data.
If receive 2047 data is selected, the data is searched for the pseudo-random 2047 pattern.
The error rate is measured after finding the pattern. If the error rate exceeds one error in
a hundred bits (over a 16 kbit/s interval), the pattern search is resumed. The measured
error rate can be monitored from the front panel of the modem. During the pattern
search, “no data available” is reported. An alarm on AIS or a data stable state can be
selected as in the transmit path.
An optional plesiochronous buffer removes Doppler shift and clock differences from the
data. The frame size presumed to implement the plesiochronous slips is implied by the
data rate. There are two standards for the 34.368 Mbit/s data rate (see table below). The
selection must be made from the front panel.
Data Rate Frame Structures
8.448 Mbit/s None
G.704
G.742
G.745
32.064 Mbit/s None
G.752
34.368 Mbit/s None
G.751
G.753
44.736 Mbit/s None
G.752
51.840 Mbit/s None
STS1
The buffer depth can be programmed from 2 to 32 ms, in 2 ms increments. The terrestrial
side of the buffer can be clocked by one of the following sources:
•
Clock recovered from the TX terrestrial data (dejittered)
•
Clock recovered by the demodulator (RX satellite)
•
External clock
•
Internal clock
•
High stability clock (if installed on the modulator)