Comtech EF Data SDM-300A - Manual (Page 54)

SDM-300A Satellite Modem Revision 6
Functional Description MN/SDM300A.IOM
2–14
2.5.3 Closed Network
Typically, the closed network operation does not add overhead to the terrestrial data. The closed
network operation is not dictated by a specification. The terrestrial data and clock are passed
through the baseband loopback relay and are translated from the selected baseband format to
TTL. The data is re-synchronized by the clock and the data stream is then output to the
modulator through the interface loopback device.
The receive data from the demodulator/decoder is input to the buffer. User data from the
DEMUX section may be optionally input to the buffer.
The front panel interface provides four clock selections clocking the data out of the buffer:
• Internal Clock (SCT)
• RX Recovered Clock (RXCLK)
• External Clock (EXT)
• TX Clock Dejittered (TXCLK)
If either RXCLK, SCT, or EXT is selected and then fails, the interface will automatically switch
to RX Sat CLK as the source. The receive data and selected clock are translated to the levels of
the selected baseband interface and output through the baseband loopback relay.
2.5.4 Open Network
The modem is a high-performance, full-duplex, digital-vector modulator/demodulator. The
modem meets the open network requirements of the INTELSAT IESS-308 and -309
specifications for IDR, IBS, and EUTELSAT SMS.