Comtech EF Data SDM-9000 - Manual (Page 126)

SDM-9000 Satellite Modem Operation
Rev.4 4–53
4.3 Clocking Options
Clocking of data between the terrestrial network and the satellite network depends on the
application. Sections 4.3.1 and 4.3.2 define the G.703 and ECL/MIL-STD-188 clocking
configurations.
4.3.1 G.703 Interface Clocking
Refer to Figure 4-21 for the clocking diagram.
4.3.1.1 TX Timing
The TX timing for the G.703 interface is always recovered from the send data supplied
by the terrestrial network. The recovered TX terrestrial clock is used as the reference to
send information over the satellite network.
4.3.1.2 RX Timing
The RX (satellite) clock is recovered from information demodulated and decoded from
the satellite network.
4.3.1.2.1 RX Timing (with Buffer)
If a plesiochronous/Doppler buffer is used, data received from the satellite network is
clocked into the buffer using the RX (satellite) clock. Data is clocked out of the buffer
and to the terrestrial network referenced to one of the four selectable clock sources listed
in the table below:
INTERNAL CLOCK Generated internal to the modem
TX TERRESTRIAL CLOCK Recovered from the terrestrial send data
EXT. REF. CLOCK External reference supplied by the terrestrial network
RX CLOCK (See note) Recovered from the RX satellite network
Note: When the RX clock is selected as the buffer clock source, the input and output
of the buffer are clocked by the same source. In this mode, the buffer is virtually
bypassed with a delay defined by the selected buffer size.
4.3.1.2.2 RX Timing (without Buffer)
If a plesiochronous/Doppler buffer is not used, RX data from the satellite network is
clocked to the terrestrial network using the RX (satellite) clock.