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SLM-5650 Satellite Modem Revision 2
Configuration MN/SLM5650.IOM
3–11
3.2 Clocking Options
Clocking of the data from the terrestrial equipment to the satellite (and vice versa) will depend on the
application. This section describes the most common options and recommended configurations.
SCT (INTERNAL) clock no longer applies when the modem has loop timing on. The TX clock source is
now recovered from the RX satellite data. This recovered clock is put out on the ST line and is used to
clock the terrestrial equipment. The transmit terrestrial clock is now essentially the same as the RX
satellite clock, except that it has been buffered by the terrestrial equipment.
Select TX TERRESTRIAL for the TX clock source when in loop timing, if the user equipment is being
slaved off the modem.
3.2.1 IDR/IBS G.703 Master/Master
Use this application when both earth stations have high stability clocks and the received data is to be
clocked to the local network. Refer to Figure 3-1for:
• Clocking block diagram
• Transmit clock options
• Buffer clock options
The disadvantage of the master/master application is that the receive data will slip, as the clocks will not
be synchronized. If the buffer is properly set up, the slips will be an exact frame length, causing minimum
loss of data. By using very high stability clocks, the expected time between slips can be several days.
Loss of the buffer clock will mean the buffer will not be emptied and data will not be available. The
buffer clock will normally revert to the low stability internal reference automatically.
3.2.2 IDR/IBS G.703 Master/Slave
Use this application when the far end earth station does not have local access to a high stability reference
clock, or when it is not required to synchronize with a local clock. Refer to Figure 3-2 for:
• Clocking block diagram
• Transmit clock options
• Buffer clock options for using external loop timing
Modem loop timing does not apply for G.703 operation. The terrestrial equipment must select loop timing
to recover the clock off the receive data and use that recovered clock for the transmit data.
The disadvantage of the master/slave application is that the signal received at the slave station is subject
to Doppler shift. The length of the buffer at the master end will need to be twice the length that is
normally required, compensating for the Doppler shift on the outward and return paths.