SDM-300L3 Satellite Modem
Revision 1
Clocking Options
MN/SDM300L3.IOM
6–2
The use of loop timing in the modem is an option for both EIA-422 and V.35 operation.
SCT (LOOP): 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. The SCT (LOOP) indication serves as a reminder that the SCT internal clock is
now the recovered clock, not the internal oscillator. Select SCT (LOOP) when the
terrestrial equipment does not provide a transmit terrestrial clock.
6.2 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 6-3 for:
•
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.
6.2.1 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 6-4 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.
