Comtech EF Data SDM-300A, Installation And Operation Manual

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SDM-300A Satellite Modem Operation
Rev. 4 3–87
3.5.3 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 3-29 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.
3.5.4 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 3-30 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.