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SLM-3650 Satellite Modem
Operation
3.6.5
D&I G.703 Master/Master
In the D&I configuration, the most typical clocking option is the master/master
application. Refer to Figure 3-31 for:
•
Clocking block diagram
•
Transmit clock options
•
Buffer clock options
3.7
Buffering
The purpose of a receive buffer is to provide:
•
Plesiochronous buffering of two dissimilar clock frequencies (normally the far
end transmit clock vs. the local network clock). The clocks may be very close in
frequency to each other and will normally slip at a constant rate. Figure 3-32
shows plesiochronous operation for dissimilar clocks. If incoming traffic is too
fast, an occasional bit will be lost. If incoming traffic is too slow, an occasional
bit will be repeated.
•
Doppler buffer of the signal of the satellite. The Doppler shift results from the
"figure 8 Station-keeping movement" (Figure 3-33) performed by the satellite in
space over a period of one day. Doppler shift should not result in a clock slip, as
the buffer will constantly fill and empty.
If the two earth stations are configured as master/slave, then the buffer need only be
configured for Doppler operation. The buffer will then have sufficient capacity for the
Doppler shift on the outward and return paths.
A buffer set up for Doppler operation only will typically require less depth than one
intended for both Doppler and plesiochronous operation.
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Revision 3
MN/SLM3650.IOM
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