Comtech EF Data SDM-300A, Инструкция по установке и эксплуатации

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SDM-300A Satellite Modem Operation
Rev. 4 3–93
3.5.5 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:
•
Blocking diagram
•
Transmit clock options
•
Buffer clock options
3.6 Buffering
The purpose of a receive buffer is two reasons:
•
Plesiochronous buffering of two dissimilar clock frequencies (normally the far
end transmit clock verses 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” (Figure 3-33) station keeping movement 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.