Comtech EF Data SDM-650B - Manual (Page 86)

Configuration SDM-650B Satellite Modem
3–44 Rev. 5
3.3.1 Buffer Setup
The receive buffer allows for:
• Plesiochronous buffering of two dissimilar clock frequencies: the far end transmit
clock and the local network clock frequency. The clocks may be very close in
frequency to each other and will normally slip at a constant rate. Figure 3-15
shows plesiochronous operation of 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 buffering of the signal from the satellite. A Doppler shift results from
the satellite’s figure 8 shaped orbit over a period of one day. A Doppler shift
should not result in a clock slip; the buffer constantly fills and empties.
If two earth stations are configured as a master/slave unit, then the buffer need only be
configured for Doppler operation. The buffer has sufficient capacity for the Doppler shift
on the outward and return paths.
A buffer setup for the Doppler operation will typically require less depth than one
intended for both Doppler and plesiochronous operations.