SLM-5650 Satellite Modem Revision 2
Configuration MN/SLM5650.IOM
3–14
3.3 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 3-3 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
3-4) 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.