Buffering; Buffer Size - Comtech EF Data SNM-1001L Installation And Operation Manual

SNM-1001L Satellite Modem
Clocking Options
The use of loop timing in the modem is an option for both EIA-422 and V.35 operation.
SCT (LOOP): SCT (INTERNAL) clock no longer applies when the modem has loop
timing on. The TX clock source is now recovered from the RX satellite data. This
recovered clock is put out on the ST line and is used to clock the terrestrial equipment.
The transmit terrestrial clock is now essentially the same as the RX satellite clock, except
that it has been buffered by the terrestrial equipment. Select TX TERRESTRIAL for the
TX clock source when in loop timing, if the user equipment is being slaved off the
modem. The SCT (LOOP) indication serves as a reminder that the SCT internal clock is
now the recovered clock, not the internal oscillator. Select SCT (LOOP) when the
terrestrial equipment does not provide a transmit terrestrial clock.
6.2

Buffering

The purpose of a receive buffer is two reasons:
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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.
6.2.1

Buffer Size

The depth of the receive buffer will depend on four parameters:
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•
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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 6-3 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 6-4) 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.
Doppler shift caused by satellite
Stability of each clock (plesiochronous/Doppler operation)
Frame/Multiframe length of multiplexed data format
Allowable time between clock slips
6–2
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MN/SNM1001L.IOM