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CDM-625 Advanced Satellite Modem
Clock Modes and Drop and Insert (D&I)
emphasized that the synthesis is exact. The net result is that the E1 or T1 timing signal used at
the local end is reproduced at the distant end, regardless of the link data rate.
The only thing that affects the overall accuracy and stability of the copy of the clock is the
Doppler shift of the link itself. This will be very dependent on the particular satellite used, and
the accuracy of the orbital station keeping (often referred to as orbital inclination). Typically the
Doppler variations are in the order of parts in 10
that, over a 24 hour period, the net error would be zero due to a fundamental characteristics of
geostationary orbits.
The T1 or E1 signal, available on the rear panel of the modem on the G.703 connectors, is now
used to provide a synchronizing source for equipment connected to the modem. The form of
this is an 'all ones' signal, which provides the maximum transition density in the AMI signal.
Note: This scheme is sufficiently flexible to permit an E1 signal to be used at the local end, and a
T1 signal to be reproduced at the distant end, or vice versa.
9.11.2
Clock Extension Mode 2
Figure 9-8 shows Clock Extension Mode 2. This is for situations where clock extension needs to
be performed, but there is no local G.703 reference. In this case, the local modem now operates
in an EXTERNAL clock mode and the accuracy of the TX Clock is determined solely by the
accuracy of the equipment connected to the modem.
At the distant end, an E1 or T1 synchronizing signal is generated regardless of the link data rate,
as in Mode 1.
9.11.3
Clock Extension Mode 3
Figure 9-9 shows Clock Extension Mode 3. This is very similar to Mode 1 but now, instead of the
EIA-530/V.35 serial interface being used, everything is based around the 10/100 Base T Ethernet
interface.
At the distant end, an E1 or T1 synchronizing signal is generated regardless of the link data rate,
as in Mode 1
.
-8
, but more importantly it should be recognized
9–12
Revision 13
MN-CDM625
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