CDM-625 Advanced Satellite Modem
Revision 1
Clock Modes and Drop and Insert (D&I)
MN-0000036 (Ref MN/CDM625.IOM)
9–11
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 dependant 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
-8
, but more importantly it should be recognized
that over a 24 hour period the net error will 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.
