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CDM-625 Advanced Satellite Modem
Clock Modes and Drop and Insert (D&I)
9.11
G.703 Clock Extension
There are some applications where it becomes necessary, at the distant end of a satellite link, to
provide a high-stability G.703 timing reference for timing equipment connected to the modem.
For instance, in cellular backhaul applications the BTS equipment may require such a reference,
even though the satellite link itself may be operating at a data rate other than 1.544 Mbps or
2.048 Mbps. Sometimes this is accomplished by adding a specialized GPS receiver at the distant
end, which then provides the G.703 synchronizing signal. However, with the G.703 clock
extension mode, this may become uneccessary, as the CDM-625 – operating at either end of the
link, where the local modem has access to a high-stability G.703 signal – can provide an almost
perfect copy of this signal at the distant end. The presence of Doppler shift on the link is the only
factor affecting the overall accuracy. If Doppler shift were not present, the copy of the clock
would be perfect.
This is accomplished by the use of a novel frequency synthesis and phase locking scheme. This
feature of the CDM-625 permits the distant end to generate a G.703 synchronizing signal that,
depending on a sufficiently accurate local reference, has short term accuracy to within parts in 10
and is solely dependant on link Doppler shift.
The subsections and figures that follow illustrate three possible G.703 clock extension modes.
Details of how to set up the modems for these various operating modes are given in Chapter 5.
FRONT PANEL OPERATION.
9.11.1
Clock Extension Mode 1
Figure 9-7 shows Clock Extension Mode 1. The local modem is assumed to be operating on
INTERNAL clock. A T1 or E1 G.703 signal is applied to the rear panel connector of the modem,
where the clock is recovered.
(Note: the G.703 signal is not intended to convey data – its function is only to provide a
synchronizing clock. The data is transferred using the EIA-530/V.35 serial interface.)
The internal clock reference generator locks, in both frequency and phase to this recovered clock,
and a special synthesizer generates an ST clock of ANY ARBITRARY FREQUENCY, over the
range 2.4 kHz to 9.98 MHz, with a resolution of 1 Hz. The synthesis is exact – there is no
approximation or residual error. For example, if the user selects 168.231 kbps as the transmit data
rate, and an E1 reference, there will be exactly 168,231 clock cycles generated for every
2,048,000 cycles of the E1 reference.
The internal ST clock is now used, as in the standard Internal Clock mode, to provide the timing
reference for the externally-connected equipment. The data is then transmitted, at the desired data
rate, to the distant end (or distant ends – this works for broadcast applications as well).
Now, at the distant end modem (timing mode: Rx Satellite), the Rx signal is received, demodulated,
and the clock is recovered. A second synthesizer, very similar to the one used at the local modem,
is now used to generate an E1 or T1 timing signal. Again, it should be emphasized that the
synthesis is exact.
MN-0000036 (Ref MN/CDM625.IOM)
9–10
Revision 1
-8
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