Comtech EF Data CDM-570A - Manual (Page 512)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix F Revision 2
F–6
F.4 X.21 Notes
The central feature of X.21 is that the DCE supplies the clock for both directions. Therefore, the
first requirement is that TX and RX data rates are the same. Because the DCE doesn’t get an
input clock moving in the same direction as its input data, the several nanoseconds of delay
through long cables and the modem’s data transceivers can cause the clock phase to change
relative to the data. This is why X.21 is not recommended above 2048 kbps.
If Modem = DCE (normal assumption), then X.21 mode is just a matter of ignoring ST and TT
and, instead, using RT to run both directions. Using the clock settings TX Clk = Loop and RX
Buffer Clk = RX assumes that the desired timing is coming from the far-end modem. The
modem’s ST is forced to be a copy of RT, because the latter assumes use by the terrestrial DTE
to drive SD data into the modem.
If, instead, the local modem supplies the timing, the clock settings should be TX Clk = Int and RX
Buffer Clk = TX or Int. These settings instead force RT to be a copy of ST. In either case, it is SD
that has an uncertain phase relationship to the clock due to round-trip delays, so the modem’s
TX clock invert feature may be necessary to avoid clocking in SD on its transitions.
If Modem = DTE, then ST and RT are ignored, and TT from the terrestrial DCE is used. The
correct modem settings are TX Clk = TT and RX Buffer Clk = TX. These settings force RT (which
drives out the modem’s RD) to be a copy of TT. It is now RD that has an uncertain phase
relationship to the clock, so the modem’s RX clock invert may be necessary to please the
terrestrial DCE.
F.5 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 example, 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. This is sometimes 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 unnecessary, as the CDM-570A/570AL – 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-570A/570AL 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 -8 , and is solely dependent 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 6.
FRONT PANEL OPERATION.