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CDM-625 Advanced Satellite Modem
Clock Modes and Drop and Insert (D&I)
9.4
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.
9.5
Drop and Insert
The Drop and Insert (D&I) multiplexer works in conjunction with the G.703 interfaces to enable
the modem to transmit or receive fractional parts of a T1 or E1 data stream.
The D&I option provides fully compliant baseband processing in accordance with Intelsat
IESS
309 for the terrestrial information rate of 2048 kbps (E1) and 1544 kbps (T1), using G.703
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interfaces. The data rate sent over the satellite link is n x 64 kbps. See the Frame Formats
diagram for the permissible values of n. The modem provides the interface to transmission level
framing compliant to IESS
309 Data Type 2.
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9–6
Revision 13
MN-CDM625
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