SCPC--Radyne DMD20 LBST and Cisco

It is a puzzle why changing the modulation type from QPSK to 8PSK should cause clocking issues.

Does the router serial interface problem arise in both directions ?

What if you change only one direction to 8PSK ?

Do you have differential encoding enabled and TPC scrambler selected ?.  A wrong configuration might work between two modems on a test bench joined with L band cables, but fail either on real satellite link of variable length or with higher order modulation that loses lock due to its higher sensitivity.  Maybe your QPSK also has clocking problems but these might be evident only if left working for a long time.

There are many factors involved in getting the links working. See the DMD20 manual here: https://www.satsig.net/global-teleports/dmd20-operation-installation-manual.pdf

I would have thought that both links would have identical configuration - not as you show.

Why does the second link use the modem internal clock (SCT) for transmit ?   If you do this, the modem clock needs to be sent back to the Cisco so the Cisco can control its output speed.  Is the Cisco configured to use an external clock for its transmit data ?  Alternatively the clock may originate internal to the Cisco along with the transmit data (with modem tx clock set to SCTE external).

Best regards, Eric.  

With VIT coding do not use TPC scambling.
With TPC coding use TPC scambling.
When you change from VIT to TPC you need to change the scrambling at the same time.

In theory, having differential encoding enabled causes a worse BER assuming the degradation is pure gaussian white noise.  In real life however amplitude/phase distortions may well make differential encoding superior, particularly with 8PSK in a closely spaced circle. Try Differential Encoding enabled.

I don't understand why both directions need to be synchronised.  I am assuming that both ends start and terminate on packet switching Cisco routers where packet buffering occurs.  If this case the originating clocks (in the Cisco send ends) may be quite independent and quite different e.g. 512kbit/s one way and 178kbit/s the other way.  If you use the modems as the clocks then the clock signal must go backwards into the originating Cisco so that the data comes out at the right speed.  Sorry I don't know about configuring Cisco serial interfaces, but I'm assuming the nominal interface speed can be specified and that you can select either internal clock or external clock wired from the satellite modem.

Best regards, Eric.