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SCPC--Radyne DMD20 LBST and Cisco

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Ex Member

Aug 7th, 2010 at 12:09am  
Hi Guys,

Need HELP!!!

Problem : I have made 512K point to point SCPC link using Radyne DMD20 LBST modem and SAT link is working fine but we are getting clocking issue for WAN link when terminating this with cisco router at both ends. 1st i tried with VIT 7/8 with QPSK mod and link works fine then I have changed the coding to TPC 7/8 with 8PSK then SAT link is fine but ROuter serial interface is going up/down.
Clocking is provided from Radyne modem for link.

One end clock settings: TX Interface: SCTE , Auto Internal; Rx Interface Rx SAT 1 of 1
Other end clock settings: Tx Interface : SCT, Auto, SCR; Rx Interface Rx SAT 1 of 1

Tried with other clock settings too but its not working.
Also tried with 5 other same model modems to ensure that modem fault is not there.
Does anyone faced such kind of issues??
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Eric Johnston
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Reply #1 - Aug 7th, 2010 at 1:40pm  
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: http://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.  
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Reply #2 - Aug 8th, 2010 at 4:17am  
Hi Eric,

Thanx for your reply.

Clocking issue is not because of mod change. It is due to coding change from VIT to TPC because Link also not stable with QPSK and TPC 3/4. But It works great with VIT and QPSK.
Diffrential coding is disabled and Scrambler is TPC on both end, and I have tested in satellite link only not in L band setup.
had refer the manual too but clocking options are not given in details.

I am using modem as DCE and router as DTE so providing clock from one end modem and other end modem using the same clock and receive it with satellite signsl. Because As I think its a single WAN synchronous serial link and use a single accurate clock to work.

Sandeep K
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Reply #3 - Aug 9th, 2010 at 12:55pm  
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.
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« Last Edit: Aug 9th, 2010 at 6:55pm by Admin1 »  
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Ex Member

Reply #4 - Aug 11th, 2010 at 5:55pm  

Thanx Eric..

I have used TPC scrambling with TPC only, not with VIT...

Ya Ihave configured both sides with same clocking config and link is working fine now...

Thanx again

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« Last Edit: Aug 11th, 2010 at 9:44pm by Admin1 »  
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