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Remote TxTDMATraffic CRC Errors

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Ex Member
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Dec 18th, 2013 at 7:22am  
Hello Good People,

I have been observing Remote TxTDMATraffic CRC errors on imonitor for some remotes which end up facing packet loss sessions lasting between 30sec and 3 minutes.

Any ideas on what is causing this?..i have changed BUC & Connectors but problem persists and its erratic, no clear pattern.
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Reply #1 - Dec 18th, 2013 at 6:37pm  
Measure and record, at the hub, the C/N of bursts from all your remotes.  They should all be the same as the iDirect system makes the remotes adjust their uplink BUC power to achieve steady values at the hub.

Was there heavy rain or dish mispointing due to high winds at the sites ?

Assuming the sites are all working well then the problem is likely due to intermittent interference into the hub TDMA burst receivers.

At your hub observe the return link burst spectrum on both the wanted and cross-polarisation.  If your hub dish has single receive polarisation, set up a VSAT dish, receive only, to observe the cross-pol traffic, if any. A supplementary receive dish like this is useful also to observe the co-pol spectrum on adjacent satellites.

Ideally, you want a spectrum analyser with video averaging, peak hold, single shot, zero-span, trace capture and plot facilities. Observe the noise floor across all the return link carriers.  Do you see any spikes or humps that should not be there ? Does anything sweep across occasionally ? Do you see anything on a wrong frequency ?  Observe in 'peak hold' mode  for several minutes or hours; this will capture occasional interfering signals.  Do the gaps between the carriers remain clean ?  If multiple sites are affected simultaneously for 30 sec - 3 minutes I would suspect one of your burst received being jammed by an accidental carrier.

Is it possible that one of your sites is transmitting a burst at the wrong time or going into CW transmit mode? Check the burst timing and lat/long data for all sites, including any sites that are supposed to be turned off.

Tuning to a return link carrier frequency and selecting zero span will provide an amplitude versus time view of the sequence of bursts arriving.  Look for any bursts overlapping.  You can also see abnormal level bursts, and any with unusual shape, possibly due to BUC power supply problems.

Have a look similarly at the cross-pol transponder. Does anything here match interference you can see in your co-pol wanted spectrum.  If it contains similar TDMA bursts as yours then it is likely that one of their sites is not polarisation aligned correctly.  When that site transmits a burst it jams whatever site on your network is using the relevant frequency at that moment. Not easy to find !  If you are thought to be interfering into them, then you need to make each of your remote sites transmit in turn on specific frequncy and get the interfered with hub to measure each site in the cross-pol transponder to determine which of your sites need polarisation adjustment.

Best regards, Eric.
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« Last Edit: Dec 19th, 2013 at 10:54am by Admin1 »  
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Reply #2 - Dec 19th, 2013 at 8:06am  
Hi Eric,

Many thanks for your detailed response. I will check on all the areas you have highlighted and revert with an update.

Merry Christmass  Smiley
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Reply #3 - Dec 26th, 2013 at 7:12pm  
It also can be a cable - I faced many issues when cable problem was causing CRC errors with unpredictable pattern.
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