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Periodically taking major CRC errors

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Ex Member
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Apr 13th, 2012 at 3:53pm  
For a couple months now we have had some of our TDMA links dropping out periodically and taking CRC errors.  One site in particular drops out more than any other but I have seen almost all of our sites drop out.  Changing equipment hasn't seemed to eleviate our problems.

Here are the specs:

Linkway S2(s) with 8.3 FW at the MRT and distant ends
CRC errors incrementing at nearly 200 per minute
.000315 BER at trouble site
.00166 BER back to MRT (no crc errors here)

We have tried raising power but have had no luck.  Any suggestions would be great.  Thanks!
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Eric Johnston
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Reply #1 - Apr 13th, 2012 at 7:24pm  
Connect spectrum analysers to observe both the outlink and return link TDMA carriers.

Use 'peak hold' on and off periodically, to record what happens during the events and plot each result.

Investigate what you see and try to determine the source of the interference.

Is one of your remote sites getting stuck ON ?.

Is some site stuck on and sweeping across the spectrum ?

Use an antenna to look at the opposite polarisation and look at the co-pol spectrums of adjacent satellites also.  Do any of these spectrum match the interferer?

The site with particularly bad reception may be:
- suffering interference locally
- may have its polarisation poorly aligned
- its location is far away from the other sites and may be adversly affected by higher power downlinks from an adjacent satellite.
- have its location incorrectly recorded in your database and is suffering from burst overlap from one of your other remote sites.

I would recommend all VSAT hubs to have a spectrum analyser permanently connected to the return links frequency range and to chase any interference seen. The quicker you chase the better chance that there will still be an installer on site at the uplink that is causing the problem.  If you leave it for months/years you will find your noise floor full of intermittent cross-pol bursts or continuous, from poorly polarisation aligned terminals in other people's systems and you will have great difficulty working out which site in their system transmits the occasional bad cross-pol burst that unacceptably affects your service ...

Remember however that satellite systems are designed on the basis of mutually acceptable interference levels, both cross-polar and adjacent satellite.

Best regards, Eric
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Ex Member
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Reply #2 - Apr 23rd, 2012 at 3:49pm  
Eric,

Thanks for the info.  What exactly causes CRC errors?  I have a good BER and power levels.  Using circular polarity.  I'm taking CRC errors at an extremely high rate about 30-60/second.

I'm not too familiar with circular polarisation, but thought that there was less possibility of interference using a circular pole.
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Eric Johnston
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Reply #3 - Apr 23rd, 2012 at 5:56pm  
A Cyclic Redundancy Check (CRC) error occurs when a frame or packet of data bits contains one or more bits in error. At the end of the frame or packet is inserted a count based on the content of the frame so far. If the count does not match the content when checked at the receive end, an CRC error alert is given.  It is a very simple error detection method.  The faulty frame or packet may well be dropped. It is not an error correction system.

CRC is applied to ethernet cable links as well as satellite links.

You need to clarify where the error is occurring.  Is it at the hub or the remote site(s) ?  Does it relate to the satellite link or to the local ethernet links between your LAN equipment and the modem ?

Are both ends configured the same? same bit rate, same FEC, same scrambling.?

You mention circular polarisation so it may be that you are using C band. If so, note that C band suffers local interference in many places due to WiMAX wireless systems operating on the same or close by frequencies. Use a spectrum analyser with a hand held feed horn/LNB to wave around and identify the source.

What does a spectrum analyser show at the hub ?  At the remote ?
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