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CRC errors on router interface

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Ex Member

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Sep 15^th, 2009 at 6:55am

Hi,

I would like to get some feedbacks from experienced guys who encoutered CRC errors on router interface and what are the most related issues causing this?

assuming a link with 2 sites, site A & site B. The router at site A shows CRC errors on the interface connected to the satellite modem, meaning there are errors coming from site B data stream.

the most know reasons of that are low reception at site A on site B transmission causing BER on the satellite link.
but, assuming my link has a very good margin, high EbNo, what may be the reasons of the problem?
1) Interference on the frequency transmitted by site B?
2) hardware failure on one of the equipment on site B? such as faulty serial cable, router interface, satellite modem interface?

what do you think?

Thanks!

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Eric Johnston

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Reply #1 - Sep 15^th, 2009 at 8:53am

Some possibilities.

Brief interferences such as sweeping carrier or cross pol TDMA bursts. Local interference due to electric switching, neon signs, air conditioners, door electrics, car ignition, radar from passing ships. Detect using spectrum analyser in very fast sweep mode and max hold on.

Receive errors may also happen every time you transmit a burst either due to tx to rx radio interference or receive phase hits due to power supply problems. Check the BUC drive level for overloading, power supply to the BUC and earth return on BUC power supply (e.g. BUC power coming back via LNB cable sheath).

SMR5000: The receive modem occasionally reloads the tuning synthesiser to the home state. This causes a break lasting few mS and the RX green flashes off for a moment. Fixed by turning off this configuration option.

Some network interfaces have auto-negoiate to resolve 10 or 100 Mbit/s on the ethernet. These auto-negoiate processes may repeat themselves from time to time and cause a brief outage on the ethernet. Fixed by setting the Cisco port speed to what is actually required (not auto). Simple hubs can also cause similar occasional intermittent symptoms if different speed internet devices are connected.

Try noting time of day when the errors occur and watch to see if the RX LED goes out for a moment. Turn on error logging if possible.

Best regards, Eric.

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Ex Member Ex Member	Reply #2 - Sep 15^th, 2009 at 10:54am Thanks Eric!
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Eric Johnston

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Reply #3 - Dec 3^rd, 2009 at 11:00am

Have you tried replacing the ethernet cable between the modem and the router ?

Look in the ethernet sockets and make sure the 8 springs are normal.

If it is Cisco router, set the port speed (10 or 100 Mbit/s) the same as the modem (not auto).

Make sure that the modem and the router both have good common earth connections so that there is no superimposed mains residual voltage or local RF interference into the ethernet cable.

Is the antenna structure earthed ? Are the outer sheaths of the coax cables earthed where they enter the building ?

If you have a passive hub in the cable between the modem and the router, remove it.

If the modem itself is sending out corrupt packets to the ethernet I would suspect a hardware fault in the modem network interface.

I'm not sure any of the above will solve the problem but they are some ideas to think about.

Best regards, Eric.

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Ex Member Ex Member	Reply #4 - Dec 9^th, 2009 at 7:08am Hi, Eric. Do you have any C band satellite guide, ebook etc? i need some documents of C band satellite baseline performance, installation etc.
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Eric Johnston

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Reply #5 - Dec 9^th, 2009 at 9:50am

If you want text books about satellite communications I recommend:

'VSAT Networks' by Gerard Maral.

Satellite Communications Systems by Barry Evans.

If you need instructions about installing a large C band antenna then I would recommend reading the manufacturers installation manual. If you have no experience then you will need help from civil engineer/architect/electrician regarding ground strength, concrete, wind loading and building structures, earthing, building regulations, crane operations etc. For larger C band antennas I suggest you contract the entire installation task to the antenna supplier. Panel and sub-reflector alignment is a highly skilled task and if done wrong your entire dish is useless.

Best regards, Eric

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Eric Johnston

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Reply #6 - Dec 11^th, 2009 at 1:06pm

Sorry, no free information. Please buy my recommended text books from Amazon, by clicking on the above links. I need the commission.

The "Cisco Rate Based Satellite Control Protocol (RBSCP)" sounds impressive but I would not recommend using it with iDirect, HughesNet etc systems without first discussing the matter with iDirect, HughesNet etc. The reason being that iDirect, HughesNet etc already have complex processing to help improve TCP/IP traffic flow. Adding extra outer processing may make matters worse. The RBSCP might work best with older serial satellite modems with no IP traffic processing or advanced FEC. I note that the RBSCP is claimed to be particularly beneficial in links with high error rates. Satellite links do not have high error rates. They tend to work with no errors for the vast majority of the time and then transition to failed quite quickly in very heavy rain. Modern FEC codes work extremely well but have very sharp transition from perfect operation to failed. The ability to work in high error rate links may be attractive on low bit rate HF radio (wireless) links used for IP transmission.

Best regards, Eric.

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Eric Johnston

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Reply #7 - Dec 14^th, 2009 at 12:11pm

If CRC errors are occurring on the etherent cable linking your satellite modem and router I can see no reason why this should be associated with outdoor temperature. Such symptoms are related to ethernet connector faults, 10/100 auto speed negotiations, passive hubs and NIC hardware faults.

From what you say it maybe that you are experiencing a high error rate on the satellite receive demodulator in the modem. This would show up as a high receive BER count, low Eb/No and accumulated high receive error count. The Rx LED may occasionally go out. If it gets worse at very low temperatures then suspect:

1. The LNB local oscillator frequency has drifted so far that the demod is tending to lose lock. Look to see if there is some readout of the frequency error. Increase the receive acquisition range. Adjust the nominal receive frequency. Get a more stable LNB such a PLL type.

2. The cold is causing contraction of the cables and there is a bad LNB cable joint that is breaking.

Best regards, Eric.

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« Last Edit: Dec 14^th, 2009 at 2:38pm by Eric Johnston »

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Ex Member

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Reply #8 - Dec 22^nd, 2009 at 11:29pm

I'm locating A site.
So here Site A:
Tx: 57.5370M
Rx: 57.1330M
- 512K
Carr: Normal
Eb/No: 11dB
Tx path ok for: 9.4 days
Rx path ok for: 33 mins
BER: < 1.0E - 12
Rx path ok since: 22/12/2009 BER:< 1.0E - 12 BUFF: +0 -> 99 006%
Tx path ok since: 13/12/2009
Demod: Eb/No 11dB Final BER: < 1.0E - 12 OFFSET -5.4Khz

So looks Rx path has package lost so is that mean the failure is on site A or B?

And it is worked normal for 4, 5 years. Only having this error in last 2, 3 months.

Site B:

Eb/No: 9.5dB
RX Offset: -3314Hz
Chan Err Rate: 0x10-5
Tx Clk 512.000kHz
Rx Clk 512.000kHz
AGC Level 133
RxDataRate 512000bps
RxFreq 57537.000kHz
RxSymRate 292571 sps
RxCodeRate Vit 7/8
RxModType QPSK
RxDiffDecoding ON
TXFreq 57133.000kHz

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ahmedmaher30

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Reply #9 - May 23^rd, 2012 at 7:22am

Hi All,

sorry if reply after 3 years of discussion but it just raised my interest.

i knew from my Experience that CRC errors comes from 1 of 5 reasons

1) Poor antenna alignment
2) Bad LNB
3) Bad Connector
4) Bad Cables
5) Bad weather

as i see in your case i see the downstream signal is fine at both sides

so you may face some outage because of weather or the cables,Connectors needs to be replaced.

please update me if you found what was causing the issues?

Thanks

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