All possible causes of CRC errors in ADSL connection

[Islam Ghunym]

I keep asking the same question everywhere and I never once had a satisfying answer.

 

What causes CRC erroring to happen?

This is my current DSL log.

I have examined the wire from the local box to the end. I spotted no single sign of defect. The wire was pretty solid even tho it barely contains any copper in it. I have examined and cleaned the contacts also near the DSL modem and around the splitter.

 

The SNR margin isn't great, but looks fine. The line attenuation is beyond outstanding. I have 2 phones connected with an ADSL splitter across the line (one near the modem and the other somewhere else [both of them on a splitter]).

 

I get these CRC errors occasionally and sometimes even DSL interruptions. However I am still getting the same errors with/without phones connected and even without any splitter (just the modem connected and nothing else)

 

Assuming that the line is connected properly from the start to the end and assuming that the local box have no issues, what could be causing CRC errors on my DSL line?

 

CRC can be anything from 1 to thousands on upstream or downstream (mostly downstream)

 

Another thing to mention is that I should be getting 2045 kbps upload, but the current low SNR on the downstream doesn't allow the ISP to activate Annex M modulation which would drop the downstream SNR in order to left the upstream SNR up

[Eigenvektor]

I think you're kind of answering your own question. You have a bad signal to noise ratio, which means there's a lot of noise, which in turn causes transmission errors, which you notice in the form of CRC errors.

 

This noise could be caused by bad wires/shielding in your home, but it could just as well be from a source outside your home. Anywhere between the modem and the ISP, really. Unless you're able to isolate the source to something in your home, you're unlikely to be able to fix it.

[manikyath]

making a really long post really short:

 

bad signal strength.

[Donut417]

4 hours ago, Islam Ghunym said:

what could be causing CRC errors on my DSL line?

Depending on where you live in the world it could be due to negligence on the ISP's part. I know many American telecoms have pretty much given up on copper. AT&T stopped selling ADSL service and Verizon has left its copper to rot and refuses to maintain it. If the issue is on the ISP's side there might not be much you can do. 

 

Depending on how the ISP hooked up the service you might be able to eliminate your home wiring and test. I know a common thing in the US was to installed a NID on the side of the customers home. The NID would connect the ISP's wiring to the customers wiring. If you can gain access to the NID you might be able to disconnect the home wiring and test to see if the issue still persists. If the problem isnt present when you bypass you home wiring then you know its the wring in your home. If its still present then its the ISP's problem. 

[PyCCo_TyPuCTo]

To answer your question CRCs increment when a corrupted packet is detected (and dropped). A ton of different things can corrupt a packet.

You actually have really good SNR and attenuation values, unless these numbers fluctuate you should be good there. Are your CRCs intermittent or is it a slow trickle? If they come in in bunches, are you saturating your link during those time frames? Saturation will cause CRCs as well.

Have you tried complaining to the ISP and have them run their checks remotely? There could be a bridge tap somewhere on the path between the CO and your home.

They can also try and play with the DSL parameters to try and improve things, but that's granted they see something not quite right on their end, and actually care about doing something. I'd imagine residential DSL is not going to be looked at by higher tier technicians.

[Alex Atkin UK]

2 minutes ago, PyCCo_TyPuCTo said:

To answer your question CRCs increment when a corrupted packet is detected (and dropped). A ton of different things can corrupt a packet.

Also given packets are small, unless you have a LOT of CRC errors back to back.

 

It can be as simple as someone idling in their car near your phone line with a bad alternator, used power tools, turned their washing machine on, etc.  So there is no point trying to prevent them unless your line is permanently reducing in speed significantly, as you'd have to reduce to line speed permanently, rather than a burst of errors reducing the speed temporarily.

[Islam Ghunym]

Thank you all. First of all 6-7 SNR Margin isn't bad and the signal strength is very fine. I also mentioned that the issue is in my home line not from the outside box, but I was unable to determine it. Even tho I believe that my country telecom company is very negligence, they don't care since they are the only option in the entire country. The CRC count can be as low as 1 error per hour, a thousand at once, a continuous counting, burst of 200 followed by another 300 or 100....etc. Saturation isn't related to CRC errors in my case.

 

However, I probably found the issue. The DSL line is going through an area near electricity wires + iron ,and sounds like when electricity runs through these wires, CRC starts to count. I verified this situation 3 times, and sounds like the CRC is random (may or may not happen), but in 3 situations the CRC initiated by electricity starting to flow through the nearby wires. the problem is that there is no way for me to let the wires move anywhere else to avoid this problem, but if there is anything I can do to fight this area interference, I may successfully fix the problem.

 

I am thinking about replacing the part that goes through this area with a more proper wire. The problem is my country "Syria" have almost nothing in stock unless I travel to get it and there aren't so much options when it comes to telephone wires and everything is low quality...

 

If I replaced the wire with 16 AWG copper wire which is quite thick, will that fight the interference? or the wires thickness have nothing to do with that? I can do some DIY if it takes to shape something because as I said I am living in a dead country with not so much options to work with.

[Alex Atkin UK]

2 hours ago, Islam Ghunym said:

If I replaced the wire with 16 AWG copper wire which is quite thick, will that fight the interference? or the wires thickness have nothing to do with that? I can do some DIY if it takes to shape something because as I said I am living in a dead country with not so much options to work with.

CAT5e ethernet cabling is generally much much better than what they will use for phone lines, it may help.
Conductor size reduces attenuation (usually negligible difference in a house), the number of twists in the cable is what helps reduce noise.  Shielded cable might help, but you need a good ground at one end.

Generally if you can move the cable even slightly further away from the mains cabling it may help and try to avoid it running parallel to the mains cabling if possible.

[Islam Ghunym]

Thank you all for the information. I don't really know what answer I should be marking as the best for the question . Many answers were useful and informative. I guess the last one because it gives actual possible solutions for my case