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What can data corruption be due to?

G

Goi

Diamond Member
Hi guys...
I have 3 systems on Win2k Pro connected to an SMC7008BR router which in turn is connected to my SB3100 cable modem to the wall jack. I'm primarily on my main PC, which often experiences corrupted file downloads. I'm wondering what can be the cause of it? So far I've performed the following troubleshooting

1) removed the router from the picture, and connecting the cable modem directly to my PC
The result was still corrupted downloads, thus eliminating the router as a potential problem

2) tried different download managers(various browser defaults, Freshdownload) and different protocols(HTTP, FTP, Kazaa, emule)
Still corrupted downloads for each case, except emule's corruption handling manages to produce good downloads at the price of massive redownloads.

3) tried a different computer in the network
still corrupted downloads, hence totally eliminating it as a software problem

4) Changed Cat5 cables and Coaxial cables connecting the wall jack/cable modem/router/pc
Still corrupted downloads, so its not a cable connecting from the wall jack onwards

5) connected my cable modem directly to the main cable point downstairs with the huge cable(larger than RG6 type), and pulling a long Cat5 cable to my router
No more corrupted downloads on 2 separate occassions I tried on 2 separate nights. Right before and right after I tried this, I experience corrupted downloads

So, I've basically pinpointed it to the cable connection between the main cable point and the wall jack - a physical problem. However, I'm skeptical of this being the problem since theoretically this shouldn't be the case. If there were any problems with the physical cable, such as improper wiring, EMI/RF interference, etc, wouldn't this either cause disconnections, or cause lost packets? In the latter case, wouldn't TCP/IP simply retransmit the packets? I don't see how this could cause corrupted downloads, and bits could be flipped w/o the TCP/IP layer knowing. Its supposed to do error correction right?

Any other ideas as to what could be the problem? Perhaps the broadband registry tweaks? I don't think so either...

Thanks!
 
No, sounds like a pretty clear case of a bad cable. Physical layer is usually the problem when it comes to something like this. You did an excellent job of troubleshooting, by the way - I wish many my peer network engineers were so thorough.

It's very possible that you're not seeing some of the physical layer errors, like corrupted packets, retransmits, etc. Many NIC's hide those from the upper layers of the stack - Even if you did spin up a sniffer, you probably wouldn't see them. So, you could actually be getting massive retransmissions on the network and not see a single one.

Networks do use CRC (Cyclic Redundancy Checks) to do corruption for error checking, but they aren't perfect. You're trying to squeeze error checking for up to ~1500 bytes of traffic into that 4 byte CRC. It's going to miss some corruption.

Sometimes the cable company is responsable for the wiring in your house. This is especially true if they ran the cable for that problem jack as part of the install of your cable modem. Call them up, tell them what you did for troubleshooting and they will probably be reasonable.

- G
 
Well, I did call up my cable company. It was the first thing that I did. However, as usual they aren't very helpful in this regards since they don't support the usage of routers. Even if I remove the router from the picture they'd still be reluctant to help. I emailed them as well and they insisted that there was nothing wrong with my connection after testing over at their side and at the hub where I'm connected. They insist that its either a software or a server problem, and they want me to pay for any technical assistance should the problem not be replicable. The problem is that the problem isn't always replicable. Its random, and I don't want to have to pay them if they come and don't find a problem, when the problem actually still is there.

Also, the cable wasn't laid by them, but by other people when I renovated the house, so they probably don't wanna have anything to do with it either.

Do you think that a cable signal booster might help? Or anything else short of ripping the cable from the walls and rewiring everything?
 
Hi Need4Speed, thanks for the input, however if you've read my troubleshooting process, you'd see that its highly unlikely for it to be either of the possibilities you've mentioned, because I've also tried file transfers on 2 other computers, neither of which were overclocked.

Thanks for the input though.
 
An easy (well, available) way to check this out would be to download Ethereal (a packet / traffic analyzer) and see what the actual traffic flow looks like.

If you see things like Late Collisions, excessive re-transmits, etc ... then there's a strong possibility of a bad segment of cabling.

If it shows a lot of timeouts, then the problems could be with the carrier (like congestion at the carrier's Head-End).

Ethereal is free, and tools like this are most helpful for finding hidden problems.

Good Luck

Scott
 
OK, I guess I'll try Ethereal out. Right now the connection is so bad I can't even download the Det FX drivers, or even the latest McAfee DAT files, grr... 🙁

However, if its due to a physical cable problem, can the problem be solved with a cable signal booster that I can buy from my cable company? They have the "Distribution AMP model AM-2FORWARD/15 (15dB)" indoor signal booster available. I'm wondering if it'll help

Also, what could the problem be due to? a damaged portion of the cable? EMI interference? Bad connections at the wall jack?

My main cable point is split 8 ways with a Regal GRS8DGV 8-way splitter if that matters.
 
Hi,

once again I've confirmed that its the cable connection between the main cable point downstairs and my room's walljack. For the entire afternoon I've had problems downloading a few multi-megabyte files(ranging from 5-18MB). I decided to do something about it and connected the cable modem to another room's walljack, and pull a Cat5 cable over to my room's router, and lo and behold, the files download fine now. At least 1 of them are, the other 2 are still downloading...

Anyway, it seems that my renovation contractor did a really botched up job with the network cables as well. I was opening up my wall jack by unscrewing them, and just moving around the cables a bit would cause intermittent disconnections from my room's router to my HTPC upstairs. I verified this by looking at my router's status LEDs as well as pinging the HTPC.

Still, I would like to continue using my room's walljack for the cable modem since the network cable topology is such that this is the most convenient setup. I would have to pull extra Cat5 or coaxial cables around the house instead of using the in-wall ones if I decide to use another cable point to connect my cable modem. So, what would help the signal of my walljack? A cable signal booster perhaps?

Thanks!

Edit: Turns out I spoke too soon - that file was just a lucky break, I'm still experiencing the same corruption now...but I've managed to download Ethereal.

How exactly do I go about using it? I've started a capture in real time and I see a lot of info but they all seem to look very normal to me, nothing out of the ordinary. I don't see any timeouts, etc...
 
Hi,

I'd really appreciate some input here on the reasons, and possible workarounds...

Thanks 🙂
 
the only way to know for sure at this point, would be to take the pc and cable modem to where your main coax line comes in and connect directly to that. start working your way back until you can recreate the problem, then should tell you if its a physical cable issue.

if that doesnt cure the problem, your main coax could be "leaking". Have your cable co. come and test it out.
 
OK, I was thinking along the same lines, but was getting opinions...

I have another theory though, I'm not sure if it makes much sense. I'm wondering if the 8-way splitter is causing a significant drop in signal? I was looking at it, and each output terminal specifies a "-11dB" signal change. AFAIK, normal 2-way splitters only drop around 3.5dB? Does the drop increase as you add the number of output terminals? Is a -11dB too much to handle for broadband connections? If so, perhaps a cable signal booster would help.

Thanks again.
 
IT would depend on when the splitter appeared on the system. If it was part of the original install, then chances are the installer cranked up the gain to compensate. If the splitter was installed afterwards, then you probably have a significant drop.

Each 3 db represents half (or double) the signal .... so -3 = 50%, -6 = 75%, -9 = 87.5 (of LOSS) .... @ -11db (not counting the losses of the cable & connectors, you're probably running on less than 5% of the original signal.

Probably a good thing to do would be to put in a two-way splitter, with one branch feeding the cable modem, the other branch feeds the video. Personally, I'd take the video side and feed a distribution amp. An eight-way, unamplified split puts the signal down into the noise floor (or very close). _*_* Assuming *_*_ that there has been no previous compensation to the system (tweaking the gain out in the post or pole).

As suggested above, try connecting the system ahead of the split and see what your performance looks like.

Good Luck

Scott
 
Hi Scott, thanks for the info, and now a bit of info of my specific cable setup

The 8 way splitter is installed right at the big RG11 cable entry point in my house. This RG11 cable comes from outside the house into the house, and is fed into the input of the 8-way splitter, each of the output terminals feed a walljack in the house. One of these output is routed to my room's walljack obviously, with no additional splitters that I know of in the middle. If I'm indeed losing 11dB, which according to you leaves me with only ~5% of my original signal, wouldn't I be unable to get any signal at all, whether on cable TV or my cable modem?

In any case, If I were to get a distribution amp, would I want to connect it before the RG11 cable goes into the 8-way splitter, or afterwards? Does it matter?
 
A bi-directional amp is ~$100 or so. The typical setup is as noted above: A two-way split, one side to the cable modem, the other to the video. The branch going to the video can be fed to a one-port , one-way amp (~12-16db should be ok, maybe even a little less). Then you can do whatever you want to the video and not affect your data connection.

The other way to do it would be to call the cable company and have them tweak your system. The tech will have a signal strength meter and can add the proper amount of amplification for proper operation. If they put in an amp they'll be responsible for the install and any consequences arising from the install (if there'd be any).

If you put stuff in, and the cable folks find out about it, they may get their shorts in a knot and be mean in your general direction.

Have you tried connecting directly to the in cable yet?

Good Luck

Scott
 
Yeah, I'm gonna get the cable guy to come down and check out the signal with a meter and see what he says...

Thanks
 
Originally posted by: Goi
Yeah, I'm gonna get the cable guy to come down and check out the signal with a meter and see what he says...

Thanks
Click to expand...

a bad cabling job is not going to be better with a booster, only if the wires are TOO long. it is very easy to mess up cabling jobs. improper cutting and termination (port, plug, whatever) are common even if you have a lot of experience. all the experience gives you is less error, and knowledge that you made an error before it gets detected.

there are 2 kinds of meters, one gives output in the form of sound or lights. the other measures voltages and such. of all of them, the one with the lights gives you less information. the others gives information as to the integrity of the wiring, not just continuation.
 
Thanks Mday. AFAIK the cable guy has a signal strength meter that measures the signal strength in dB. I've checked out the cable termination job, it looks ok to me, however the connection to the wall jack wasn't optimal to my layman eyes. I unscrewed the wall jack to take a look, but what I found was a bit too much excess cable that was looped at what seemed to me like excessive bending. Is there a minumum bending radius for RG6 coaxial cable before the impedance gets screwed up? Can I use a regular multimeter to measure the impedance at one point? Or, can I use a multimeter to do any checking on myself? That's about the only tool I have right now.
 
instead of monkeying around with all this signal strength stuff, I'd still first hook up directly to the main cable feed (as i suggested above) before i went through all this other trouble. that will quickly tell you were you need to concentrate....
 
NFS4, thanks for the suggestion. However, if you've read my initial post, that was one of the things I've tried in my troubleshooting process to determine where the problem was...and in my case the problem is somewhere between the main cable point and the individual wall jacks, which leaves only either the terminations or the cables themselves.
 
OK, I was on the phone with my cable company, and I explained my problem to him and what I've done in my troubleshooting process to determine that its a cable/connection problem. He was quite dumbfounded though, and claims that as long as the modem LED says "online" the connection should be on and there shouldn't be a case of corrupted downloads, its either on or off. He says he's never come across such a problem before, and he asked an engineer to test my signal strength from their side(apparently they can do that w/o coming down?) and says that there were no problems with the signal strength.

Anyway, they've asked me to leave my modem on overnight(which I do 24/7 anyway) and then connect it to my main cable point today(which I've just done) and they will monitor both cable points to see if there are any anomalies. When they say "monitor" I wonder if they'll know exactly where I'm at online. I wouldn't wish for them to find out that I'm using P2P software and all that...while they were checking my signal strength they mentioned that I had pretty high traffic and asked me if I was downloading something...hehe
 
Hmmn, turns out its not that particular cable that I thought at all. Now I'm just confused, I don't know what's wrong. I connected my cable modem to the main cable point downstairs and ran the long Cat5 cable just like I did the last time, and tried various downloads for a few hours. I'm now getting the corruption 🙁

Now I really don't know what's wrong.
 
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