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I've been having some problems with my home LAN. Downloaded executables won't run, my remote desktop sessions keep getting interrupted due to encryption errors, flash video streams show visible corruption (both Hulu and YouTube), and I've had a couple downloads for which the md5 hashes don't match. The problem has even occurred with a couple images embedded in webpages, though that's rare enough (presumably because images are relatively smaller files). I've had this problem across two Windows machines and a Mac, so it's neither machine-specific nor at the app or OS level. Comcast claims it's nothing to do with them, and my Linksys/Cisco RV016 router is out of warranty, so I have no access to official support.

When I log into my router, it shows no error packets or dropped packets received. I plugged a laptop directly into the router and was able to download a 5.5 MB file and verify its MD5 hash, which is not proof that the problem is downstream of the router, but makes it seem quite likely, since I failed to download the same file several times from two desktops (one Mac, one Windows).

Could this be a wiring problem? If so, is there any way clever/elegant to determine which wiring is faulty with just software? If I can avoid tracing all the wires throughout my entire house it would make my life quite a bit easier.

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What else is downstream of the router? Do you have a hub/switch? – ChrisF Jan 27 '11 at 9:03

disconnect all internal cables on the router. using a laptop and a new cable go right to the router, plug in to a switch port, and attempt a download. If it still fails, your router is bad. Ive seen routers and switch interfaces die many times. Perhaps even bypass the router. You said comcast. Thats cable right? try plugging your computer directly into the comcast cable modem. Make sure to reboot your cable modem right after plugging your computer directly into it. If you can download fine in this config, chances are your router is toast. You could try a firmware upgrade, but its probably hardware at that point. You will rarely see this mentioned. Electronics always die after so long. Electrons slowly eat away the conductors in your microprocessors. 3 nanometers of conductor is not hard to break.

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Based on your test, it does sound more like a wiring issue than anything else. Did anything change before this started happening (rewiring, remodeling...). When did this start happening?

Are there any high voltage power lines that run near/parallel with the Ethernet cables? Do you know if you are using cat5, cat5e, or cat6 cables? Do you have any network switches that split off the network down stream from the router? What speed is it currently connected at (10/100/1000)?

In windows you can monitor network packet errors. Open up Performance Monitor from the run prompt (perfmon.msc) Then Right click -> remove all counters. Click on the green plus button at the top and then scroll down to "Network Interface" and add "Packets Received Errors" and "Packets Outbound Errors".

In windows you can force the LAN link speed down to 10 mbps in the network adapter properties. See if you still get corrupt downloads after that.

You can force the link speed to 10 mbps by right clicking on the network icon on the lower right and clicking "open network and sharing center". Then click "Change adapter settings" in the top left. Right click-properties on the network adapter that is trying to connect. Then click "Configure". Go to the Advanced tab. Look for something in the list that looks like "Link Speed & Duplex". From there you can select a speed in the drop down menu.

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No config changes. I did discover a switch hanging, suspended by routers - the boxes it was stacked on partially collapsed. I have no clue how long it's been like that, but I'm guessing that's what caused the problem. Trying to work out which cables to replace is going to be a pain but hopefully that'll resolve it. Interestingly, perfmon didn't pick up packet errors as they occurred - not sure what that says about the problem. I'll try forcing the speed down and see if that helps - thanks! – rfrankel Jan 29 '11 at 4:58
I'm not sure my network adapter supports forcing the link speed down - I looked up instructions for how to do it but apparently my NIC doesn't support the relevant property. – rfrankel Jan 29 '11 at 5:02
@rfrankel Ok I'll add instructions to lower the speed and pictures just to make sure. I would first try switching out the network switch/hub that you discovered. Network cables don't usually start randomly malfunctioning unless they have undergone some heavy stress or were created incorrectly to start with. If the new network switch doesn't help, you can try testing the cables with a cat6 cable tester. – James T Jan 29 '11 at 7:12

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