Ethernet network issues… any brilliant ideas?

  • Posted: 30 August 2012 12:59 PM

    JUst recently started having issues with one machine (2011 iMac) on my network.  Can’t figure out what the problem is, though at this stage I’m thinking it’s the iMac (either bad hardware or software config), not the network itself or its hardware components.

    So my network looks like this: Room 1 [DSL Modem -> Wireless router -> CAT6 wall jack]; Room 2 [CAT6 wall jack -> Mac Mini media server]; Room 3 [CAT6 wall jack -> iMac].  In the basement is a network switch, which is the wired hub for these three rooms.

    This setup has worked great; the mac mini stores/our music and video files and is connected to our big screen TV.  But I also like to move those files around or stream them to the iMac sometimes, thus the wired network.  In the last week or so—perhaps since the 10.8.1 update, though I can’t attest to that—the iMac drops the network connection.  It loses internet, it loses local network connections (visually evidenced by seeing shared libraries in iTunes appear and disappear randomly), and streaming video has gotten choppy as the connection drops and picks back up.  I’ve kept the network preferences pane open and watched the green “connected” light on the ethernet panel switch to “red” and back again, seemingly at random intervals.  I did system reboots, of the computers and of the networking hardware… nothing changed.

    I’m a bit of a networking dunce, but I did realize that the problem could in theory be anywhere in the chain, so I did some tests to isolate the issue.

    First, I checked the wireless connection.  I disconnected the iMac from ethernet, turned on its Wi-Fi connection to the network, and did some network file transfers to and from the mac mini, streaming movies from the mini, web use, etc.  All during this test, the Mac Mini was still connected to the network via ethernet.  The network connection never dropped, it worked great.  For the sake of completeness, I did the same on my MacBook Pro (wi-fi connection and same tests).  All great.  So that would seem to absolve the modem and Airport Extreme router from any role in the problems.  it would also seem to confirm that the ethernet cables, in-wall wiring, and the switch were operating fine as well - at least those cables and wires running between the mac mini and the switch, and the switch and the Airport Extreme.

    Second, to test the in-wall wiring, ethernet cable, and switch that run to the iMac, I hooked the MacBook Pro to the same wall jack that the iMac has been connected to, using the same ethernet cable that the iMac uses.  I turned off wi-fi and then did file transfers, video streaming, etc from the mac mini—entirely over the ethernet network.  And I did some internet browsing and downloads.  Everything worked great!

    These above tests have been repeated several times and the results have been consistent.

    Since it seems that the network behaves itself all the time on wireless, and does great over ethernet when anything but the iMac is connected… I conclude the issue is probably with the iMac itself, either configuration or the hardware is getting flakey.  I checked my networking settings (but again, I’m a bit of a network dummy) and they all looked “normal”.  Certainly they matched (other than the IP address) the settings on the MBP, and the MBP worked great over ethernet.

    So I’m stumped.  And I’m afraid that if I just go to the Genius Bar at the Apple Store… well, that level “genius” won’t really be able to help, and they would be more likely to conclude it’s my network, not the iMac at fault.

    Any obvious ideas jump out at anyone?  Thanks!!!

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    Posted: 30 August 2012 02:33 PM #1

    Use shift-cmd-G in the Finder (that is “go to folder”) and enter

    /Library/Preferences/SystemConfiguration 

    find com.apple.network.identification.plist, NetworkInterfaces.plist and preferences.plist and throw them in the trash.

    Reboot. It will recreate these plist files.

    See if that fixes the problem.

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  • Posted: 30 August 2012 11:32 PM #2

    Intruder - 30 August 2012 05:33 PM

    Use shift-cmd-G in the Finder (that is “go to folder”) and enter

    /Library/Preferences/SystemConfiguration 

    find com.apple.network.identification.plist, NetworkInterfaces.plist and preferences.plist and throw them in the trash.

    Reboot. It will recreate these plist files.

    See if that fixes the problem.

    Thanks for the idea!  Just gave it a try.  Unfortunately, it’s still happening.

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    Posted: 31 August 2012 07:37 PM #3

    +

    Have you tried hooking the iMac into the Room 2 Ethernet jack and see if you have Ethernet performance there? If not, it might be a simple problem with the iMac’s Ethernet jack itself. In Room 3, have you tried a different Ethernet cable for the iMac, despite the MBPro working properly?

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  • Posted: 01 September 2012 04:23 PM #4

    mrmwebmax - 31 August 2012 10:37 PM

    +

    Have you tried hooking the iMac into the Room 2 Ethernet jack and see if you have Ethernet performance there? If not, it might be a simple problem with the iMac’s Ethernet jack itself. In Room 3, have you tried a different Ethernet cable for the iMac, despite the MBPro working properly?

    Thanks again, both of you, this troubleshooting has been very helpful so far!!!

    Here’s what I’ve done this morning:
    (1) I hooked the iMac in room three up via a different ethernet cable (a CAT5 cable, not that it should matter).  Problem persisted.

    (2) I moved the iMac in to room 1 and hooked it up to an ethernet port on the Airport Extreme, using the original CAT6 cable and moved some multi-gig files around.  No drops!  So now I became suspicious of the wall jack or in-wall cabling leading to room 3 again… so just to be redundant:

    (3) I connected the MBP to the jack in room 3, using the original (CAT6 cable), and repeated the same tests.  Works great!

    So now I don’t know what to think.  Clearly the iMac is the machine that has issues, but it seems to only have issues when connected via the room 3 gear.  I thought about the switch being the problem (i.e. the switch and the iMac not liking each other for whatever reason), but on the face of it, that shouldn’t be the issue—in the iMac-in-room-1 test, the iMac is still communicating with the Mac Mini in room 2 via the network switch.  The only difference I can see is that the router is between the switch and the iMac now, which would not be the case when the iMac is in room 3.  Could this be it?  Why in the world would THAT be a problem?  And if that’s a possible culprit, what can be done short of moving the router to the room 3?

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    Posted: 01 September 2012 05:50 PM #5

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    Engine Joe - 01 September 2012 07:23 PM

    Clearly the iMac is the machine that has issues, but it seems to only have issues when connected via the room 3 gear.  I thought about the switch being the problem (i.e. the switch and the iMac not liking each other for whatever reason), but on the face of it, that shouldn’t be the issue—in the iMac-in-room-1 test, the iMac is still communicating with the Mac Mini in room 2 via the network switch.  The only difference I can see is that the router is between the switch and the iMac now, which would not be the case when the iMac is in room 3.  Could this be it?  Why in the world would THAT be a problem?  And if that’s a possible culprit, what can be done short of moving the router to the room 3?

    Is it possibly a signal strength issue in Room 3? I’m thinking that maybe the Ethernet signal in Room 3 isn’t as strong as it is in the other rooms, yet strong enough for the MBPro to handle it. I had a similar problem with my cable modem dropping my Internet connection from time to time, until I picked up a cable signal strength booster at Radio Shack. Haven’t had a problem since.

    Can you measure the Ethernet signal strengths in all three rooms, to see if maybe Room 3 has a weaker signal?

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    mrmwebmax (formerly mrmgraphics)
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  • Posted: 04 September 2012 10:07 AM #6

    mrmwebmax - 01 September 2012 05:50 PM

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    Can you measure the Ethernet signal strengths in all three rooms, to see if maybe Room 3 has a weaker signal?

    Labor Day weekend got in the way there… I can’t seem to find a (non-horrendeously expensive) ethernet signal strength measurement device.  There are all kinds of apps and tools for measuring wi-fi signal strength, but I couldn’t find anything that measures wired signal strength (well, I found one $30 item that just measures whether or not there IS signal, but is strictly binary in that regard).  Other tools I came across are in the three figures, so I’m not exactly keen on the outlay.

    I would have though there was a software way to measure this?  But even if not, there must be a reasonably priced tool out there, somewhere.

    P.S. since this is a recent issue and the network used to work fine, I would assume if the signal strength was weak now that something has changed physically, yes?

    Thanks for the input!

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    “We think we own the world and own the stars… in fact we barely even own our cars.” - Sparks, “A Song That Sings Itself”