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  1. #1

    Question Need help diagnosing network problem

    Hi, I recently "upgraded" my broadband connection, and it seems to no longer work properly. I am having trouble identifying the source of the problem, so I'd appreciate any help.

    I'm using Ubuntu 11.04, but the same problems are present on an older version on my other laptop, and with all probability on my roommate's XP machine (so, it is likely not caused by Ubuntu). The problems only appeared after "upgrading" the connection to 60 Mbit/6 Mbit. The provider is UPC and the new modem (and probable source of problem) is a "Cisco EPC3925 EuroDocsis 3.0 2-PORT Voice Gateway". I am connected to this modem through a hub and a very long cable (40m), which may contribute to the issues (but worked fine before).

    The symptoms are:
    Websites do not load half of the time, or do not load entirely (often, part of an image is missing, or the whole page is cut halfway down and some of the elements or text don't load). Bigger, complicated sites seem to cause problems more often.
    Small files (like torrents) require several tries before they download - the failed attempts do not even start.
    Large files (direct downloads) always stop after downloading several megabytes, although the exact number seems random.
    Torrents seem to download fine, maybe a little bit slower - I'm assuming this is because the client compensates for any failed attempts and the files actually transferred are small pieces.

    Please let me know if you have any ideas what could cause this or how to fix it. Also, if I can provide any additional information. Thanks in advance!

  2. #2
    Linux Guru Rubberman's Avatar
    Join Date
    Apr 2009
    I can be found either 40 miles west of Chicago, in Chicago, or in a galaxy far, far away.
    A 40m cable (120+ feet) is VERY long for what I assume is a gigabit connection. The cable has to be at least a cat 5e cable for this distance. If not, then you will definitely experience data dropouts. Again, it has to be at LEAST a 5e category cable, and a cat-5 cable WILL NOT WORK reliably over these distances.

    All that said, if a 5e cable (certified) is what you have, then try a cat-7 cable. The number refers to the number of twists in the wires per unit of measure (I think it is inches). These devices use (or should use) balanced line-drivers to send the signals, but it may be that your hub is out of spec. If changing the cable doesn't work, try changing the hub and/or cable between hub and modem.
    Sometimes, real fast is almost as good as real time.
    Just remember, Semper Gumbi - always be flexible!

  3. #3
    Just Joined!
    Join Date
    Jan 2011
    Fairfax, Virginia, USA
    Hi Jendra,
    If you want, you could use a tcpdump to show you what is going on. My best guess is you have some packet loss and TCP is trying to fill in the gaps as well as throttle back the rate of the connection. Wireshark is a newer graphic program that is almost like tcpdump.

  4. $spacer_open
  5. #4
    Just Joined!
    Join Date
    Nov 2004
    New Zealand
    Some suggestions:
    40M is perfectly ok for an ethernet cable _IF_ it is spec'd for the link speed you are trying to run & it's not munted.
    "According to the ANSI/TIA/EIA standard for category 5e copper cable (TIA/EIA 568-5-A[5]), the maximum length for a cable segment is 100 meters (328 feet). " - Wikipedia.
    If you have a cat5 cable (no official gigabit certification) you may want to try forcing your network card to 100mbit.

    The only real way to diagnose a cable, short of replacing it, is with a proper cable analyser.

    Google "TCP window scaling fix".
    Try different DNS servers.
    Use ping, traceroute etc. to determine where packets are being lost.

    Transfer a file over the LAN to another machine on the hub to eliminate cables / hub as a source of trouble.

    Plug a machine directly into the router / modem (IME the most common source of trouble).



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