Strange network problem, collisions and fragmentation - Network

This is a discussion on Strange network problem, collisions and fragmentation - Network ; We have a 10MB fiber connection to the Internet and have some strange collision problems when communicating with a specific host. Our setup is as follow: fiber cable --> LanCast converter --> switch --> PC As I understand the LanCast ...

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Thread: Strange network problem, collisions and fragmentation

  1. Strange network problem, collisions and fragmentation

    We have a 10MB fiber connection to the Internet and have some strange
    collision problems when communicating with a specific host. Our setup
    is as follow:

    fiber cable --> LanCast converter --> switch --> PC

    As I understand the LanCast only converts the optical signal to
    ethernet, it does not do any routing. The switch is manageable and we
    put it there to be able to analyze the traffic (and the errors).

    When uploading data to one specific server which is with another ISP
    we see a lot of collisions, fragmentations and 'late events' errors on
    the switch and some CRC. When downloading we don't see these problems
    and uploading is about three times longer than downloading the same
    file. We made the same test (upload and download of a file) with
    other hosts (with different ISPs) and there is no errors.

    As I understand collisions can only occur between two directly
    connected nodes and do not 'cross routers' so the collisions we see on
    our switch are between our switch and our ISP switch or router at the
    other side of the fiber. However why do these only occurs when
    transmitting to a host 10 hops away?

    Any clues?

    olig

  2. Re: Strange network problem, collisions and fragmentation

    Errors and CRC can occur in any transmission link. If the delay only occurs
    when you traceroute the 10th hop and not the 9th, then the problem lies
    between the 9th and 10th hop.



  3. Re: Strange network problem, collisions and fragmentation

    "Erwin" wrote in message news:<1078612596.396248@seven.kulnet.kuleuven.ac.be>...
    > Errors and CRC can occur in any transmission link. If the delay only occurs
    > when you traceroute the 10th hop and not the 9th, then the problem lies
    > between the 9th and 10th hop.


    But would these errors show up on my switch? I thought that as far as
    layer 2 is concerned, once the frame is delivered to the next switch,
    it is as if the frame was delivered succesfully, then it is up to
    upper layers protocols to deal with the fact that some packets can be
    dropped. But I can be wrong.

    For example, if we have the following setup:
    station 1 -- switch 1 -- router -- switch 2-- station 2

    Station 1 sends a frame to station 2 and the frame is dropped between
    switch 2 and station 2 because of a collision, who is responsible for
    re-sending the frame, and how it is notified?

    I read that 'collision fragment' are frames smaller than the minimum
    size (64 bytes) what can cause that?

    olig

  4. Re: Strange network problem, collisions and fragmentation


    > But would these errors show up on my switch? I thought that as far as
    > layer 2 is concerned, once the frame is delivered to the next switch,
    > it is as if the frame was delivered succesfully, then it is up to
    > upper layers protocols to deal with the fact that some packets can be
    > dropped. But I can be wrong.


    I think errors and CRC on some hops away would occur in your switch as
    collision or delay.

    > For example, if we have the following setup:
    > station 1 -- switch 1 -- router -- switch 2-- station 2
    >
    > Station 1 sends a frame to station 2 and the frame is dropped between
    > switch 2 and station 2 because of a collision, who is responsible for
    > re-sending the frame, and how it is notified?


    In this case station 2 sees error and asks (the sender) to resend the frame.
    Switch 2 and router can also see errors, but do nothing with them. Switch 1
    only sees delay, I don't think it sees errors. Station 1 sees delay, got a
    resend frame request from station 2, and then is responsible for re-sending
    the frame.

    The strange part is that you also have CRC only when you communicate with
    certain ISP. It seems that the routing table only chooses that one link to
    communicate with that ISP?

    > I read that 'collision fragment' are frames smaller than the minimum
    > size (64 bytes) what can cause that?


    I just found an interesting link,
    http://www.techfest.com/networking/lan/ethernet3.htm
    It seems that your 10Mbps ethernet requires 512 bit time slot. This time
    slot establishes the minimum size of an Ethernet frame as 64-bytes, and any
    frame less than that is automatically discarded by receiving stations.

    I also wonder what's the answer of your problem. If you finally solve the
    problem, please post it





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