csma/cd do packets or frames or signals collide?

Discussion in 'Cisco' started by jameshanley39, Apr 5, 2005.

  1. with csma/cd, is it more correct to say that signals collide, packets
    collide, or frames collide? are any of those wrong?

    The detection must occur at the physical layer, so it's signals there.
    But once there's a collission, i'd have thought it's frames. and all
    frames contain packets.

    i know that cisco refers to signals colliding. But most other sources
    refer to packets colliding, and many - but fewer - refer to frames
    colliding. IF it is packets colliding then that's more specific than
    signals. But is it as correct?
    jameshanley39, Apr 5, 2005
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  2. All of them are technically wrong, but "signals" is the least so.
    Unlike "signals" and "frames", "packets" is very wrong, albeit
    common usage.
    Not all frames contain packets (consider LLC2, Spanning tree BPDUs,
    etc). Frame is an informal name for a "MAC protocol data unit," but is
    also an informal name for a "LLC protocol data unit." In the latter
    sense, it could be incorrect because a collision could occur before
    the LLC portion of the frame ever gets put on the Ether.
    My personal preference is to just state that a collision occurs, because
    the MAC PDU which is being sent could be colliding with ANYTHING,
    including noise on the line.

    Whether or not signal is correct depends upon your definition of
    "signal." In common dictionary usage, a "signal" is something that
    serves as a means of communication, so noise and line faults would
    be erroneously excluded if we required that "signals collide." Even
    in the absence of failure, a jam or flow control signalling from
    a switch would not be considered a "frame," so there does not
    have to be another frame present on the line to cause a "frame"
    collision. And, as already stated, the presence of a network PDU
    (packet) is optional, so a "packet" collision is clearly misleading.

    As always, YMMV. If prepping for a test, the correct answer is whatever
    the book/lecturer/test writer says it is.
    Vincent C Jones, Apr 5, 2005
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