Re: Dial-up Modem Question

Discussion in 'A+ Certification' started by w_tom, Sep 18, 2005.

  1. w_tom

    w_tom Guest

    I strongly encourage you to ignore most replies and
    concentrate mostly on what Michael A. Terrell has posted. To
    rephrase what is what he also posted. Modems 'train' for each
    phone line. They adjust outputs on an assumption that a
    dedicated line between two modems always remains same. It is
    called Circuit Switched technology. No connection change. No
    timing changes. A DSP will adjust for each unique connection
    and should not need to retrain for each data packet.

    With VoIP, the connecting characteristics, including timing,
    are constantly changing - Packet Switched technology. Modems
    above 2400 baud cannot adapt to constant changes. That
    'retraining only at the start of a connection' is why 33K and
    56K modems are possible. But many modems will no longer
    understand 300 baud and 1200 baud connections - that don't use
    DSP retraining. 300 and 1200 baud modems may work on VoIP.
    But that means modems at both ends must understand those 30
    year old protocols.

    Michael also demonstrated another important technique in
    debugging the problem. He first breaks a problem down into
    parts. IOW he does a simple modem to modem connection ...
    no PPP software, no authentication. He simply has one
    conventional modem talk directly to another AND he watches on
    screen each byte exchanged one alphanumeric character per
    keystroke. To understand what is happening with a modem over
    VoIP, if you are not using simple BBS type software (such as
    Hyperterminal or ProComm), then the problem is made too
    complex to comprehend. Simplify. Break the problem down into
    parts, then analyze each part. Carefully reread what Michael
    A. Terrell has posted since it is a far more comprehensive
    solution - literally targets potential reasons for no
    connection, breaks the variables out into separate tests, and
    should provide symptoms that others here can explain further.

    Barry Watzman wrote:
    > DSL was introduced into this thread by others who didn't understand or
    > were not responding to the original question, but in fact simply was not
    > part of the original question, in any way.
    >
    > A 56K modem WILL, (fact) work on the analog component of a DSL phone
    > line with DSL filters installed. I've done that.
    >
    > But in the original question, the phone service was not DSL, rather it
    > was VoIP over a cable modem. And it's not at all clear that any high
    > speed modems (even 28k or 33k, much less 56k) will work over a VoIP
    > connection of any type.
    >
    > I want to come back to another question that I posed as part of this:
    > Does anyone know of a modem test phone number, a (free) number that you
    > can dial just to find out if a modem is working and at what speed?
    w_tom, Sep 18, 2005
    #1
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