DP Tool Club 19/CD_ASCQ_19_010295.iso/vrac/whyv34.zip/WHYV34.TXT

Text File | 1994-12-18 | 18KB | 350 lines

               ASYNCHRONOUS COMMUNICATION AT 28.8K BPS          17 Dec 94

                        By Paul Munoz-Colman

    Background.
    ~~~~~~~~~~

         What are these terms V.FC and V.34?

    V.FC ("FC" stands for "Fast Class") is a proprietary implementation
    of an early version of the 28.8kbps asynchronous international
    communications standard, designated V.34.  For the past two years due
    to marketplace pressure, V.FC implementations have been rushed to the
    public, while the international standards body ITU-T (formerly known
    as the CCITT) was slowly and carefully working on the design and
    development of V.34.  Those who released the early V.FC modems were
    participants of the ITU-T Study Group who cooperatively developed
    V.34.

         The current state of the technology.

    V.34 is now a reality.

    It was ratified late this past Summer, and is now the international
    standard for asynchronous communication at modulations up to 28.8k
    bits per second (bps).  V.34 operates at a top speed which is twice
    that of the previous generation of high speed modems (which were
    called V.32-bis and operate at 14,400bps), and three times the speed
    of the generation before that (called V.32, which operates at
    9,600bps).

    At this writing, the marketplace is very volatile.  Today, some
    vendors have V.34 modems on the market, and several more are in
    various stages of testing.  Within six months or more, nearly all
    vendors will have V.34 modems readily available.

         The high-speed chaos which this has created.

    In the haste to get modems to the marketplace and supply chipsets to
    other modem manufacturers, there have been many releases of V.FC,
    even within the same manufacturer of modems.  This has caused
    terribly confusing difficulty in interoperability between modems, in
    establishing connections, maintaining them properly, and in
    transferring data across them.  To a much lesser degree, the
    implementations of V.34 also suffer from some compatibility problems,
    due to some difference in interpretation by modem vendors of the high
    complex specification for this transmission rate.


    Why is it such an issue?
    ~~~~~~~~~~~~~~~~~~~~~~~

         Because of the design limits of 28.8k.

    It is not only perfectly normal, but even typical in a V.34 or V.FC
    connection to see a less than 28.8kbps connection.  V.34 and V.FC are
    not fixed-speed standards, and make/change their connections based on
    phone line quality.

    Very few people can get consistent 28.8kbps connections.  Speeds of
    28.8kbps require pristine phone line quality along the entire length
    of the connection.  But VFC and V.34 modems are capable of pushing
    the limits of analog phone lines, commonly offering connection speeds
    of 21.6k, 24k, and even 26.4kbps.

    The bandwidth (or "bandpass") of a voice-grade phone line is about
    3,000Hz to 4,000Hz (3-4KHz).  Because the mathematics of compressing
    28.8kbps pushes the phone line to near its theoretical limits, V.34
    was designed to accommodate a variety of phone line conditions.  V.FC
    and V.34 are both smart enough to do what is called a "channel
    probe", which is a frequency response and signal-to-noise ratio test
    of frequencies at various points across the bandpass.  During the
    modem handshake, the modems send a series of tones to each other, at
    known signal levels and specific frequencies.  The modem calculates
    the level of the received signal at each frequency, and therefore can
    determine the maximum bandwidth available for use.

         So, just how good does a line have to be?!

    In reality, it takes line clarity at about -44dB or better (about the
    sound level of a clearly whispered conversation across a medium size
    room) at the top of the phone line's "bandpass" to obtain and
    maintain a 28.8kbps connection.  At about -46dB and below, modem
    receivers tend to "go deaf".  The typical long distance connection
    can be much worse than this at that frequency; it is not unusual to
    see -55dB to -70dB (closer to the background hiss level of a
    factory-fresh medium-grade audio tape).

    Standard transmit levels for domestic (US/Canada) modems are
    approximately -10 dB, although V.34 and V.FC negotiate these levels
    during the initial connection attempt.  Receiving levels can vary
    widely, depending on the conditions on your local phone line, the
    line at the remote modem, and any long-distance or inter-office
    carrier facilities.

    Typical receiving levels range from -40 dB at the low end, to -15 dB
    at the high end, with figures in the -20dB to -35dB range being most
    common.  Extreme values in either direction probably indicate a
    problem in the connection from your modem to your local phone
    company, which in some cases the phone company may be able to adjust.

    However, be aware that Ma Bell and the long distance carriers are not
    required by law, statute, or tariff to "fix" this "problem" on
    unconditioned voice grade lines, because it is not really a
    "problem"!

         Why does it get bad?

    Simple line impairment.

    Variations in line quality are typically the culprit for low connect
    rates.  Line impairments can result in link timeouts (when the error
    control protocol does not receive a block of data within its expected
    timeframe), link naks (when the error control protocol requests
    retransmission of data), blers (block errors; errors in received
    error control protocol or data blocks), and resent data blocks.
    Everyone occasionally gets "a bad line" and has to hang up and call
    again to get a better connection.  However, if you find that you
    never or rarely connect at rates above 19.2kbps, you will want to
    investigate the line quality of your connections.


    All right, so how is V.34 more robust?
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

         Recovery from adverse line conditions.

    The goal of 28.8 modem protocols is not only to have a high top
    speed, but to spend as much of that time as possible operating at the
    highest possible speed under inevitably changing conditions.  The
    V.34 protocol has advanced procedures for training and for recovery
    from transient disturbances during training.  There are several
    retrain and speed switching procedures to insure link integrity under
    adverse conditions.

         The line (channel) probe.

    Both V.FC and V.34 "probe" the phone line for quality.  The line (or
    channel) probe quickly examines line conditions and selects the best
    transmission strategy to optimize data transmission (there are a
    variety of such strategies available).  This technique can detect
    certain unusual non-linear distortion mechanisms present on some
    phone circuits, particularly international ones.  The modems can then
    select the operational modes that better combat distortion.

         V.FC's weak implementation of probing.

    The Channel Probe determines proper connection speed.  V.34 measures
    signal levels every 150 Hz across the entire channel, whereas V.FC
    measures only 6 points, concentrated at the upper end of the
    frequency range.  This provides V.34 with a much more accurate sample
    of the channel bandwidth, and greater accuracy in selecting the
    appropriate symbol rate.

    Thus, in V.FC, the weak implementation of the probe can generally
    result in a "retrain" (when the two modems lose synchronization with
    each other), which usually ends up lowering the speed to where it
    should have been in the first place!!

         The retrain is a Killer!!

    A retrain is where the two modems suspend operations and renegotiate
    the best possible connection all over again.  V.FC retrains are
    extremely slow, and can take 5 to 60 seconds, during which time the
    modems appear "dead" to the network, host, or PC to which they are
    connected.  With V.FC, a retrain is generally required to change the
    speed.  This might be tolerated by some PC-to-PC connections, but it
    is rarely tolerated in a network environment, particularly a
    packet-switched one.  The "timeouts" which will be sensed by a
    variety of network software packages simply won't tolerate them, will
    perceive them as disconnects, and will act accordingly, interrupting
    end user service.

         V.34's improvement of the probe and rate renegotiation.

    A first major factor is that V.34 probes 25 frequencies across the
    channel (vice 6 concentrated at the high end for V.FC).  Because the
    frequencies are spaced closer together, the frequency response
    profile (ie the channel probe) is more accurate.  That is a main
    reason why V.34 connections are more reliable than V.FC connections
    (more accurate line problem detection).  The channel probe occurs
    during initial modem negotiation, and during training and retraining.
    Additionally, line noise and the line's signal-to-noise ratio is
    remeasured continually during the connection.

    Besides a better probe, rather than retrain, V.34 does a cooperative
    and nearly instantaneous speed shift (also called a "fallback"),
    which hosts can better tolerate.  This rate renegotiation procedure
    allow rapid switching ranging from 4.8kbps up to 28.8kbps, as line
    conditions vary.

    V.34 speeds will usually be slightly lower, more truthful, and more
    reliable than V.FC.

         Other reasons why V.34 is a more robust standard.

    V.34 has a number of features which may be implemented to a lesser
    degree, a poorer degree, or may not available at all in V.FC:
    precoding (changing the transmitted signal to reduce the effects of
    noise multiplication in adaptive equalization, which compensates for
    severe amplitude distortions); powerful multidimensional trellis
    coding; constellation shaping and other innovations that give V.34 a
    greater immunity to noise; and nonlinear coding (changing the
    transmitted signal to improve operation in the receiver, which
    addresses the problem of signal peaks being distorted due to
    nonlinear circuit elements).

    A key improvement in V.34 is independent receive and transmit channel
    speeds (and their associated "symbol rates").  This allows the
    receive and transmit channels of the modem to be adjusted
    independently and operate at different speeds, thus making maximum
    use of available bandwidth in the face of channel impairments.  V.FC
    forces both the receive and transmit channels to operate and the
    lowest of the two speeds (and thus symbol rates), so a channel
    impairment in either direction drops both speeds to that tolerated by
    the impairment.

    V.34 has more robust Trellis Coding in use by the modem's receiver
    and transmitter.  Trellis coding is a mathematical operation
    performed on the transmitted data which improves the system's noise
    immunity.  The type of coding may vary significantly when connecting
    modems from different manufacturers.  V.34 supports a 64 state 4
    dimensional coding scheme for greater noise immunity than the V.FC
    protocol.


    All right, you convinced me!  I just bought a V.34 modem and am
    still having problems!  What can I do to get a better connection.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    *Try calling a different location.  Line quality differs from region
    to region, and it may be a problem with the lines or modem at the
    other end of a particular call.

    *Try connecting with a local call.  Sometimes the connections within
    a long distance call can cause impairments.  (If this isolates the
    problem, you can try switching long distance companies.)

    *Try plugging the modem to a different phone line or wall jack.

    *Try eliminating all telephone extensions, phone line surge
    suppressors, line switches, utility monitoring devices connect to the
    phone line, and anything else on the line with the modem.

    *If you know someone else in your area with a high speed modem, ask
    what type of connections they make.  Try making the connection from
    their location.  If you encounter the same low connection rates, the
    problem may be resulting from impairments along the lines running to
    the local telephone company or within your home or office.  Your
    telephone company or a private consultant may be able to help.

         Dropped V.FC Connections and V.FC Rate Switching.

    VFC connections can only switch rates down to 14,400 bps.  If you
    connect using VFC and line quality drops below that allowable for a
    14,400 connection, the modems will disconnect.  If this occurs
    frequently for a particular call, you will want to disable VFC before
    calling that modem again.  A slower modulation, (V.32-bis at
    14,400bps, for example) will be established and will allow the modems
    to switch to lower bit rates as line quality warrants.  If the
    problem is severe, use the modem's command set to disable V.FC, so
    that V.34 (or a lower speed modulation on those modems which don't
    have V.34) is forced. Some VFC modems from some manufactures do not
    support rate switching (it's a tossup as to who does and in what
    version they do).  These connections are more likely to drop.  For
    these calls, you can force a lower connect speed by locking the modem
    to a lower link rate.

         Dropped V.34 Connections and V.34 Rate Switching.

    Dropped connections can occur when there is a sharp decrease in line
    quality during a call.  V.34 modems will switch to rates as low as
    4,800 bps to compensate for these changes.  If the loss of quality is
    extremely severe, even V.34 will drop the connection.


    Technical phone line bandwidth requirements, and how a connection's
    bandwidth and symbol rates are determined.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    As already stated, V.34 and VFC connection rates are based on the
    available bandwidth over the phone line.

    The modems use the channel probe to test the phone lines before
    establishing a connection rate, and will select the highest "symbol
    rate" allowable.  V.34 and V.FC modulations allow adjusting the
    symbol rate to any of six possible values, to obtain the best match
    with the available bandwidth.  Other protocols only allow a single,
    fixed value for the symbol rate, regardless of the bandwidth of the
    link.

    A "symbol" is a waveform transmitted by the modem, which contains a
    certain number of encoded bits of data to be moved across the link.
    The receiving modem decodes this waveform, recovers the package of
    bits, and re-assembles it.  The noise levels in the channel determine
    how many bits are encoded in each symbol; lower noise levels allow a
    greater number of bits per symbol.  The bandwidth of the channel
    limits how many of these symbols may be sent each second.

    Symbol rate is directly related to overall connection speed.  In
    general, a higher "symbol rate" allows greater data transfer speeds,
    but requires greater bandwidth.  Once a symbol rate is determined
    through negotiation, it remains constant.  The bit rate then is
    adjusted on-the-fly to maintain low error rates, based on the modem's
    tracking of noise and the signal-to-noise ratio.

    The approximate bandwidth requirements for each symbol rate are shown
    in the chart below.  Thus, based on the connections you make, and/or
    by diagnostics contained in the better brands of modems, you can
    determine the approximate bandwidth detected by the modem.  The
    connection can be made at any of the frequency ranges for any of the
    given symbol rates.  This allows it to select the frequency range of
    best quality for that call.

     Symbol                 Carrier       Bandwidth         Maximum
      Rate      Protocol    Frequency    Requirements      Bit Rate

      2400      V.34        1600Hz        400-2800 Hz        21600
                V.34/VFC    1800Hz        600-3000 Hz        21600

      2743      V.34        1646 Hz       274-3018 Hz        24000
                VFC/V.34    1829 Hz       457-3200 Hz        24000

      2800      V.34        1680 Hz       280-3080 Hz        24000
                VFC/V.34    1867 Hz       467-3267 Hz        24000

      3000      V.34        1800 Hz       300-3300 Hz        26400
                V.34/VFC    2000 Hz       500-3500 Hz        26400
                VFC         1875 Hz       375-3376 Hz        26400

      3200      V.34        1829 Hz       229-3429 Hz        28800
                VFC         1920 Hz       320-3520 Hz        28800

      3429      V.34        1959 Hz       244-3674 Hz        28800

    NOTE:  These are maximum bit rates. V.34 will connect at speeds as
    low as 4,800 bps with any of the above symbol rates.  VFC will only
    connect down to 14,400 bps.  If the bit rate is much lower than the
    maximum bit rate supported by the symbol rate, the phone line has
    lots of noise or other impairments on it.

    =====================================================================

    Permission is granted to reprint and redistribute this information only
    in its entirety.

    Acknowledgement for selected source materials to:

     - Paul Gebert, Joe Frankiewicz, and Dale Walsh of US Robotics, Inc.