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                              V34PLUS.TXT
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            ANALOG ASYNCHRONOUS COMMUNICATION AT 33.6K BPS      
  
  
                             30 SEP 96
  
                        By Paul Munoz-Colman
                          FunStuff Software
  
        http://Ourworld.CompuServe.Com/Homepages/FunStuff_Software/
  
  
  BACKGROUND.
  ~~~~~~~~~~
  
     What are the terms V34, V.34+, and VFC?
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
     V.34.
  
  The newest international asynchronous data communications standard
  for analog modems in today's marketplace is "V.34".  V.34 was
  ratified in the Summer of 1994 by the international standards body
  ITU-T (formerly known as the CCITT).  As originally published, V.34
  offered modulation rates up to 28.8k bps (28,800 bits per second),
  with the opportunity for future extensibility.
  
     V.34+
  
  That future is here today.  Recent technology improvements enabled
  the implementation of optional features of V.34.  These improvements,
  crafted by AT&T, US Robotics, and others, have been temporarily
  designated "V.34+" in today's marketplace (the ITU-T hasn't announced
  the final "V" label, but it is likely that the new improvements will
  still be called "V.34".)  The improvements raise the maximum
  modulation rate from 28.8k to 33.6k bps.  A typical 2,400 bps
  increase in connection rates occur, as well as improved performance
  at lower speeds.
  
  Even though they are still unratified, the 33.6k bps options which
  are now implemented in millions of modems, are well understood and
  well documented by major industry suppliers.  As such, V.34+ is
  living proof of the high value of a well-designed, extensible, open
  systems communications standard.
  
     VFC
  
  For historical purposes, the early work on V.34, which began about
  four years ago, was called "VFC".  "FC" stands for "Fast Class", and
  was an early, proprietary, and less robust implementation of V.34.
  
     What has happened to technology?!
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  Older modems operated at 9,600 bps, using the V.32 protocol.  The
  next generation of modems operated at 14,400 bps, and used the
  V.32-bis protocol.  The V.34+ protocol operates at a top speed 2.5
  times that of V.32-bis, and 3.5 times that of V.32.
  
  In sharp contrast to the extremely volatile marketplace that existed
  when I wrote the previous version of this article ("Asynchronous
  Communication at 28.8k bps, dated 17 Dec 94"), as I predicted then,
  the dust has settled.  Nearly all vendors have V.34 modems on the
  market, and many have V.34+.
  
     There is still some high-speed chaos.
  
  Three years ago, prior to V.34, much haste occurred to get 28.8k bps
  modems to the marketplace.  There were many different early versions
  of 28.8k bps, even within the same manufacturer of modems.  This
  haste caused extraordinary confusion and lack of interoperability
  between modems.  Connections were difficult to establish difficult to
  maintain, and unreliable to transfer data.
  
  Early implementations of V.34 also suffered compatibility problems. 
  These problems were due to differences in the interpretation of the
  high complex specification.  Some modems still suffer from this
  problem today.
  
     What is the big deal about V.34's implementation?
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
     V.34's design limits.
  
  It is not only perfectly normal, but even typical in a V.34
  connection to see less than 33.6k bps in a connection.  As V.34 is
  not a fixed-speed standard, it makes or changes its connections based
  on phone line quality.
  
  It is very rare to get consistently perfect connections.  Speeds of
  33.6k bps require pristine phone line quality along the entire length
  of the connection.  However, V.34+ is certainly capable of pushing
  the practical and ordinary limits of analog phone lines, commonly
  offering connection speeds of 24k, 26.4, and 28.8k bps.
  
  Analog modems communicate over voice-grade (unconditioned) telephone
  lines.  To support speech, the minimum bandwidth (or "bandpass") of a
  voice-grade line must be at least 3,000 Hz (cycles per second).  To
  make 3,000 Hz available on the phone line, the laws of physics
  require supplying more than 3,000 Hz of bandwidth.  Because more than
  3,000 Hz is available, the technological implementation of advanced
  mathematics in modems can make use of that extra bandwidth to give
  greater connection speeds.
  
  Where does this "extra" come from?  To supply the 3,000 Hz in analog
  circuits, there must be a natural "rolloff" in the amplitude of the
  signal.  This rolloff occurs at the lowest frequencies (near 0 Hz or
  DC), and at the highest frequencies (near 4,000 Hz).  Even at the
  reduced levels, there is significantly usable available bandwidth,
  but under a variety of line conditions.  The phone lines are pushed
  to the limit, by applying various mathematical techniques to compress
  the signal.  This achieves the highest possible rates, at nearly
  theoretical limits.
  
  V.34 employs a smart method, called a "channel probe," which measures
  the frequency response and signal-to-noise ratio at various points
  across the bandpass.  During the "handshake," modems send a series of
  tones to each other, at known signal levels and at defined
  frequencies.  The modem calculates the level of the received signal
  at each frequency.  By a set of rules defined by the V.34 protocol,
  the modem determines the maximum bandwidth available for use.
  
     So, just how good does a line have to be?!
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  To obtain and maintain a 33.6k bps connection, it takes a clear line
  which does not drop below about -44 dB (deciBels) or better, measured
  very close to 4,000 Hz, the upper limit of the rolled-off portion of
  the bandwidth.  (-44 dB is the sound level of a clearly whispered
  conversation across a medium size room.)  At -46 dB and below, modem
  receivers start to "go deaf."
  
  But at 4,000 Hz, the typical long distance telephone connection can
  be much quieter than -46 dB.  At 4,000 Hz, it is not unusual to see
  rolloffs of -55 dB to -75 dB, which is 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 below reference level (-10 dB).  During the
  initial transmission attempt, the actual transmit levels are
  negotiated.  Receive levels vary widely, depending on the conditions
  of the local phone line, the line at the remote modem, and the
  long-distance or inter-office carrier facilities.
  
  Typical receive levels range from -40 dB at the low end, to -15 dB at
  the high end, with the -20 dB to -35 dB 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.
  
     So how come V.34 is so robust?
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
     Recovery from adverse line conditions.
  
  The goal of 33.6k bps modem protocol is a simple one:  under
  inevitably changing conditions, it should have a high top speed, and
  should spend as much time as possible operating at the highest
  possible speed.  The V.34 protocol has advanced procedures for
  "training" (synchronization between modems), and for recovery from
  transient disturbances during training.  There are several retrain
  and speed switching procedures to ensure the integrity of the link
  under adverse conditions.
  
     The line (channel) probe.
  
  V.34 "probes" 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 examination consists of
  measuring the amplitude of the signal at various frequency levels
  ("frequency response") and aspects related to signal distortion. 
  
     This concept can be captured by instrumentation within a modem.
  
  In the case of US Robotics modems, visible instrumentation is built
  into the modem's command set, and the data from the commands is
  recallable to the user.  Live data on the condition of a connection
  can be captured in numeric form.  The numeric data can then be passed
  to Joe Frankiewicz's fabulous program called USRSTATS, which draws a
  graphical representation of the line's frequency response.  USRSTATS
  is available on my web site, on Joe's BBS, and on USR's BBS or web
  site.
  
  The drawing below is the frequency response from a typical long
  distance connection.  This one was from my home in Reston, Virginia,
  to the US Robotics bulletin board in Skokie, Illinois, using MCI as a
  long distance carrier.  The rate for this particular connection was
  24,000 bps (receive channel), and 19,200 bps (transmit channel).  In
  this example, note the modem receiver "went deaf" above 3,000 Hz, so
  there wasn't much opportunity for highest speeds here.
  
  The measured signal level (in dB) is shown on the left Y-axis, and
  the corresponding attenuation (or drop, also in dB) from the
  practical usable signal level is shown on the right Y-axis.  The
  frequencies in the bandpass (in Hz) are shown along the X-axis.
  
  ====================================================================
  
                      The V.34 Channel Probe
  
    Ŀ
     -28                              0 
     -30                              2 
     -32                              4 
     -34                              6 
     -36                              8 
     -38                             10 
     -40                             12 
     -42                             14 
     -44 ==========================  16 
     -46                             18 
     -48                             20 
     -50                             22 
     -52                             24 
     -54                             26 
     -56                             28 
     -58                             30 
    LevelAtten
      dB   0 0 0 0 0 0 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3    dB 
           1 3 4 6 7 9 0 2 3 5 6 8 9 1 2 4 5 7 8 0 1 3 4 6 7       
     Freq  5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5  Freq 
      Hz   0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0   Hz  
    
                         Frequency, Hertz (Hz)
  
  ====================================================================
  
  The Channel Probe determines proper connection speed.  V.34 measures
  signal levels at 25 frequencies across the entire channel, in
  intervals of 150 Hz.  This provides a highly accurate sample of the
  channel bandwidth, and in selection of the appropriate  "symbol
  rate."
  
  Because of this close spacing of the probe samples, the accurate
  profile (and its ability to provide problem detection) is a main
  reason why V.34 connections are so reliable.  The channel probe
  occurs during initial modem negotiation, and during training and
  retraining.  Additionally, the line's quality and noise levels are
  measured repeatedly during the connection.
  
  One of the objectives of the probe is to 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.
  
  Using these kinds of measurement tools, with practice and perhaps
  some technical consultation, it is possible to become adept at
  determining different kinds of problems with phone lines.  Even
  better, if you are lucky enough to have a cooperative local phone
  company or long distance carrier, the tools can even be used to help
  them troubleshoot and pinpoint adverse situations.
  
  However, please be aware that on unconditioned voice grade lines, Ma
  Bell and the long distance carriers are not required by law, statute,
  or tariff to "fix" this "problem."  Why not?  Because it is not
  really a "problem", but simply a fact of nature and of technology.
  
  Besides a quality line probe, V.34 does a cooperative (and nearly
  instantaneous) speed shift, also called a "fallback," which host
  computers can tolerate well.  This rate renegotiation procedure
  allows rapid switching ranging from 4.8k bps up to 33.6k bps, as line
  conditions vary.
  
  Rate renegotiation is a tremendous improvement from earlier modem
  "retrains" (where the modems would isolate themselves from the host
  computer for up to a minute, while they recomputed the line
  parameters).  Unlike renegotiations, host computers do not tolerate
  retrains well at all.  Often, they think the line has gone dead, and
  so will themselves disconnect from the modem.
  
     So why does it get bad?
     ~~~~~~~~~~~~~~~~~~~~~~
  
     Simple line impairment.
  
  Variations in line quality are typically the culprit for low connect
  rates.  Line impairments result in several conditions:  
  
    . link time-outs (the error control protocol does not receive a 
      block of data within its expected timeframe), 
  
    . link naks (the error control protocol requests retransmission of
      the data),
  
    . blers (block errors, or errors in received error control protocol
      or data blocks), and
  
    . retransmitted 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 19.2k bps, you will want to
  investigate the line quality of your connections.
  
  There are significant differences in the reliability and stability of
  modems from different manufacturers, and in their available
  instrumentation.  Those which are most reliable come from
  manufacturers who take the greatest care in implementation of the
  V.34 standard.  In many cases, modems so produced will operate
  reliably at speeds that are slightly lower than others.
  
  Some modem manufacturers place greater store in reporting exceedingly
  aggressive initial connection rates.  Doing this allows the modem to
  report a high connection rate, knowing that the switching protocol
  will quickly adjust the rate down to proper operating range. 

     Other reasons why V.34 is a robust standard. 

  V.34 has a number of features that make it the most reliable 
  communications standard published to date:
    
    . precoding (which changes the transmitted signal to reduce the
      effects of noise multiplication in adaptive equalization,
      compensating for severe amplitude distortions);
        
    . powerful multidimensional trellis coding;
      
    . constellation shaping (which gives greater immunity to noise); and
      
    . nonlinear coding (which changes the transmitted signal to improve
      operation in the receiver, thus addressing the problem of 
      distorted signal peaks due to nonlinear circuit elements).
  
  VFC, and earlier ITU-T standards at lower speeds, kept both the
  receive and transmit channels operating at the lowest of the two
  speeds (and their associated symbol rates).  A channel impairment in
  either the transmit or the receive direction would drop both speeds
  to the single level tolerated by the impairment.
  
  In contrast, a major improvement in V.34 is first international
  standardization of independent receive and transmit channel speeds
  (and their associated "symbol rates").  This allows the receive and
  transmit channels of the modem to adjust independently and operate at
  different speeds, thus making maximum use of available bandwidth in
  the face of channel impairments.  (Historically, independent transmit
  and receive channel speeds were first introduced many years ago by US
  Robotics in their proprietary High-Speed Transmission (HST)
  modulation, and were submitted and eventually incorporated in V.34 by
  the ITU-T.)
  
  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 that 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 high noise immunity.
  
     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
       suppressers, 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 communications 
       consultant may be able to help.
  
     . If you're a troubleshooter by nature, and happen to own a US
       Robotics modem, download the USRSTATS program from my web site,
       their BBS or web site, capture some statistics, and methodically 
       study your own connections!
  
     Dropped Connections and Rate Switching in Early Protocols.
  
  Earlier protocols could only switch rates down to 14,400 bps.  If you
  connected using one of these protocols, and the line quality dropped
  below that allowable for a 14,400 bps connection, the modems would
  disconnect.  If this occurred frequently for a particular call,
  disabling the protocol was a way to make a connection when calling
  that modem again.  As line conditions warranted, establishing a
  slower modulation would allow the modems to switch to lower bit
  rates.  If the modems did not allow rate switching, the connection
  would likely drop.  In those severe cases, locking the modem to a
  lower rate could complete the call.
  
     Dropped V.34 Connections and V.34 Rate Switching.
  
  Even though V.34 rarely drops connections, they will occur during a
  call, when there is a sharp decrease in line quality.  In contrast to
  the early protocols, V.34 modems will switch down to rates as low as
  4,800 bps to compensate for these changes.  If the loss of quality is
  extremely severe, however, even V.34 will drop the connection.
  
  ====================================================================
  
                                Summary:
                                ~~~~~~~
  
            Technical phone line bandwidth requirements, and
        how bandwidth and symbol rates are determined for a connection:
  
  As already stated, connection rates are based upon the phone line's
  available bandwidth.  Modems use the channel probe to test the phone
  lines before establishing a connection rate, and then select the
  highest "symbol rate" allowable.  V.34 and VFC 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.  The waveform
  contains a certain number of encoded bits of data to move across the
  link.  The receiving modem decodes this waveform, recovers the
  package of bits, and re-assembles it.  Noise levels in the channel
  determine the number of bits encoded in each symbol.  Lower noise
  levels allow a larger number of bits per symbol.  The rate at which
  symbols are sent is limited by the bandwidth of the channel.
  
  Symbol rate is directly related to overall connection speed.  A
  higher "symbol rate" generally allows greater data transfer speeds,
  but requires greater bandwidth.  Once negotiation determines a symbol
  rate, it remains constant.  To maintain low error rates, by
  considering both the changing characteristics and the levels of
  noise, the modem adjusts the bit rate dynamically.
  
  The chart below shows the approximate bandwidth requirements for each
  symbol rate.  Thus, based on the connections you make, and/or by the
  quality of the diagnostics contained in the better brands of modems,
  you can determine the approximate bandwidth detected by the modem. 
  For each symbol rate, a connection can be made from the choice of
  frequency ranges.  Thus, the modem selects the best quality for each
  call.
  
  These are maximum bit rates.  V.34 will connect at speeds as low as
  4,800 bps with any of these 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.
  
  ====================================================================
                                                   Maximum    Maximum
  Symbol  Protocol    Carrier      Bandwidth       Bit Rate   Bit Rate
  Rate     Range      Frequency    Requirements   (V.34/VFC)  (V.34+)
  -----   ---------   ---------    ------------    --------   --------
  2,400   V.34/V.34+  1,600 Hz     400-2,800 Hz     21,600     21,600
          VFC /V.34+  1,800 Hz     600-3,000 Hz     21,600     21,600
                     
  2,743   V.34/V.34+  1,646 Hz     274-3,018 Hz     24,000     26,400
          VFC /V.34+  1,829 Hz     457-3,200 Hz     24,000     26,400
                     
  2,800   V.34/V.34+  1,680 Hz     280-3,080 Hz     24,000     26,400
          VFC /V.34+  1,867 Hz     467-3,267 Hz     24,000     26,400
                     
  3,000   V.34/V.34+  1,800 Hz     300-3,300 Hz     26,400     28,800
          V.34/V.34+  2,000 Hz     500-3,500 Hz     26,400     28,800
          VFC         1,875 Hz     375-3,376 Hz     26,400
                     
  3,200   V.34/V.34+  1,829 Hz     229-3,429 Hz     28,800     31,200
          VFC /V.34   1,920 Hz     320-3,520 Hz     28,800
                     
  3,429   VFC /V.34   1,959 Hz     244-3,674 Hz     28,800            
               V.34+  1,959 Hz     244-3,674 Hz                33,600
  
  ====================================================================
  
   Permission is granted to reprint and redistribute this information 
   only in its entirety.
   
    Greatful acknowledgement for selected source materials is given to:
    
     Joe Frankiewicz, Paul Gebert, and Dale Walsh of US Robotics, Inc.  
  
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