Домосет

part 19 from «The Winn Rosch HARDWARE BIBLE»


                               M O D E M S
                               -----------

              [part 19 from «The Winn Rosch HARDWARE  BIBLE»]
              [Brady, New York, 1988                        ]

         © Copyright  1991  by  Sergey Shulgin [SHS].  You can copy
         this document for non-commercial use without changes only.

         This document  was  printed  by  Sergey  Shulgin from Moscow
         Institute of Physics and Technology,  Laser Center  lab.  If
         you have  some  interest to this problem,  please call me by
         modem or voice.
               Coord:  Dolgoprudny,  Moscow region, Moscow Institute
                       of Physics and Technology, Institutsky per, 9
                       Laser Center lab.
               Phone:  (095) 408-79-88 from 10:00 till 19:00 by voice
                       (095) 408-79-88 from 19:00 till 08:00 by modem

          [connect to «Player's Dream» BBS and send message for me]
         +-----------------------------------------------------------+
         |        Welcome to «Player's Dream» BBS station!!!         |
         |     [1st BBS Station of United Group Games in Russia]     |
         |                                                           |
         |   Operating time: 19:00–08:00 Mon.-Sat.                   |
         |                   24h Sunday.                             |
         |                                                           |
         |   phone: (095) 408-79-88                                    |
         |                                                           |
         |   modem:up to 9600 with MNP protocol                      |
         |                                                           |
         |   SysOp: Yaroslav Levashov, Sergey Shulgin                |
         +-----------------------------------------------------------+


                               INTRODUCTION
                               ------------

             For Some  reason known to God and the infidels who figure
         out the most profitable hardware packages to call  computers,
         the modem  -  perhaps  the  most  desired  and  used computer
         peripheral — remains an option in all but a  random  sampling
         of portable computers.  The modem is the one computer feature
         that lets  you  display  the  personality  of  your  personal
         computer to  the  outside  world.  It  puts you in touch with
         on-line databases,   remote   computer   systems,   far-flung
         friends, and  even  those  who  are still flinging around the
         country.
             The purpose  and function of the modem seem almost absurd
         in their simplicity.  The modem merely connects your computer
         to the telephone line. The need for the extra device seems so
         absurd because both computer and  telephone  use  obstensibly
         the same  stuff  for  making and moving messages — electrical
         signals. Were not  the  giant  corporations  specializing  in
         computers and telephones (we won't name names) not such avowed
         rivals, you might suspect that they were in cahoots to  foist
         such a contrived accessory on the computer marketplace.
             Step back and look at what a  modem  does,  however,  and
         you'll gain  new  respect for the device.  In many ways,  the
         modern modem is a miracle worker.  For instance,  the best of
         today's modems  can  squeeze  more  than  a  dozen  data bits
         through a cable where only one should  fit.  Even  the  least
         expensive generic  modem  operates  like  a  specialized time
         machine that  can  bridge  the  century-wide  chasm   cutting
         between state-of-the-art  computers  and  stone-age telephone
         technologies.
             Far from   the   hatchlings   of   some   plot   by   the
         Military-industrial complex -  or  that  even  more  sinister
         force, the  telephone company — modems are a necessary bridge
         between digital and analog signals.  The modern modem usually
         does much  more than connect.  Most are boxes chocked full of
         convenience features that can make using  them  fast, simple,
         and automatic.  The  best of today's modems not only make and
         monitor the connection but even improve  it.  They  dial  the
         phone for  you,  even  remembering  the  number  you want and
         trying again and again.  They listen in until they's sure  of
         good contact,  and  only  then  let  you  transmit across the
         telephone line.  Some even have built in circuits  to  detect
         and correct  the inevitable errors that creep your electrical
         conversations.



                       1. MODEM OPERATING PRINCIPLES
                       -----------------------------

             A modem  is  a  signal  converter   that   mediates   the
         communications between  a computer and the telephone network.
         The very name «modem» indicates the role that it  plays.  The
         term is a  foreshortening of the words MOdulator/DEModulator.
         As a modulator,the modem converts the digital, direct current
         pulses used  by  the  computer  system  into an analog signal
         containing the   same   information,   a    process    called
         «modulation».

         1.1 Modulating and Demodulating
         -------------------------------
         Modulation is  necessary  because  the  telephone  system was
         designed even before electronics was invented and solid-state
         digital circuitry  lay almost a hundred years off.  The first
         pained words out of Dr.Bell's speaking telegraph were  analog
         electrical signals,  the  same  juice  that flows through the
         receiver of your own telephone.  Although strictly  speaking,
         digital communications are older — the conventional telegraph
         predates the telephone by nearly three decades  (Samuel  F.B.
         Morse wondered  what  God  had  wrought  in  1844)  — current
         digital technology is only a recent phenomenon.
             The telephone  system  was designed only to handle analog
         signals because that's all that speaking  into  a  microphone
         creates. Over  the  years,  the  telephone system has evolved
         into an elaborate international network capable  of  handling
         millions of these analog signals simultaneously and switching
         them from one telephone set to another, anywhere in the world
         and possibly   beyond.   Although   telephone  companies  are
         increasingly using  digital  signals  to  move   trunk   line
         communications between   switching  centers,  the  input  and
         output ends  of  the  circuit  still  end   in   conventional
         analog-based telephones.
             Modulation, and hence modems, are necessary because these
         analog telephone  connections will not allow digital,  direct
         current signals to pass freely — or at  all.  The  modulation
         process creates  analog  signals  that  code  all the digital
         information of the computer original but can  be  transmitted
         through the voice-only channels of the telephone system.
             DEMODULATION reverses  the  modulation  process.  At  the
         other end  of  the  connection,  the  modem  as a demodulator
         receives that analog-coded signal and converts it back to its
         original digital   form   while  preserving  its  information
         content.

         1.2 The Carrier
         ---------------
         The actual processor of modulation superimposes one signal on
         another. The modem as modulator starts its modulation process
         by generating a constant signal which is called the «carrier»
         because it  carries  or  bears  the  load  of  the modulating
         information. In most systems,  the carrier is a  steady-state
         signal of  constant  amplitude (strength)  and frequency,  and
         coherent phase.

         1.3 Modulation
         --------------
         The signal that's electrically  mixed  with  the  carrier  to
         modify some  aspect  of  it  is  given  the  same name as the
         process, «modulation».  Changes in the modulation result in a
         change in  the carrier-and-modulation mix.  The change in the
         modulation makes a corresponding change in  the  carrier  but
         not necessarily  a  change in the same aspect of the carrier.
         For instance,  in FM or frequency modulation, a change in the
         «strength» of  the modulation is reflected as a change in the
         «frequency» of the carrier.
             Modulation brings  several benefits,  more than enough to
         justify the  complication  of  combining   signals.   Because
         electronic circuits  can  be tuned to accept the frequency of
         one carrier  wave  and  reject  others,  multiple   modulated
         signals can  be  sent through a single communications medium.
         This principle  underlies   all   radio   communication   and
         broadcasting. In   addition,   modulation   allows   digital,
         direct-current-based information to be transmitted through  a
         medium, like  the telephone system,  that otherwise could not
         carry direct current signals.
             In demodulation,  the  carrier  is  stripped away and the
         encoded information  is  returned  to  its   original   form.
         Although logically   just   the   complement  of  modulation,
         demodulation usually involves entirely different circuits and
         operating principles, which  adds  to  the  complexity of the
         modem.

         1.4 Short-Haul Modems
         ---------------------
         Some so-called  modems  aren't  even  modems  at   all.   The
         inexpensive «short-haul  modems»  advertised  involve minimal
         circuitry, definitely not enough to modulate  and  demodulate
         signals. So little, in fact, that often it is entirely hidden
         inside the shell of a simple cable  connector.  All  that the
         short-haul modem  does  is  convert  the  digital output of a
         computer to another digital form that an better withstand the
         rigors of a thousand feet of wire.



                              2. CHANNEL LIMITS
                              -----------------

             Like all great works of art, the modem is constrainted to
         work within the limits of its medium,  the telephone channel.
         These limits are imposed by  the  characteristics  of  analog
         communications and  the  communications  medium  that's  used
         (primarily unshielded twisted-pair wire).

         2.1 Signal Bandwidth
         --------------------
         All communications  channels  and  the  signals  that  travel
         through them   have   a  characteristic  called  «bandwidth».
         Bandwidth merely specifies a range of  frequencies  from  the
         lowest to  the  highest  that  the  channel  can carry or are
         present in the signal.
             An unmodulated  carrier  wave  has  a  nominal  operating
         frequency. For  example,  in  radio  broadcasting,  it's  the
         number you  dial  in  when you tune in your favorite station.
         Without modulation,  the carrier  wave  uses  only  that  one
         frequency and has essentially zero bandwidth.
             The modulation  that's  added  to  the  carrier  contains
         information that  varies  at  some  rate.  Traditional analog
         signal sources — music  or  voice  signals,  for  instance  -
         contain a  near-random  mix  of  frequencies  between  20 and
         20,000 Hz.  Although digital signals start off as  DC,  which
         also has  no bandwidth,  every change in digital state adds a
         frequency component.  The faster the states change — the more
         information that's  squeezed  down  the  digital channel,  as
         measured in its  bit  rate  (bits  per  second)  -  the  more
         bandwidth the signal occupies.

         2.2 Sidebands
         -------------
         In the  simplest  modulation  systems,  a  modulated  carrier
         requires twice  the  bandwidth  of  the  modulation   signal.
         Although this  doubling  sounds  anomalous,  its  the  direct
         result of the combining  of  the  signals.  The  carrier  and
         modulation mix  together  and result in «modulation products»
         corresponding to the frequency of modulation both «added  to»
         the carrier together with the frequency of the modulation and
         «subtracted from» the carrier.  The  added  result  is  often
         called the  «upper  sideband»  and  the  subtracted  result i
         correspondingly called the «lower sideband».

         2.3 Channel Bandwidth
         ---------------------
         The bandwidth  of  a  communications  channel   defines   the
         frequency limits  of  the  signals  that  it can carry.  This
         channel bandwidth may be physically  limited  by  the  medium
         used by  the channel or artificially limited by communication
         standards. For example,  the bandwidth of radio transmissions
         are limited  artificially,  by  law,  to allow more different
         modulated carriers to share the air  waves  while  preventing
         interference between them.
               In wire-based  communications  channels,  bandwidth  is
         often  limited  by  the  wires  themselves.  Certain physical
         characteristics of wires cause  degradations  in  their  high
         frequency  transmission abilities.  The «capacitance» between
         conductors  in  a  cable  pair,  for  instance,  increasingly
         degrades  signals as their frequencies rise to the point that
         a high frequency signal might not be able  to  traverse  more
         than a few centimeters of wire.  «Amplifiers» or «repeaters»,
         which boost signals so that they can travel longer distances,
         often  cannot  handle  very  low  or  very  high frequencies,
         imposing more limits.
             Most telephone channels also have an artificial bandwidth
         limitation imposed by  the  telephone  company.  To  get  the
         greatest financial  potential  from  the  capacity  of  their
         transmissions cables,  microwave  systems,  and   satellites,
         telephone carriers regularly limit the bandwidth of telephone
         signals. One reason bandwidth is  limited  is  so  that  many
         separate telephone  conversations  can be stacked atop of one
         another through multiplexing techniques,  allowing  a  single
         pair of    wires    to   carry   hundreds   of   simultaneous
         conversations. Although the effects of  bandwidth  limitation
         are obvious  — that's why your phone doesn't sound as good as
         your stereo — the telephone  company  multiplexing  equipment
         works so  well  that  you  are  generally  unaware of all the
         manipulations made on the voice signals as they are  squeezed
         through wires.

         2.4 Bandwidth Limitations
         -------------------------
         One of   the   consequences   of   telephone  company  signal
         manipulations is a severe limitation in the bandwidth  of  an
         ordinary telephone  channel.  Instead  of  the full frequency
         range of a good-quality stereo system,  from 20 to 20,000 Hz,
         a telephone  channel  will only allow frequencies between 300
         and 3000 Hz to freely pass.  This very narrow bandwidth works
         well for  telephone  because frequencies below 300 Hz contain
         most of the power of  the  human  voice  but  little  of  its
         intelligibility. Frequencies   above   3000  Hz  increase the
         crispness of  the  sound  but  don't   add   appreciably   to
         intelligibility.
             While intelligibility is the primary concern  with  voice
         communications (most   of   the   time),   data  transfer  is
         principally oriented to bandwidth.  The comparatively  narrow
         bandwidth of   the  standard  telephone  channel  limits  the
         bandwidth of the modulated signal it can carry, which in turn
         limits the amount of digital information that can be squeezed
         down the phone line by a modem.
             Try some simple math and you'll see the harsh constraints
         faced by your modem's signals.  A telephone channel typically
         has a useful bandwidth of about 2700 Hz (from 300 to 3000 Hz)
         At most a carrier wave at exactly the center of the telephone
         channel, 1650 Hz, and burdened by both sidebands, could carry
         data that varies at a rate of 1650 Hz.  Such a  signal  would
         fill the  entire  bandwidth  of the telephone channel without
         allowing for a safely margin.

         2.5 Safely Margin
         -----------------
         A safely margin is necessary, however, because the quality of
         telephone lines   varies   greatly,  particularly  when  long
         distance connections are involved.  Because poor  connections
         can't handle  the nominal 300 to 3000 Hz telephone bandwidth,
         it's ill-advised for a modem to try to take advantage of that
         entire frequency  spread.  If  the connection is substandard,
         when the data rate reaches  the  fringes  of  the  bandwidth,
         errors are likely to crop in.

         2.6 Duplex
         ----------
         The usable bandwidth of a data communications channel through
         a modem is also limited because most modem communications are
         handled in   «duplex»   mode.   The  term  «duplex»  -  often
         redundantly called «full-duplex» — describes the ability of a
         communications channel  to  simultaneously handle two signal,
         usually (but not necessarily) going in  opposite  directions.
         Using these  two  channels,  a full duplex modem can send and
         receive information at the same time.  Two carriers are  used
         to simultaneously   transmit  and  receive  data.  Using  two
         carriers, of course, halves the bandwidth available to each.

         2.7 Half-Duplex
         ---------------
         The alternative to duplex communications is «half-duplex». In
         half-duplex only  one  signal  is  used,  and  to  carry on a
         two-way conversation  a  modem  must  alternately  send   and
         receive signals.  It  allows more of the channel bandwidth to
         be put to use  but  in  practice  slows  data  communications
         because a   modem  often  must  switch  between  sending  and
         receiving modes after every block of data crawls through  the
         channel.

         2.8 Echoplex
         ------------
         The term «duplex» is often,  and mistakenly, used to describe
         «echoplex»  operation.  In  echoplex  mode,  a  modem sends a
         character down the phone line,  and the distant modem returns
         the same  character thereby echoing it.  The echoed character
         is then displayed on the originating terminal as confirmation
         the character was sent correctly.  Without echoplex, the host
         computer usually writes the transmitted character directly to
         its monitor screen.

         2.9 Guard Bands
         ---------------
         Duplex does more than cut the  bandwidth  available  to  each
         channel in  half.  Separating  the  two  channels is a «guard
         band», a width of unused frequencies that isolate the  active
         channels and   prevent   confusion   between  their  separate
         carriers. The safety margin  is,  in  effect,  also  a  guard
         between the carriers and the varying limit of the bandwidth.
             Once you add in the needs of duplex communication and the
         guard bands,   the   practical   bandwidth  limit  for  modem
         communications over real  telephone  channels  that  have  an
         innate 2700  Hz  bandwidth  works out to about 2400 Hz.  That
         leaves 1200 Hz for each of the two duplex channels.



                          3. MODEM MODULATION METHODS
                          ---------------------------

         For the job of making modulation, a modem has several methods
         available to  it  much  as  AM  and  FM  radio  stations  use
         different modulation   methods.   The   different   forms  of
         modulation all  are  based  on  the  characteristics  of  the
         carrier wave that can be changed to encode information.
             Three of the primary carrier characteristics  that  might
         be used for modulation are its amplitude,  its frequency, and
         its phase.

         3.1 Amplitude Modulation
         ------------------------
         The amplitude is the strength of the signal or  the  loudness
         of a  tone  carried  through the telephone wire.  Varying the
         strength of the carrier in response to modulation to transmit
         information is called «amplitude modulation».
             One way that digital  information  could  be  coded  with
         amplitude modulation  is  as  two  discrete  strengths of the
         signal corresponding to the two digital states.  In fact, the
         most rudimentary  form  of  amplitude  modulation — which has
         earned the special name «Carrier Wave» or  «CW»  transmission
         uses the  two  limits of carrier strength for its code:  full
         power and zero power.  The loudness of a telephone signal  is
         its most likely characteristics to vary,  however,  with both
         changes in the telephone line and noise that might be  picked
         up by  the line.  Consequently,  pure amplitude modulation is
         not used by modems.

         3.2 Phase Modulation
         --------------------
         Another state  of the carrier that can be altered  to  encode
         information is   its  phase.  An  unmodulated  carrier  is  a
         constant train of  identical  waves  that  follow  one  after
         another precisely  in  step.  If  one  wave  were delayed for
         exactly one wavelength,  it would fit exactly atop  the  next
         one. The  peaks  and troughs of the train of waves flow by at
         constant intervals.
             By delaying   one  of  the  waves  without  altering  its
         amplitude or frequency,  a detectable state change  called  a
         «phase shift» is created. The onset of one wave is shifted in
         time compared to those that preceded it.  Information can  be
         coded as  «phase modulation» by assigning one amount of phase
         shift from the constant carrier to a digital one and  another
         to a digital zero. Although this form of modulation is useful
         in modem communications, it is most often used in combination
         with other modulation techniques.

         3.3 Frequency Modulation
         ------------------------
         The other   alternative   modulation   technique  alters  the
         frequency of the carrier is response to the  modulation.  For
         example, a  higher  amplitude  of modulation might be made to
         cause the  carrier  to  shift  upward  in   frequency.   This
         technique, called «frequency modulation», is commonly used in
         radio broadcasting by familiar FM stations.

         3.4 Frequency Shift Keying
         --------------------------
         In the most rudimentary digital form of frequency modulation,
         a digital one would cause the carrier wave to change from one
         frequency to another.  In other words,  one  frequency  would
         signify a  digital  one  and  another  discrete  frequency  a
         digital zero.  This form of modulation is  called  «frequency
         shit keying»  or FSK because information is encoded in (think
         of it being «keyed to») the shifting of frequency. The keying
         part of  the  name  is  actually  left  over from the days of
         telegraphy when  this  form  of  modulation  was   used   for
         transmitting Morse code and the frequency shift came with the
         banging of the telegraph key.  Frequency shift keying is used
         in the    most    rudimentary    of   popular   modems,   the
         once-ubiquitous 300 bit per second modem that operated  under
         the Bell 103 standard.

         3.5 Baud Rates
         --------------
         With such  modems  one  bit  of data causes one corresponding
         change of frequency in the  carrier  wave.  Every  change  of
         frequency or  state  carries  exactly one bit of information.
         The unit of measurement used to describe the number of  state
         changes taking  in place in one second is the «baud».  In the
         particular case of the FSK modulation,  one change  of  state
         per second   -   one  baud  -  conveys  exactly  one  bit  of
         information per second.
             Depending on   the   number   of   states   used  in  the
         communication system,  a single transition — one baud  -  can
         convey less  than  or  more than one bit of information.  For
         example, several different frequencies of tones might be used
         to code  information.  The  changing  from  one  frequency to
         another would take place at one  baud,  yet  because  of  the
         different possible changes that could be made,  more than one
         bit of information could be coded by that transition.  Hence,
         strictly speaking,  one  baud  is not the same as one bit per
         second, although the terms are often,  and incorrectly,  used
         interchangeably.
             The number of bits that can be coded by  baud  varies  by
         the inverse  logarithm  of  the  number  of  available states
         )tones, voltage or phases).  Most 1200 bit per second  modems
         operate at 600 baud with four different states available, and
         most 2400 bit per second modems operate at 600 baud  with  16
         different states.
             (In case you're interested,  the term  «baud»  was  named
         after J.M.E.  Baudot, a French telegraphy expert. His name is
         also used to describe a 5-bit digital code used  in  teletype
         systems).

         3.6 FSK Modems
         --------------
         This 300  bit  per second rate using the simple FSK technique
         requires a bandwidth of 600 Hz.  The two  300  baud  carriers
         (which require  a 1200 Hz bandwidth,  two times 600 Hz) and a
         wide guard band fit comfortably within the 2700 Hz limit.
             Under the  Bell  103 standard,  which is used by most 300
         bit second modems,  the two carrier frequencies are 1200  and
         2200 Hz.  Space  modulation (logical zero) shifts the carrier
         down by 150  Hz,  and  mark  modulation  pushes  the  carrier
         frequency up by and equal amount.
             Because the  FSK  modulation  technique   is   relatively
         simple, 300  baud  modems are generally inexpensive.  Because
         they don't push out to the  limits  of  available  bandwidth,
         they are generally reliable even with marginal connections.
             Using the same simple modulation technique and exploiting
         more of  the 2700 Hz bandwidth of the typical telephone line,
         modem speeds can be doubled to 600 baud.  Beyond  that  rate,
         however, lies the immovable bandwidth roadblock.



                    4. MODEMS FASTER THAN 300 Bits Per Second
                    -----------------------------------------

         A data communications rate of 300 bits per second is  slow  -
         slower than  most  folks  can  read  text  flowing across the
         screen. Even the slowest PC can absorb information  at  least
         32 times  faster,  limited  by  the maximum serial port speed
         that IBM supports.  Were long distance communications limited
         to the 300 bit per second rate,  the only people who would be
         happy would be the  shareholders  of  the  various  telephone
         companies. Information  could,  at best,  crawl slowly across
         the continent.
             By combining several modulation techniques, modern modems
         can achieve much higher data rates through  ordinary  dial-up
         telephone lines.  Instead  of merely manipulating the carrier
         one way.  they may  modify  two  (or  more)  aspects  of  the
         constant wave.  For  instance,  today's most popular 1200 and
         2400 bit  per  second  modems  combine  frequency  and  phase
         modulation to achieve faster data flow.

         4.1 Quadrature Modulation
         -------------------------
         These more complex forms of modulation add no extra bandwidth
         (remember, that's a function of the  communications  channel)
         but they  take advantage of the possibility of coding digital
         data as changes between a variety of states  of  the  carrier
         wave. For example, the carrier wave can be phase modulated so
         that it assumes one of four states.
             In the   «quadrature   modulation»   (a   form  of  phase
         modulation) used by most 1200 bit  per  second  modems,  each
         state of  the  carrier  differs  from the unmodulated carrier
         wave by a phase angle of 0,90,180,  or 270  degrees  -  while
         operating at 600 baud.

         4.2 Group Coding
         ----------------
         The four  different phase states are sufficient to encode the
         four different patterns of two digital bits.  Each  baud  can
         hold two bits of data,  thus, a quadrature-modulated 600-baud
         modem can communicate at its  date  rate  of  1200  bits  per
         second. This  bit-packing  is  the key to advanced modulation
         techniques. Instead of dealing with data one bit at  a  time,
         bits of  digital code are processed as groups.  Each group of
         data bits is encoded as one particular state of the carrier.
             The ultimate  speed  of  the  mode  is  determined by the
         number of  states  that  are  available   for   coding.   The
         relationship is not linear, however. As the number of bits in
         the code increases by a given figure (and thus the  potential
         speed of  the modulation technique rises by the same figure),
         the number of states required increases to the  corresponding
         power of two.  Twice as fast requires four states; four times
         faster requires 16 states;  eight times as fast requires  256
         states; and so on.  Data rates of 2400 bps can be achieved by
         using an even more complex modulation that yields 16 discrete
         states while still operating at 600 baud.  Each state encodes
         one of the 16 different patterns of four  digital  bits.  One
         blip on the  telephone  line  carries the information of four
         bits going into the modem.
             More complex  methods  of  modulation  allow  even higher
         modem speeds-dial-up modems operating at 4800 bps and  beyond
         are already   available.  Most  higher  speed  modems  -  for
         example, today's 9600 bps products — get  an extra  boost  by
         foregoing duplex  transmission  and alternate between sending
         and receiving.



                              5. HIGH SPEED MODEMS
                              --------------------
         Modems that operate at data rates in excess of 2400 bits  per
         second are  generally  classed  as  «high speed modems».  The
         distinction is as qualitative as it  is  quantitative:  Above
         2400 bps, squeezing more information into the confines of the
         telephone line  becomes  increasingly  difficult,   requiring
         inventive modulation  techniques  quite  unlike those used at
         lower rates.
             According to  the  free  lunch principle,  this system of
         seemingly getting  something  for   nothing   using   complex
         modulation must  have a drawback.  With high speed modems the
         problem is that the quality of  the  telephone  line  becomes
         increasingly critical   as   the   data  rate  is  increased.
         Moreover, as modem speeds get faster,  each phone  line  blip
         carries more  information,  and  a single error soon can have
         devastating effects.

         5.1 Leased-Line Modems
         ----------------------
         One way to coax higher speed from a modem is  to  forego  the
         one part  of the connection that imposes the severe bandwidth
         limitation — the telephone line.  Special high-grade circuits
         can be  rented  from  telephone  companies to whisk data from
         point to point at almost unbelievably high data  rates  (from
         ten thousand  to  millions  of bits per second).  The special
         lines are semi-permanently  installed  and  stretch  directly
         from one  location  to  another,  never  allowed  to  venture
         through the rigors of the telephone switching system. Because
         these special  lines  are  leased  by  the  month  (or  other
         period), they are called «leased lines» and the  modems  that
         use them are termed «leased-line» or «dedicated-line» modems.
         They usually lack  the  dialing  and  answering  features  of
         dial-up modems, and are meant for continuous connections.

         5.2 Dial-Up Modems
         ------------------
         In contrast,  the  modems  that  you are likely most familiar
         with — the ones that tie into the telephone switching  system
         — are  distinguished  as  «dial-up»  modems.  They  face  the
         constraints of the telephone system and must  be  capable  of
         dealing with  its  special problems and shortcomings.However,
         they are the  most  useful  because  they  can  reach  nearly
         anyone, anywhere  -  as long as the modems at the two ends of
         the call are compatible with one another.

         5.3 Line Compensation
         ---------------------
         Although a  long  distance  telephone  connection  may  sound
         unchanging to  your ear,  its electrical characteristics vary
         by the moment.  Everything from a wire swaying in the Wichita
         wind to  the  phone company's automatic rerouting of the call
         through Bangkok when the direct circuits fill up  can  change
         the amplitude,  frequency, and phase response of the circuit.
         The modem then faces two challenges — not to  interpret  such
         changes as  data and to maintain the quality of the line to a
         high enough standard  to  support  its  use  for  high  speed
         transmission.

         5.4 Switching Modems
         --------------------
         Perhaps the   biggest  limit  imposed  on  high  speed  modem
         communications is the  use  of  full  duplex  communications.
         Because a  complete  duplex  modem circuit is essentially two
         complete channels,  each can have (at  most)  only  half  the
         telephone line's bandwidth available to it. Most of the time,
         however, communications go only in one direction.  You key in
         commands to  a  remote  access  system,  and  only  after the
         commands are received does the remote system respond with the
         information that  you  seek.  While  one end is sending,  the
         other end is more than likely to be completely idle.
             To make  better  use of the available bandwidth so-called
         «switching modems» are designed  to  make  use  of  the  full
         bandwidth of  the telephone channel,  switching the direction
         of the signal as each end of the line  needs  to  send  data.
         Such modems  are  able  to  achieve  a  doubling of data rate
         without adding any complexity to their modulation.  In remote
         mainframe access  situations,  where the protocol of the call
         fits the mold of the two ends of the connections taking turns
         using the  phone  line,  switching  modems can give a genuine
         boost to the cross-country throughput of a modem system.

         5.5 Asymmetrical Modems
         -----------------------
         Switching doesn't  always  work,  however.  The   change   of
         direction of communication isn't instantaneous. The modem has
         no way of knowing when to switch other  than listening for  a
         pause in  the  data  stream.  Some  delay to recognize such a
         pause must be  built  into  the  system.  Further,  when  the
         direction of  the  call  changes,  the modem may be called to
         adjust for  line  differences  (the  characteristics   of   a
         telephone connection  are  not  necessarily  the same in both
         directions because the two directions  of  communication  may
         take entirely   different   paths).  In  all,  switching  the
         direction of the data movement can take a full second.
             While a  second  pause  may not be burdensome when you're
         simply sending characters and  watching  a  response  on  the
         screen, it can be overwhelming when transferring a file. Most
         file transfer protocols (for example,  XMODEM and Kermit)  are
         designed to  send a small block of data to the remote system,
         which then checks it for accuracy and finally sends  a  brief
         return message  that  the data was received intact or that it
         was bad.  A switching modem may require a full second or more
         for each turn-around and confirmation. In that some protocols
         use blocks only 256 or 512 bytes long between  confirmations,
         sending a  file  amounts  to  the  classic  hurry-up-and-wait
         syndrome. The modems blast a  block  across  the  line,  then
         lolly around for a much longer period awaiting a confirmation.
             In an  attempt  to  get  the   best   of   both   worlds,
         «asymmetrical modems»   cut  the  waiting  by  maintaining  a
         semblance of two-way duplex communications  while  optimizing
         speed «in  one  direction only» by shoehorning in a low speed
         (typically 300 bps) channel in addition  to  the  high  speed
         one. As  with  a  switching  modem,  asymmetrical  modems can
         flip-flop the direction of  the  high  speed  communications.
         They rely  on  algorithms  to determine which way is the best
         way. Typically,  the  high  speed   channel   is   used   for
         transferring blocks  of  data while the confirmations trickle
         back on the lower speed channel.

         5.6 Fallback
         ------------
         Most modems  use,  at   most,   two   carriers   for   duplex
         communications. These  carriers are usually modulated to fill
         the available bandwidth.  Sometimes,  however, the quality of
         the telephone  line  is  not  sufficient  to  allow  reliable
         communications over the full bandwidth expected by the modem.
         In such  case,  most high speed modems incorporate «fallback»
         abilities. When the top speed does not work,  they attempt to
         communicate at   lower  speeds  that  are  less  critical  of
         telephone line quality.  The pair of modems might  first  try
         9600 bps and be unsuccessful;  they might then try 4800, then
         2400, and so on until reliable communications are established.

         5.7 Multiple-Carrier Modems
         ---------------------------
         While most modems  rely  on  a  relatively  complex  form  of
         modulation on  one  or two carriers to achieve high speed,  a
         few (notably the Telebit Trailblazer) use instead  relatively
         simple modulation  on  multiple  carriers.  One  of the chief
         advantages of this system  used  by  these  «multiple-carrier
         modems» comes  into  play  when  the quality of the telephone
         connection deteriorates. Instead of dropping down to the next
         incremental communications rate, generally cutting data speed
         in half,  the multiple-carrier modems  just  stop  using  the
         carriers in  the  doubtful  regions  of  the  bandwidth.  The
         communication rate may fall off only a  few  percent  in  the
         adjustment. (Of  course,  it could dip by as much as a normal
         fallback modem as well).

         5.8 Data Compression
         --------------------
         Although there's no way of increasing  the  number  of  bits
         that can  cross  a telephone line beyond the capacity of the
         channel, the  information  handling  ability  of  the  modem
         circuit can be increased by making each bit more meaningful.
         Many of the bits that are sent through the telecommunication
         channel are  meaningless  or  redundant  -  they  convey  no
         additional information. By eliminating those worthless bits,
         the information content  of the data stream is more intense,
         and each bit is more meaningful. The process of  paring  the
         bits is called «data compression».
             The effectiveness of compression varies with the tye  of
         data that's being transmitted.One of the most prevalent data
         compression   schemes   encodes   repetitive    data — eight
         recurrences of  the  same  byte  value might be coded as two
         bytes, one signifying the value,  and the second the  number
         of repetitions.This form of compression is most effective on
         graphics, which often have many  blocks  of repeating  text.
         Other compression  methods  may strip out start,  stop,  and
         parity bits.  Modem  manufacturers  often  claim that  their
         proprietary data compression methods might reduce the number
         of bits  that  need  to  be  transferred  by   50   percent,
         effectively doubling communications speed.

         5.9 Error-Checking
         ------------------
         Because all higher speed modems operate closer to the limits
         of the telephone channel,  they are naturally more prone  to
         data errors.  To better cope with such problems,  nearly all
         high speed modems have their own  built-in  «error-checking»
         methods. These  work like communications protocol — grouping
         bytes into blocks and sending cyclical  redundancy  checking
         information. They   differ   from   the  protocols  used  by
         communications software in that they are implemented  in the
         hardware instead  of  your  computer's software.  That means
         that they don't load down your computer when  it's straining
         at the limits of its serial ports.
             It can also mean that software protocols  are  redundant
         and a  waste  of  time.  As  mentioned before in the case of
         switching modems,  using  a  software-based   communications
         protocol can   be  counterproductive  with  many  high-speed
         modems, slowing the transfer rate to a crawl. Most makers of
         modems using  built-in  error-checking  will  advise against
         using such software protocols.



                               6. MODEM CONTROL
                               ----------------

         Besides its  basic  purpose  of converting digital data into
         modulated audio signals,  the modem is often called upon  to
         handle other chores of convenience.  For example,  it may be
         called upon to automatically dial or  answer  the  phone  or
         report the  condition of the telephone line.  These features
         of the modem must be able to be controlled by your computer,
         and the  modem  must  be  able to signal your computer about
         what it does and what it finds out.

         6.1 Dual Modes
         --------------
         Most modems operate alternately in  one  of  two  modes.  In
         «command mode»,   the   modem   receives   and  carries  out
         instructions sent  by  your  computer.  In   «communications
         mode», it  operates as transparently as a modem can,  merely
         converting data.
             Changing modes  is  mostly  a  matter of sending control
         characters to the modem. The characters can only be received
         and processed in command mode.  In communication mode,  they
         would be passed along down the telephone line.

         6.2 Hayes Command Set
         ---------------------
         Today, most modems use a standardized  set  of  instructions
         called the  «Hayes  command  set»,  after  Hayes  the  modem
         manufacturer (which was,  in turn, named after Dennis Hayes,
         its founder).  For  the  most  part,  the  Hayes command set
         comprises several dozen modem instructions that begin with a
         two character  sequence  called  «attention character».  The
         sequence is almost mnemonic — the letters AT,  which must be
         capitals. Other  characters  specifying  the command  follow
         the attention character.  Because the AT is part  of  nearly
         every command, the  Hayes command set is also termed the «AT
         command set»,  most often by Hayes' competitors  that  don't
         want to   give   the   competition   credit.  A  modem  that
         understands the Hayes command set (or the AT command set) is
         said to be «Hayes-compatible». (The basic Hayes command  set
         is listed in Appendix A of this document).
             Most AT  commands  follow  the attention characters with
         one letter that specifies the  family  of  the  command  and
         another character  that indicates the nature of the command.
         For example,  H stands for Hook.  H0 means put the phone «on
         the hook»  or  hang  up.  H1 indicates that the modem should
         take the phone off the hook,  that is,  make a connection to
         the line.
             Several commands and their modifiers can be  combined on
         a single  line  after  an  initial  attention  command.  For
         example, to command a Hayes  or  Hayes-compatible  modem  to
         dial information on a tone-dialing line, the proper sequence
         of commands  would  read:  ATDT15511212.  The  AT   is   the
         attention signal,  D  is  the Dial command,  the T tells the
         modem to use tones for dialing,  and  the  15511212  is  the
         number of the telephone company information service.
             All AT commands must be followed by a  carriage  return.
         The modem waits for the carriage return as a signal that the
         computer has sent the complete command and  that  the  modem
         should start processing it.
             At first   it   would   appear   that   shifting    into
         communications mode would be a one-way street for the modem,
         particularly were it only able  to  receive  instruction  in
         command mode.  Fortunately, the Hayes command set allows the
         modem to react to  exactly  one  command  in  communications
         mode, a  command  that  instructs  the  modem  to  break off
         communications and shift back to command mode.
             The tricky  part  of designing such a command it that it
         must be a character sequence that will never appear  in  the
         data that  the  modem is supposed to be communicating across
         the telephone line.  Although it's impossible  to  guarantee
         that any  command  sequence  will never appear in the normal
         progress of communications,  the command in the Hayes set is
         specifically designed  to  be  statistically unlikely.  This
         command simply consists of a string of three «plus  signs» -
         that is,  +++.  To make the command stand out from data, the
         Hayes command set also specifies that the three  plus  signs
         be isolated  from  any  other  characters  by  at  least one
         second, before and after.  Such a pause  followed  by  three
         specific characters,  followed  by  a  pause  should   never
         occur (well,  almost) except when  the  command  is   really
         meant.

         6.3 Extended Hayes Command Set
         ------------------------------
         At the time the Hayes command set was developed,  modems had
         relatively few  special  features.  As  modems  became  more
         sophisticated, they  became  more  loaded with abilities and
         features. The original Hayes command set had to be  extended
         to handle    all   the   possibilities.   Note   that   many
         Hayes-compatible modems recognize only the  original command
         set. All  of  their  features  — if they have them — may not
         work with software that expects the extended Hayes set.

         6.4 S-Registers
         ---------------
         The extensions to the original  Hayes  command  set  include
         sufficient new  functions  that  the command language  would
         become ungainly and confusing.  After all, there are only 26
         letters in  the  alphabet  that might be used for one-letter
         commands. Hayes added the facility of a special  register or
         memory area  called  the «S-register» inside its modems that
         allows the settings of the modem's operating  parameters. By
         setting the value contained by the S-register, a  variety of
         modem functions can be controlled.  (S-register settings are
         shown in Appendix B of this document)

         6.5 Response Codes
         ------------------
         Commands sent to a Hayes-compatible modem by their very name
         and nature are one-way.  Absent some means of  confirmation,
         you would  never  know  whether  the modem actually received
         your command,  let alone acted upon it.  Moreover,  you also
         need some  means for the modem to tell you what it discovers
         about your connection to the telephone  line.  For  example,
         the modem  needs to signal you when it detects another modem
         at the end of the line — and when that connections is broken.
             Part of  the  Hayes command set is a series of «response
         codes» which serve that feedback function.  When  the  modem
         needs to  tell  you something,  it sends back — via the same
         connection used to send data between your computer and modem
         — code numbers or words to appraise you of the situation. In
         the Hayes scheme of things,  you can set the modem  to  send
         simple «numeric» codes, consisting solely of codes (which you
         can then look up in your modem manual,  if you have one)  or
         «verbose» responses,  which may be one or more words long in
         something close to everyday English.
             Typical responses include «OK» to signify that a command
         has been received and acted upon, «CONNECT 1200» to indicate
         that you've linked with a 1200 bit  per  second  modem,  and
         «RING» to  show  that  the  phone  at  the  other end of the
         connection is ringing.  (Hayes response codes are listed  in
         Table below).
             Note that because the  response  codes  flow  from  your
         modem to  your  computer as part of the regular data stream,
         you may accidentally confuse them with text  being  received
         from the far end of your connection.

         +---------+----------------+-------------------------------+
         | Numeric |    Verbose     |                               |
         |  code   |     vcode      |         Definition            |
         +---------+----------------+-------------------------------+
         |    0    | OK             | Command executed without error|
         |    1    | CONNECT        | Connection established (300bps)|
         |    2    | RING           | Phone is ringing              |
         |    3    | NO CARRIER     | Carrier lost or never detected|
         |    4    | ERROR          | Error in command line or line |
         |         |                | too long                      |
         |    5    | CONNECT 1200   | Connection established at 1200|
         |    6    | NO DIALTONE    | Dialtone not detected in      |
         |         |                | waiting period                |
         |    7    | BUSY           | Modem detected a busy signal  |
         |    8    | NO ANSWER      | No silenced detected while    |
         |         |                | waiting for a quiet answer    |
         |   10    | CONNECT 2400   | Connection established at 2400|
         +---------+----------------+-------------------------------+



                              7. MODEM FEATURES
                              -----------------

         The broad  term  «features»  describes  various subtle — and
         some not-sosubtle — ways in which  modems  differ  from  one
         another. For  the  most part,  the features of a modem taken
         together determine how easily and conveniently you  can  put
         it to work. A no-frills modem, for example, may require that
         you spin the dial of your phone with your  index  finger  or
         answer incoming  calls  before  turning  them  over  to your
         computer system when you hear the carrier tone of  the modem
         at the other end of the line. Many people are willing to put
         up with such petty inconveniences to save on the price  of a
         modem.
             Although none  of  the  tasks  that   features-deficient
         modems foist  upon you will tax your mind or constitution, a
         no-frills modem  short-changes  the  capabilities  of   your
         computer. With  a full-featured modem,  your PC can dial the
         phone faster and with fewer errors and can handle  the chore
         automatically when  you're  not  around.  Or with the latest
         memory-resident communication software,  your  PC  can  dial
         the full-featured  modem and collect your messages while you
         are in the midst of browbeating data into shape with another
         program.
             Actually, nearly every modem made today -  including the
         lowest budget models made in obscure foreign lands — has all
         the standard features that you might normally want. Once you
         start integrating features into circuit chips,  adding a few
         more features is not  an  arduous  process.  The  only  time
         you're likely  to  run  into  a  modem  deficient in today's
         convenience features is when you try to  make  do  with  one
         manufactured to   yesterday's  standards  -  the  modem  you
         inherit from some corporate higher-up,  one  that  you  find
         lying face-down  in  the  gutter and you nurse back to life,
         one that you buy used from a shady-looking  character  in  a
         trench coat on a deserted street corner.
             Of the various features of a state-of-the-art modem, the
         ones you  should  expect  in  any  new  product that you buy
         include:

         7.1 Auto-Answer
         ---------------
         An auto-answer  modem  is  capable  of  detecting   incoming
         ringing voltage (the low-frequency,  high-voltage signal that
         makes the  bell  on  a  telephone  ring)  and  seizing   the
         telephone line  as if it had been answered by a person. Upon
         seizing the phone line, the auto-answer modem sends a signal
         to its  host computer to the effect that it has answered the
         phone. The computer then can interact with the caller.
             An auto-answer  modem  allows  you or other to call into
         your computer system without anyone being present  to answer
         the telephone and make the connection to your computer.

         7.2 Auto-Dial
         -------------
         An auto-dial  modem  is  capable of generating pulse-dial or
         DTMF (dual-tone modulated frequency or  touch-tone)  dialing
         signals independent of a telephone set.
             An auto-dial modem can dial the telephone under computer
         command, for  example,  after  hours  when you're asleep and
         phone rates are low.  Without auto-dial,  you would have  to
         dial the  phone  yourself,  listen  for  the  screech of the
         far-end modem's answer, plug in your modem, and finally hang
         up the phone.

         7.3 Automatic Speed Sensing
         ---------------------------
         Before a connection is made,  you may have no way of knowing
         at what speed a distant modem will  be  operating.  Most  of
         today's modems can automatically adjust for the speed of the
         distant modem — if it is within the speed range that  can be
         handled. High  speed  modems  usually  negotiate the highest
         possible shared  speed  to  operate  at  using   proprietary
         protocols.
             Many modems also attempt to adjust to the speed at which
         you send  them data — again if is within the range of speeds
         that the modem can handle.  The attention code of the  Hayes
         command set  conveys  enough data that a modem can lock into
         the data and appropriately match its operating speed to that
         of the information flow.

         7.4 CCITT Compatibility
         -----------------------
         A branch  of  the United Nations,  the CCITT,  which roughly
         translated from  the  original  French  means   «Cooperative
         Committee for  International  Telephony  and Telegraphy» has
         created a number of communications standards that  have  won
         great worldwide acceptance. Many of these standards apply to
         modems. For   example,   many   modems   boast   of   «CCITT
         compatibility» with   the  v.22  standard,  which  describes
         operation at a data rate of 2400 bits per second.  At higher
         speeds other  CCITT standards are  gaining  popularity, such
         as v.32 for 9600 bit per second modems.
             In theory,  the  adoption  of the CCITT standard is good
         news for people who want to communicate overseas  (where the
         Bell standards  may be illegal).  The principal value of the
         CCITT standards is,  however,  that many  manufacturers  are
         taking them  to  heart  and  designing  products  to  match,
         increasing compatibility   and    eliminating    marketplace
         confusion.

         7.5 Acoustic Couplers
         ---------------------
         Really vintage   modems  made  no  electrical  contact  with
         telephone lines at all.  That's because  years  ago  hooking
         your modem  directly to the phone line was neither practical
         or legal.  It wasn't practical before the now-common modular
         telephone plug-and-jack  arrangement  allowed anyone to plug
         in telephone equipment  without  fear  of  embarrassment  or
         electrocution. It  wasn't  legal  because  telephone company
         regulations dating long before the AT&T  telephone  monopoly
         was split  up did not permit individuals to directly connect
         modems to their telephone lines.
             Instead of  electrical connections,  vintage modems sent
         their signals to telephones as sound waves.  A device called
         an «acoustic  coupler»  was  used  to  convert the tone-like
         analog signals made by the modem into sounds which  are then
         picked  up  the   microphone  in  the  telephone handset and
         passed through the telephone  network  again  as  electrical
         signals. To  make the sound connection a two-way street, the
         acoustic coupler also incorporated a microphone to  pick  up
         the squawks  emanating  from the earpiece loudspeaker of the
         telephone handset, convert them into electrical signals, and
         supply them to the modem for demodulation.
             Acoustic couplers  can  take  many   forms.   In   early
         equipment, the  acoustic coupler was integral to the modem -
         a special cradle in which you  lay  the  telephone  handset.
         Today you're  more  likely  to  see  couplers  made from two
         rubber cups designed to engulf the mouthpiece  and  earpiece
         of a telephone handset. This latter form of acoustic coupler
         persists because it allows modems to  be  readily  connected
         and disconnected  from  non-modular  telephones — those that
         you cannot  unplug  to  directly  attach   a   modem.   This
         connectability is  especially important for roving computers
         that may be called upon to tie their  internal  modems  into
         nonmodular pay station and hotel room telephones.

         7.6 Direct-Connect Modems
         -------------------------
         Modems that  directly  plug into the electrical wires of the
         telephone system    are     called,     quite     logically,
         «direct-connect» modems. Almost in tribute to the acceptance
         of the modular telephone wiring system,  nearly every  modem
         that you can buy today is direct-connect.

         7.7 Asynchronous Modems
         -----------------------
         Almost any  modem  that  you buy for normal use with your PC
         will feature «asynchronous transmission».  This odd-sounding
         term describes  a  method  of exchanging information between
         two different  computer  systems  that  operate   completely
         independently and do not share any timing information.
             Normally, the time at which a pulse occurs  in  relation
         to the  ticking  of a computer's system clock determines the
         meaning of a bit in a digital signal, and the pulses must be
         synchronized to   the   clock   for   proper  operation.  In
         asynchronous transmissions,  however, the digital pulses are
         not locked  to the system clock of either computer. Instead,
         the meaning of each bit of a digital word is defined  by its
         position in  reference  to  the  clearly (and unambiguously)
         defined start bit.  Because the timing is  set  within  each
         word in isolation, each word of asynchronous signal is self-
         contained and essentially independent of any  time relations
         beyond its self-defined bounds.
             The signals of modems that use the telephone  system are
         generally asynchronous  because  it  is  more  expensive and
         difficult to  synchronize  signals  through  the   telephone
         system through  which  signals  may  be rerouted at any time
         without any warning.

         7.8 Synchronous Modems
         ----------------------
         Most dedicated-line  modems  use  a  special   communication
         technique often  used  among  mainframes called «synchronous
         transmission». In this method of  transmitting  data  across
         phone lines, the two ends of the channel share a common time
         base and the communicating modems  operate  continuously  at
         substantially the   same   frequency   and  are  continually
         maintained in the connection  and  adjust  for  the  circuit
         conditions. Higher  speed  modem  — 2400 bits per second and
         beyond — often use synchronous transmissions.
             In synchronous  transmissions  the  timing  of  each bit
         independently is vital,  but framing bits  (start  and  stop
         bits) are    unnecessary,   which   makes   this   form   of
         communication a bit — actually two or three bits  -  faster.
         One problem  in  using it  is that before information can be
         exchanged, not just the two ends of the connection  must  be
         synchronized, but  also  the  link  between  the  modem  and
         computer must be synchronized.  Autodialing features usually
         won't work  in synchronous mode because without a connection
         being made there's nothing  to  synchronize  to  -  and  the
         connection cannot be made without dialing.

         7.9 Autosynchronous Modems
         --------------------------
         Hayes solved    the    dialing   problem   for   synchronous
         communications by adding  an  «autosynchronous»  feature  to
         their newest  higher speed modems.  This special mode allows
         the connection   between   PC   and   modem    to    operate
         asynchronously. The  modem  translates  those  signals  into
         synchronous mode before  sending  them  down  the  telephone
         line. It also works the other way and translates synchronous
         signals from the far end of the line into  asynchronous  for
         sharing with the host computer. The autosynchronous features
         can help PCs talk to mainframe and other computers  that use
         synchronous communication as easily as they communicate with
         other PCs.

         7.10 Modem Packaging
         --------------------
         Perhaps the biggest choice you have in buying a new modem is
         whether in installs inside your PC as an «internal modem» or
         connects outside your PC through a  cable  as  an  «external
         modem». Internal  modems  are like any other expansion cards
         that plug into a vacant slot inside your PC. External modems
         are additional boxes to find a place for on your desk.
             In many cases When you have  to  choose  between  actual
         products, physical  appeal  may  be  the best guiding factor
         because exactly the same circuitry is often available in the
         different packages.
             There are a few practical  reasons  for  preferring  one
         style of modem packaging over another. External modems offer
         the advantage of portability.  You can  move  your  external
         modem between  different  computer  systems (even those that
         are not IBM compatible) simply by pulling the  plug.  Moving
         an internal  modem  requires popping the lid off your PC and
         the recipient and all the folderol that follows.
             Additionally, internal   modems   can  restrict  you  to
         certain computer systems.  Some internal modems are built as
         full-length expansion   cards,  which  means  you  can  only
         install them  in  full-size  PCs,  XTs,  ATs,  and  hardware
         compatibles. You'll   need   a   different   modem   for   a
         foreshortened computer (like the Tandy 1000).  When you make
         the move  from PC to PS/2 architecture,  you'll also have to
         shell out the cash for a new internal  modem.  Most  laptops
         and the   PCjr  can  use  only  internal  modems  that  were
         specifically designed for their proprietary expansion buses.
             If you  have  a PC with its original,  minimal 63.5-watt
         power supply,  adding  an  internal  modem   -   an   older,
         full-length modem  card in particular — may limit the number
         of other expansion  boards  that  you  can  plug  into  your
         system. Such  modems  are  notoriously  power-hungry and may
         leave few watts for other cards,  such as hard disks and EMS
         boards.
             On the other hand,  internal modems tend  to  be  a  few
         dollars cheaper  than  external  models because the internal
         units don't need extra packaging or power supplies (although
         they need some extra signal circuitry).  You can also forego
         the cost of a serial cable, which might cost you $30 or more
         from a local dealer.  With an internal modem, you don't have
         to deal with a tangle of cords,  plugs or transformers vying
         for the  few holes in your wall outlet,  extra boxes on your
         desk, or another thing to  switch  off  when  you  put  your
         system to sleep at night.

         7.11 Port Assignments
         ---------------------
         Other than  matters  of  power  supply,  the  impact on your
         system resources will be the same no  matter  the  style  of
         modem you  choose.  While  external  modems require a serial
         port and cable,  internal modems also require the use  of  a
         serial port  address — which means you still lose the use of
         that address by a serial port, COM1 or COM2 (or COM3 or COM4
         in PS/2s  and the latest DOS versions).  If you use versions
         of DOS before 3.3,  you'll only be able to  add  one  serial
         port in  addition  to  the port or address used by either an
         external or internal modem.  While some internal modems  can
         have their addresses to be set as COM3 or COM4,  you must be
         sure that the communications software that  you  choose  can
         control the ports beyond COM2.
             If there is any  general  rule,  it's  that  you  should
         choose an external modem for its flexibility and its ability
         to move to new and different  computer  systems;  choose  an
         internal modem for its neatness and lower overall cost.



                        8. OTHER MODEM CONSIDERATIONS
                        -----------------------------

         Selecting one modem from the hundreds of  products currently
         available is  no  small task.  However,  it can be made more
         tractable by  making  four  separate  judgments  about  each
         particular modem's performance, compatibility, features, and
         price.
             Over a   perfect   telephone  line,  nearly  all  modems
         function perfectly  -  without  errors.   However,   perfect
         telephone lines are impossible to find,  and even getting an
         acceptable one  nowadays  seems  to   require   bribing   an
         operator. The  performance differences between modems appear
         as line quality goes down.  Better modems are better able to
         cope with bad connections. They work with worse circuits and
         can pull data through with fewer errors.
             One of  the critical parameters of the telephone line is
         the amount of noise it contains in relation to  the strength
         of the  signal  it  carries.  This  relationship  is usually
         termed the signal-to-noise ratio of  the  line.  The  higher
         this ratio is, the better the connection.
             The signal-to-noise ratio is often expressed in decibels
         (one tenth  of  a measurement unit called the Bel,  which is
         named after Alexander Graham Bell,  by the way) which form a
         logarithmic measurement scheme — that is,  a signal-to-noise
         ratio that is twice as good will only appear  three decibels
         better.
             The compatibility of a modem refers to software  and not
         its hardware connection.  Controlling all the features of an
         advanced dial-up modem requires that commands be sent  to it
         from the  computer  that it is connected to.  These commands
         are usually sent invisibly  by  the  communications  program
         that is   actually   in   control.  More  compatible  modems
         recognize the commands of a wider range of software packages.
             Obviously, to  be  useful  communications  software must
         know the  commands   that   the   modem   recognizes.   Some
         communications programs  let  you  define the modem commands
         yourself (usually during the installation  process)  and  in
         theory will    work    with   any   modem.   However,   most
         communications programs are designed to accept a one or more
         standard sets of commands.
             In dial-up modems, no commands set has received official
         sanction as  the  one and only standard.  Most modems today,
         however, follow a de facto standard, the Hayes or AT command
         set. Originally  developed by Hayes Microsystems and used in
         the company's line of Smartmodems,  the  popularity  of  the
         hardware led  to  many  software companies incorporating the
         commands in their products.  Newer modems  were  adapted  to
         accept the existing software,  resulting in an acceptance of
         the standard.  A modem that recognizes the Hayes command set
         will therefore  work  with  the  widest variety of software.
         Hayes-compatible modems are thus more versatile.
             Note, however, that the Hayes standard is not immutable.
         As new modem features and capabilities  are  developed,  the
         commands set  becomes richer.  Moreover,  varying degrees of
         Hayes compatibility exist.  Some modems only  recognize  the
         most rudimentary  of  commands,  for example,  using ATDT to
         initial the dialing sequence.  Other more elaborately  mimic
         the operation  of  Hayes  products  and incorporate the same
         registers as used by Smartmodems, which permit, for example,
         setting the  number  of  rings  required  before  the  modem
         answers.
             Unless you  have masochistic tendencies or software that
         is specially designed for another  modem  command  set,  the
         safe bet  is selecting a modem that's as Hayes compatible as
         possible.
             The price  difference  between  different  manufacturers
         dwarfs the  difference  between  the  internal  an  external
         modems of  any  given  manufacturer.  Exactly  how  much you
         should spend depends on what you're  looking  for  and  what
         you're willing to settle for.  As with any other PC product,
         you should carefully consider every  aspect  of  your  modem
         purchase before  making  your decision — select the one that
         you're absolutely sure you want, then settle for the one you
         can afford.
             Unlike other common peripherals,  modems often  are  not
         plug-and-play devices, external modems particularly. Perhaps
         that's to be expected because they plug into  ornery  serial
         ports.

         8.1 Modem Cabling
         -----------------
         The easiest  part  of  installing  a modem is hooking up the
         cable. MOdems connected to the serial ports used by  PCs and
         PS/2s use  «straight-through»  cables.  Only AT-style serial
         ports with nine-pin connectors  require  adapters  to  match
         them to most modems.
             The problems begin with software.  In  the  interactions
         various communications  packages make with modems,  a number
         of serial port control lines are  brought  into  play.  Some
         communications programs,  such  as  PC-Talk III make minimal
         use of the indications modems provide.  Others monitor every
         connection. Consequently,  the  number of control lines that
         must be connected — and thus the number of wires  that  must
         be available  in  the  cable  that  links your modem to your
         computer — depends on the communications software  you  plan
         to use.  In some situations the minimal triad — pins 2,3 and
         7 — will suffice. Other programs require the full complement
         of ten connections. The moral to be drawn from this story is
         that should you not know the type of cable required  by your
         modem, use  a straight-through serial cable equipped with at
         least ten conductors.

         8.2 Modem Switch Settings
         -------------------------
         Modems themselves can be programmed to treat  their  various
         connections in   different   ways  to  match  the  needs  of
         software. For example,  some programs require that the modem
         keep them  abreast  of  the  connection  through the Carrier
         Detect signal.  Other  programs  couldn't  care  less  about
         carrier detect  but carefully scrutinize Data Set Ready.  To
         accommodate the range of communications  applications,  most
         modems have  setup  switches  that determine the handling of
         their control lines.  In one position,  a switch  may  force
         Carrier Detect  to  stay  on continually,  for example.  The
         other setting might cause the status of  Carrier  Detect  to
         follow the state of the modem's conversations.
             These switches   take   two   forms,   mechanical    and
         electrical. Mechanical  switches  are  generally  of the DIP
         variety. In  the  prototypical  modem,  the  original  Hayes
         Smartmodem 1200,  these switches are hidden behind the front
         panel of the modem.  (To get to them,  carefully pry up  the
         trailing ears  of  the  sides of the bezel,  first one side,
         then the other of the black front panel of the  modem.  Then
         pull it forward and off).
             Most commercial  modems  that  use  DIP   switches   are
         patterned after  the Hayes Smartmodem 1200.  (Its DIP switch
         settings are shown in Appendix C of this document).
             The other  kind of switch is electrical,  exemplified by
         the Smartmodem 2400. Made from EEPROM memory, these switches
         are set by sending commands to the modem from your computer.
         Because of their EEPROM nature,  they retain their  settings
         even when the modem is turned off or unplugged.
             Other modems may follow this pattern exactly or  may use
         another memory  technology — for example,  battery backed-up
         dynamic RAM. A few don't make any effort toward removing the
         volatility. Such  modems  require  you  to  reprogram  their
         settings every time you turn them on.  While  you  can't  do
         much to  make  modem memory non-volatile when it's not,  you
         can make life easier using disk memory. Simply add the modem
         settings you  wish to enforce to the setup strings that many
         communications software packages send to  the  modem  before
         they begin to make a connection.
             (The setup commands for the industry-standard Smartmodem
         2400 are shown in Appendix D of this document).


         P.S: Appendices A-D will be available later in our «Player's
         Dream» BBS station. Make your orders, gentlemen!



[email protected] Все права защищены / Copyright 2010 domoset.ru