control information into the data stream will reduce actual throughput ).   
      
   Data compression techniques can yield additional data throughput advantages   
   over non-error-correcting links, by compressing data before the modem   
   transmits it (some transfer protocols feature this ability as well).   
   Error-correction coupled with data compression can theoretically yield data   
   throughputs which are many multiples of the DCE rate. It should be noted   
   that this is accomplished by reducing the amount of data that the modem has   
   to transmit, via compression, not by increasing the DCE rate.   
      
   The most important question associated with a communication channel is the   
   maximum rate at which it can transfer information. Information can only be   
   transferred by a signal if the signal is permitted to change. Analogue   
   signals passing through physical channels may not change arbitrarily fast.   
   The rate at which a signal may change is determined by the bandwidth. In   
   fact it is governed by the same Nyquist-Shannon law as governs sampling; a   
   signal of bandwidth B may change at a maximum rate of 2B. If each change is   
   used to signify a bit, the maximum information rate is 2B.   
      
   The Nyquist-Shannon theorem makes no observation concerning the magnitude   
   of the change. If changes of differing magnitude are each associated with a   
   separate bit, the information rate may be increased. Thus, if each time the   
   signal changes it can take one of N levels, the information rate is   
   increased. As N tends to infinity, so does the information rate.   
      
   Is there a limit on the number of levels? The limit is set by the presence   
   of noise. If we continue to subdivide the magnitude of the changes into   
   ever decreasing intervals, we reach a point where we cannot distinguish the   
   individual levels because of the presence of noise. Noise therefore places   
   a limit on the maximum rate at which we can transfer information.   
   Obviously, what really matters is the  signal-to-noise ratio (SNR). This is   
   defined by the ratio of signal power to noise power and is often expressed   
   in decibels.   
      
   There is a theoretical maximum to the rate at which information passes   
   error free over the channel. This maximum is called the channel capacity C.   
   The famous  Hartley-Shannon Law states that the channel capacity C is given   
   by:   C  =  bandwidth  x  LOGbase2 ( 1 + SNR)   
      
   The theorem makes no statement as to  how the channel capacity is achieved.   
   In fact, channels only approach this limit. The task of providing high   
   channel efficiency is the goal of coding techniques. The failure to meet   
   perfect performance is measured by  the bit-error-rate.   
      
   THE CONNECTION PROCESS:   
      
       Communications between computers using modems is a negotiated process.   
   Three data transfer links are established, the DTE at the host, the DCE   
   between the modems, and the DTE at the remote system. DTE parameters are   
   locally established under the control of communications terminal software   
   as limited by the capabilities of the modems. DCE parameter negotiation is   
   somewhat more complex.   
      
       To effect a link, several precepts must be mutually agreed to by the   
   modems. Information regarding modulation and  error-control protocol   
   support is exchanged between the modems, and a connection established ONLY   
   if there is a mutually supported modulation protocol. If the modems do not   
   incorporate a common error control protocol, the link will be established   
   without the benefit of error control. The connect speed will be the highest   
   mutually supported by the modems under the common modulation protocol with   
   the line conditions as they exist at the time of the link negotiation   
   process.   
      
   ANSWERS TO FREQUENTLY ASKED QUESTIONS:   
      
   Question: I just replaced my trusty Generic Xpress V.32bis modem with a   
   V.34 model, but it doesn't ever connect at 33.6Kbps. What's wrong?   
      
      Answer: It is not only perfectly normal, but even typical in a V.34   
      connection to see a less than 33.6kbps connection.  V.34 is not a   
      fixed-speed standard, and makes/changes its connections based on phone   
      line quality.   
      
      Very few people can get consistent 33.6kbps connections.  Speeds of   
      33.6kbps require pristine phone line quality along the entire length of   
      the connection.  V.34 modems are capable of pushing the limits of analog   
      phone lines, commonly offering connection speeds of 21.6k, 24k, 26.4K,   
      28.8K, and even 31.2kbps.   
      
      The bandwidth (or "bandpass") of a voice-grade phone line is about 300Hz   
      to 3,800Hz .  Because the mathematics of encoding 33.6kbps pushes the   
      phone line to near its theoretical limits, V.34 was designed to   
      accommodate a variety of phone line conditions. V.34 is smart enough to   
      do what is called a "channel probe", which is a frequency response and   
      signal-to-noise ratio test of frequencies at various points across the   
      bandpass.  During the modem handshake, the modems send a series of tones   
      to each other, at known signal levels and specific frequencies.  The   
      modem calculates the level of the received signal at each frequency, and   
      therefore can determine the maximum bandwidth available for use.   
      
      So, just how good does a line have to be?!   
      
      In reality, it takes line clarity at about -44dB or better (about   
      the sound level of a clearly whispered conversation across a   
      medium size room) at the top of the phone line's "bandpass" to   
      obtain and maintain a 28.8kbps connection.  At about -46dB and   
      below, modem receivers tend to "go deaf".  The typical long   
      distance connection can be much worse than this at that frequency;   
      it is not unusual to see -55dB to -70dB (closer to the background   
      hiss level of a factory-fresh medium-grade audio tape).   
      
      
   --- MPost/2 v2.0a   
    * Origin: Marsh BBS (c) Dawson Creek BC Canada (1:17/23)