Subj : Comm Primer 10
To   : All
From : Gord Hannah
Date : Fri Oct 01 2004 03:00 am


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).

   Standard transmit levels for domestic (US/Canada) modems are
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 * Origin: Marsh BBS (c) Dawson Creek, BC, Canada  (1:17/23.0@fidonet)

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