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From: Gord Hannah
To: All
Date: 2010-11-15 01:00:04
Subject: [5 of 12] Comm Primer

LOCAL LOOP - the copper wires that run between a telephone subsciber's
location and the telephone company's switching station.

MODEM - The word modem is an abbreviation for MOdulator/DEModulator. Modems
are used to modulate, or mathematically speaking to multiply, the data
signal onto a carrier signal for transport over a medium where the data
signal does not happily go.

In our case here, we are talking specifically about modulating RS232 serial
data from the serial port of our computer onto an audio carrier for
transmission on a telephone line.

Historically, modems started out by "multiplying" a slow, simple data
stream onto a set of tones.  One tone represented a logical binary ON, the
other a logical binary OFF.  By way of example, we can use the A below
middle C at  440Hz, as one tone, and the next higher A, at 880 Hz, as the
other tone.  If the serial bit stream from our computer looks like:

   1   0   0   0   1   1   1   0   1   0   1   1   0   1   0   1
   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
for example, this modem would send a sequence of:  |   |   |   |
   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
  440 880 880 880 440 440 440 880 440 880 440 440 880 440 880 440

using a simple method called frequency shift keying.  This sounds like just
what it is, a pair of tones flipping back and forth.  They are flipping
back and forth at the rate of the data being sent.

The other end, the demodulator for this simple arrangement, needs only two
tone detectors, which are looking for 440 and 880 Hz.  The signal is
reconstructed from this, and then sent on to the computer attached to it.

This basic example is at the heart of modem theory.  There are other ways
of doing modulation than this simple multiplying.  This is also called
amplitude modulation, like AM radio, which carrier an audio signal mixed
onto a radio carrier frequency.  And like radio, there are good reasons why
old AM is not necessarily the most effective way to carry a signal.

This example above shows that we are encoding our data signal straight on,
with no bit encoding.  Bit encoding is used for all higher data rates,
because if we used simple frequency shift keying, we could only use data
rates up to about 1200 bits per second (bps) before Telecom would start
complaining that we are using more frequency than we are allowed on a
telephone channel.  There are important technical reasons for keeping our
signal within the width of one telephone channel, ie. between 300 and 3700
Hz.

To use a really simple example of bit encoding, we could use 4 tones to
encode two bits.  We take the following coding, which is only illustrative,
to indicate the mechanism:

  bit1  bit 2    freq
   0     0        440
   1     0        880
   0     1       1320
   1     1       1760

and now we re-encode our bit stream:

 1   0   0   0   1   1   1   0   1   0   1   1   0   1   0   1

 \  /    \   /   \   /   \   /   \   /   \   /   \   /    \  /
  880     440     1760    880     880     1760    1320    1320


Last time we had to send 16 pieces of information to encode our 16 bits,
this time, we only needed to send 8.  This information piece is called a
symbol element. We are sending, in this example, two bits per symbol
element, whereas in the first example, we were sending one bit per symbol
element.

Extrapolating, with 8 different tones, we could send the same 16 bits of
serial information in only 4 symbols. Further, we could send it with only
two symbols if we used a 16-level encoding scheme.

This sounds wonderful, but there are tradeoffs.  For this simple example,
using only frequency shift keying, we cannot send data any faster than a
few hundred bits per second, otherwise the tone multiplication overlaps the
next tone so much we could not decode it at the other end.

In order to get away from that, these discrete levels are sent in real
modems separated in both frequency and a second dimension called phase.
This is getting analogous to FM radio.  In radio you can send a signal in
both AM and FM on the same carrier, or you can even send two separate radio
signals on the same carrier, one modulated AM and the other FM.  So too, we
can send signals modulated in two fashions, and this lets us get more
levels onto the carrier, ie. more bits per symbol. The combination of a
different amplitude and phase for each symbol gives us a modulation scheme
called Quadrature Amplitude Modulation.

    DESIGN CONSIDERATIONS

    On a _very_ basic level, a modem consists of three basic parts; the
    interface to the telephone line (DAA), a signal processing section (data
    pump), and a command processing section (controller). The parts can be
    and indeed are put together in a variety of configurations that can
    dramatically affect the capability and performance of a given device.
    This discussion shall be limited to variations in data pump and
    controller implementations, the DAA design can certainly play a
    significant role in performance characteristics.

    The data pump section of a modem is comprised of a digital signal
    processor and the programming code in which the low-level details of the
    modulation protocols supported by the device are implemented. DSP code
    is typically stored in some manner of ROM, but may be hard-coded into an
    mask programmed integrated DSP chipset [modems which utilize Rockwell
    chipsets fall into this category (eg. Boca, Cardinal, Zoom, Microcom)],
    stored in a conventional style ROM memory (eg. USR Sportster [excluding
    the DSVD model], Motorola Modemsurfr), or stored in field re-

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