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Got this from Rockwell's site.
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                   56 Kbps Communications Across the PSTN

                    A new era in dial up communications

   * The Communications Path
   * Dealing with the communications path
   * Problems in the network
   * Shannon's limit
   * The upstream channel
   * Standardization
   * Connection limitations
   * Summary
   * Footnotes

This paper describes the basics of the 56 Kbps modem technology recently
announced by Rockwell Semiconductor Systems.

The basic concept behind this communications technology is that the public
switched telephone network (PSTN) is increasingly a digital network and not
an analog network. Existing analog modems, such as V.34, view the PSTN as
an analog system, even though the signals are digitized for communications
throughout most of the network.

                                  [Image]

    Figure 1: The components of a modem connection in a digital network

Additionally, more and more, central site modems [1] are connected to the
PSTN via digital connections (T1 in the Untied States and E1 in Europe [2]
) and do not utilize a codec [3] . The modem interprets this digital stream
as the representation of the modem's analog signal.

Rockwell's announced 56 Kbps technology looks at the PSTN as a digital
network which just happens to have an impaired section in the
communications path. That impaired section is, of course, the copper wire
connection between the telephone central office and the user's home,
usually referred to as the analog local loop.

The Communications Path

When a user at his/her home calls a central site T1 connected modem, the
network situation can be represented by Figure 1, below. The user is
connected to the network via a two wire twisted pair [4] copper line. At
the central office, this twisted pair line is terminated by a special type
of transformer, called a hybrid, which converts from two wire to four wire
[5] . This four wire connection is then connected to a codec. In the United
States, this codec is called a mu-law codec, named for the technique used
to space the sample points (which are also called quantization levels or
quantization points). In Europe, a different technique is used for spacing
these points, called A-law.

The mu-law codec is, in turn, connected to the digital network. The full
duplex digital data, to and from the codec, is switched through the network
to the central site modem DSP, allowing the central site modem DSP to
communicate digitally with the mu-law codec.

The mu-law codec has 255 non-uniformly spaced quantization levels which are
closer together for small signal values and spread farther apart for large
signal values. The modem DSP at the central site can generate any
quantization point voltage on the analog line simply by sending the
appropriate eight bit sample to the mu-law codec. Since the PCM codec
sampling rate is 8-KHz, these voltage levels will be generated 8,000 times
per second.

For the modem at the user's home, the major challenge is to be able to
determine which quantization point was generated by the eight bits sent by
the central site modem, and to do it 8,000 times per second.

To do this, the modem in the home must synchronize its sample clock to the
network codec's 8-KHz clock. Clock recovery is done in existing analog
modems and equivalent techniques are used to recover the network clock in
this new application.

Now let's look at how data is sent. Assume that the modem DSP at the

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