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Internet service provider can send only two different sample values to the
mu-law codec, say the values representing the two outermost points. The two
voltage levels on the analog line which result from sending these sample
values can be used to represent two binary values (0 and 1). These sample
values will be sent 8,000 times per second, the network clock rate. Further
assume that the modem in the home can discriminate between the two
voltages, 8,000 times per second. In this case, the central site modem can
send data to the user at 8,000 bits per second (bps).

Now let's assume that the modem DSP at the Internet service provider can
send four different sample values, representing four different voltage
levels. Since there will now be four different voltage levels on the analog
line, we can assign two bits to each voltage level (00, 01, 10, and 11).
Again, sample values will be sent 8,000 times per second. If the modem in
the home can discriminate between these four different voltage levels,
8,000 times per second, then 16,000 bps can be transmitted.

Table 1, following, shows how the data rate increases as more voltage
levels can be transmitted and discriminated.

                      Number of     Bits per  Line Rate
                   voltage levels    level      (bps)

                   2                1         8,000

                   4                2         16,000

                   8                3         24,000

                   16               4         32,000

                   32               5         40,000

                   64               6         48,000

                   128              7         56,000

                   256              8         64,000

   Table 1: The relationship between the number of voltage levels on the
   analog line, the number of bits communicated per voltage level and the
                            resulting line rate.

Dealing with the communications path

To make this technology work over the analog loop, the modem must
"equalize" the line. But this is easier said than done.

Some of the problems encountered in equalizing the loop are caused by the
central office codecs, which are designed for voice and not data. Also, the
transformer hybrids connecting the transmit and receive paths to the loop
introduce spectral nulls at DC. Some of the solutions developed by RSS
engineers for these problems are being submitted as patent applications.

Once these issues are dealt with, the quantization levels on the analog
line are simply treated as symbols [6] in modem symbol space, in exactly
the same way as combinations of amplitude and phase are treated as symbols
in an analog modem QAM space [7] . And once you're in symbol space, you can
use many of the techniques already developed for traditional analog modems
to improve the modem receiver's ability to discriminate between
quantization levels, thereby improving communications accuracy and speed.

For example, new trellis8 codes, which recognize the non-uniform spacing of
the symbols, can be created and applied to allow better discrimination
between the quantization levels, especially those near the origin. While
not all of the existing modem coding techniques can be applied to this new
communications technology, a great many can.

Problems in the network

If everything could be done perfectly, this technique would allow
communications at 64 Kbps (8 bits per sample times 8,000 samples per
second). However, there are a number of problems which prevent operation at
this speed.

First of all, in the United States, the link between the network and the

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