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central site modems can be a T1 line utilizing "robbed bit signaling" for
call progress indication. Robbed bit signaling "steals" the low order
sample bit in two of the samples per frame to indicate the status of an
incoming (or outgoing) call. The use of this bit by the network means that
the central site modem cannot always access 8 bits per sample and this
reduces the achievable data rate.

Additionally, the codecs in the network are not perfect. Many have a DC
offset problem which limits the ability to utilize the quantization points
near the origin. There may also be a significant amount of nonlinear
distortion in the circuit. This further limits the achievable data rate.

Finally, there is the problem of accurately determining the quantization
point which was "sent" by the central site modem DSP. Since the
quantization points are closer together near the origin, it is more
difficult to discriminate between these points. Depending upon the channel,
more or less of these points may have to be given up.

Taken together, these limitations reduce the achievable data rate to about
56,000 bps.

Shannon's limit

Shannon's limit is determined by a number of parameters but for ordinary
telephone channels it is, to a large degree, determined by the channel's
signal to noise ratio.

Conventional modems treat the telephone network as a pure analog channel,
so the analog signals generated by these modems see a PCM codec
quantization distortion of about 36 dB. This distortion represents a
significant impairment as data rates are increased and limits the channel
to about 35 Kbps.

The effects of PCM quantization distortion are avoided by using a form of
amplitude modulation in which the amplitude levels are chosen to be the
quantization levels of the PCM codec in the central office. The user's data
is encoded into this quantization-level symbol alphabet and transmitted
across the local loop in digital form.

The problem then is to equalize the local loop such that the signal samples
seen by the user's modem are equivalent to the quantization levels at the
central office codec. This equalization problem is significantly reduced by
limiting the data transmission to a single local loop. With this approach
of "hooking" into the middle of the channel and avoiding one of the
encoding or decoding PCM steps, the PCM quantization distortion can be
treated as a deterministic impairment, and not as a random noise source,
which is the case for the conventional analog modem. This raises the
theoretical Shannon's limit very close to 64 Kbps, depending upon the local
loop.

The upstream channel

It is more difficult to equalize the upstream channel, and therefore more
difficult to achieve the same high data rates as are achieved in the
downstream channel. However, for Internet access, the data rate in the
upstream direction is less important than downstream, since the upstream
channel transmits mostly "key strokes and mouse clicks". At present, a data
rate of around 30 Kbps can be attained in the upstream direction, but
research continues toward increasing the rate.

Standardization

Like any dial modem technology, this new technology will have the greatest
value to users if it is standardized, so that products from different
vendors can interoperate. RSS will be working with partners to submit the
specifications for this technology to appropriate standards groups in an
effort to gain international acceptance. The standards bodies are attended
by some of the best minds in the modem industry so improvements should be
expected by the time the technology achieves standardization.

This technology provides so much value to bit starved Internet users that
we expect it to be rapidly addressed by the standards bodies, especially
ITU Study Group 14 which achieved the V.34 standard, and/or ANSI TR30.

Connection limitations


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