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 close to 64 Kbps, depending upon the l.l.
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 
 
For this technology to operate, several things are required: 
 
 
The modems on both ends of the link must implement this new technology. 
During startup, the modems "identify" themselves and their capabilities to 
each other.  Only if both modems have this capability do they try to 
establish a 56 Kbps connection. 
 
The central site modem pool must have a digital connection to the network, 
such as with a T1 or E1 line (which may or may not be ISDN). 
 
There must be no conversions of the digital signal within the network.
Any conversions will prevent the modem DSP in the central site from
generating the proper voltage levels on the analog line which will prevent
this technique from operating.  Examples of digital conversions are: (1)
mu-law to A-law or vice versa (this means users will not be able to use
this technology for communications between the US and Europe),
(2) conversion to ADPCM, such as occurs in transatlantic submarine cables,
(3) conversion to analog and back to digital somewhere along the link, or
(4) other signal conversions such as ATT's TrueVoice [9] .
 
To check if a fully digital path exists, with no conversions, the modems 
send a "probing" signal between themselves.  If conversions are detected, 
the connection is established at V.34 rates. 
 
These limitations are not serious, especially for Internet access which 
generally involves a local telephone call. 
 
SUMMARY 
 
The 56 Kbps technique announced by Rockwell Semiconductor Systems is 
achieved by viewing the public switched telephone network as a digital 
network instead of an analog network.  Overcoming the limitations of the 
analog loop at the customer site is not easy but can be accomplished 
through the use of standard modem techniques. 
 
It is easier to overcome these limitations in the "downstream" direction, 
resulting in an asymmetrical modem technology with higher data rates from 
the service provider to the user than in the return direction. 
 
The theoretical Shannon's limit for this technique is close to 64 Kbps, 
depending upon the local loop. 
 
Rockwell will work with partners to submit this specification to the 
appropriate standards bodies so that an interoperability specification can 
be ratified and published. 
 
This new communication technology announced by Rockwell Semiconductor 
Systems promises to provide a new era in dial up communications 
capabilities, especially for bit starved Internet power users, and another 
life extension for the "analog" modem. 
 
 
Rockwell Semiconductor Systems is the fastest growing business segment of 
Rockwell. Based in Newport Beach, Calif.
FOOTNOTES 
 
1. Central site modems are those installed at a service provider, such as 
an Internet service provider, or at a corporation to allow many 
simultaneous connections for Remote LAN access.  They are generally 
manufactured as cards containing many modems which plug into the device 
which provides the appropriate access. 
 
2. A T1 line is a digital service provided by the telephone company to 
provision the equivalent of 24 individual voice lines.  It operates at 
1.544 Mbps.  An E1 line is the European equivalent and provisions the 
equivalent of 30 individual voice lines.  It operates at 2.048 Mbps. 
 
3. Coder/decoder.  The device which sits between the digital portion of
the network and the analog local loop and converts between analog and
digital.
 
4. The physical connection between the central office and the home is two 
individual copper wires of 24 or 26 gauge twisted about each other to 
minimize crosstalk.  It's length depends upon the distance from the home
to the central office but is normally less than 18,000 feet.
 
5. The network carries the two sides of a voice call in two separate 
channels. However, the connection to the home is only two wires.  The 
hybrid converts between this separate channel system, referred to as four 
wire, and the two wires serving the home. 
 
6. A "symbol" is an information carrying token.  In this 56 Kbps 
technology, a symbol is a voltage level.  In ordinary modem technology, a 
symbol is a combination of amplitude and phase.  The term "symbol" was 
adopted after the original term, "baud", became corrupted in common usage. 
 
7. The symbols in QAM space are created by simultaneously modulating a 
carrier in amplitude and phase 
 
8. A trellis code is a technique to improve the modem receiver's ability
to discriminate between two adjacent symbols.
 
9. However, TrueVoice can be disabled using the same techniques as are
used to disable echo suppressors.
 
Copyright (R) 1996 Rockwell International, all rights reserved 
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