
============================================================================= 
* Forwarded by Chad Simmons (1:142/550.4) 
* Area : REMEMBER 
* From : Craig Ford, 1:106/2001 (25-Sep-96  22:57:47) 
* To   : All 
* Subj : 56Kbps Modems 
============================================================================= 
* Copied from: COMM 
Hello All! 
 
This is an ASCI rendering of a white paper on the new 56Kbps modems announced 
by Rockwell found at http://www.nb.rockwell.com/nr/modemsys/hispeed.html. 
 
Hopefully it will answer many of the questions that have been raised about 
the technology and its application. I have no information on availability or 
pricing of the devices. 
 
==============================================================================
 
 
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. 
 
Figure 1:  The components of a modem connection in a digital network 
 
[-------]   [        ]                [       ]                 [-------] 
| MODEM |   | LINEAR | 2-WIRE TWISTED | m-LAW | 64K         64K | MODEM | 
|       |---|        |----------------|       |-----[delay]-----|       | 
|  DSP  |   | CODEC  |      PAIR      | CODEC |                 |  DSP  | 
[-------]   [        ]                [       ]                 [-------] 
 
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 duple x 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 
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 l ine, 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 an d 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 
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 
56,0 00 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 
 
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., it comprises the Multimedia 
Communications Division and the Wireless Communications Division. The 
Multimedia Communications Division is the world leader in facsimile and PC 
modem devices for personal communications electronics. The Wireless 
-!- timEd/2 1.10+ 
 - Origin: Home of the Fidonet COMM Echo * 713-458-0237 * (1:106/2001) 
============================================================================= 
~NAME All 
Hi All , hope you are having a nice day 
 
Chad Simmons
<chads@unix.collider.com>
Written on Sun 29 Sep 1996 at 00:36:21.
... BEWARE - Tagline Thief in this echo
--- Terminate 4.00/Pro
 * Origin: Chads Point via Terminate 4.0 (1:142/550.4)


FIDO MESSAGE AREA==> TOPIC: 105 HIGH SPEED MODEM Ref: DDW00098Date: 09/29/96
From: CHAD SIMMONS                                          Time: 12:36am
\/To: ALL                                                 (Read 4 times)
Subj: 2 56Kbps Modems

============================================================================= 
* Forwarded by Chad Simmons (1:142/550.4) 
* Area : REMEMBER 
* From : Craig Ford, 1:142/550 (25-Sep-96  22:57:47) 
* To   : All 
* Subj : 2 56Kbps Modems 
============================================================================= 
* Copied from: COMM 
Communications Division offers total system solutions for advanced cordless 
telephony and global positioning system (GPS) receiver engines and is 
developing products and technologies to address the Personal Communications 
Services (PCS) and wireless packet data markets. 
 
 
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 
============================================================================ 
 
Regards... 
 
Craig 
aka: cford@ix.netcom.com 
