#Hi David,
I thought that You and the Gang here might be interested in this. I find
it ironic that I got this off a local BBS, who recieved it from another
User via the Internet, whom had found it on the FidoNet ! This is the text
of a white paper on the new 56Kbps dial-up modem technology by Rockwell.
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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 modem~bs, such as V.34, view the PSTN
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
P}P֤r 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
comm}^.f-}|unications 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 aSCnalog 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 i┊s then connected to a codec. In
the
United States, this codec is called a mu-law codeSTN via digital
onnections
(T1 in the Untied States and E1 in Europe [2] )+c, 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 bitB8) sample to the mu-law codec. Since the PCM codec
sampling rate is 8-KHz, thesw#cNje voltage levels will be generated
,000
times
per second.
For the modem at the user's home, the major challenge is to be able to
determine which quantization point wasn 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 ErC߸{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.
mB
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 representk~+ping 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
{|S (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, 8~1,000 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).
A7Bgain, 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 voRO{U~ltage levels
an
be
transmitted and discriminated.
$H 2 1 8,000
4 2 16,000
8 3l (bps)
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* Origin: Ye Olde Coffee House, Athens, GA, USA (1:370/50)
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