In <001501bf86d5$c9b4f960$d785e7d8@microsol>, on 03/05/00
   at 02:04 PM, Jim Meagher <[log in to unmask]> said:

>Matthew,

>After a deeper investigation, I see that we are both partly
>correct in our statements about 56K technology, but we were both
>more wrong than right. <g>

>There is a white paper available at the 3com web site which
>details the 56K "process", but it is written in "engineer-speak"
>.  Following is excerpt from an article I found at the WHATIS,COM
>site which (I think) explains the process in a much simpler and
>clearer manner.
>(Much better than I did anyway <grin>).

>> 56 Kbps technologies exploit the fact that most telephone company
>> offices are interconnected with digital lines. Assuming your Internet
>> connection provider has a digital connection to its telephone company
>> office, the downstream traffic from your local Internet access provider
>> can use a new transmission technique on your regular twisted-pair phone
>> line that bypasses the usual digital-to-analog conversion.
>>A V.90-equipped modem doesn't need to demodulate the downstream
>> data. Instead, it decodes a stream of multi-bit voltage pulses generated
>> as though the line was equipped for digital information.

>I still disagree on point 2.  I beleive the USR test line is of
>dubious value.

>Lets assume a very simplistic topology for the telephone network
>that has: POINT A is the dialing modem.
>POINT B is the TelCo switch center.
>POINT C is USR's test facility (or whatever you want to call it)
>POINT D is the ISP.

>As stated above (and in the white paper), the 56K data stream
>bypasses the A/D converters and feeds a "digital-like" signal
>directly to the modem. Ergo, the wiring from POINT B to POINT A
>is just as critical as the wiring from POINT C to POINT B.  Now
>consider that (in the "real" circuit) it is POINT D that is
>sending the data back to POINT A, and - obviously - no test of
>the wiring from C to B to A will ever account for the wiring from
>POINT D to POINT B.   And that is why I said the line test is not
>a complete test.

>As you pointed out, each time you make a connection from POINT A
>to POINT C you are at the whim/mercy of the TelCo network and may
>or may not use the exact same circuit.  Which supports the other
>part of my statment that the line test is inconclusive.

>Further, IMHO, unless you live in a small town, there are
>probably two or more switch centers involved in routing the
>"call" from POINT A to POINT D. In other words, for many (some?)
>of us, the full path is POINT A to POINT B1 to POINT B2 to POINT
>Bx to POINT D.

>So I still maintain that the USR test line can only confirm that
>you have a good connection - but only for this one connection -
>from POINT A to POINT C.

>Jim Meagher
>Staff Editor

Hi again:

The "WHATIS.COM" explanation is perhaps simpler and clearer but
it's also not exactly correct.

On an analog phone line, the transmission from the terminal
(phone) to the CO is analog:) Both ways. All internal Telco
signals are digital, whether on wire or fibre-optic. On your end
the signal is generated by an analog device (microphone) or a
Digital to Analog Converter (DAC). Modems (and some phones) use
DAC's to generate an analog (variable voltage level) signal. The
modem (and some phones) use an Analog to Digital Converter (ADC)
to change the incomming signals to digital signals.

In the bad old days, before 56K, a modem to modem connection thru
the telco had to follow this path:

(sending) modem DAC-analog-ADC
(Telco1)--digital---(Telco2)-DAC--analog-ADC modem (recieving)

Telco 1 and 2 might be the same, and the signal may cross the
country or remain in the same building.

The brilliance of 56K was eliminating half the conversions by
hooking the ISP up to the digital signal directly.

V.90 tech: Assume ISP sending:

(sending)ISPmodem-digital--(Telco1)--digital---(Telco2)-DAC--analog-ADC
modem (recieving)

Each conversion introduces noise, so this cuts the conversion
noise in half. The also made the modem a little smarter, so that
added a few bits/second too.

The critical point is that while the signal is in digital (0,1)
format, it does not get noisier (sort of). The critical point is
the analog section from CO to house. If you live in a new
neighborhood, with fibre-to-neighborhood you're in fat city.

The 3Com/USR test places a series of tones from approximately 1500
to 3500 Hertz on the line, and watches for distortion,
attenuation, noise and delay shifts. From these statistics, it's
possible to calculate the transmission capability of the line. The
1.5 mile limit on the analog portion is based on statistics of the
national wiring. Sometimes you can be lucky, as with new wire, few
users. Sometimes you get old wire, and lots of modems on adjacent
wires, which adds noise.

Now, if i can just get the telco to upgrade my wire a bit:)

jan lambert

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