Re: 57600 modems

Jesus Maria Arango - Ing. Comunicaciones FSM LTDA. (
Wed, 27 Nov 1996 10:27:46 +0500 (GMT)

This sound very interesting but I have a little theorical objection to
this point of view.

Telephone lines have a bandwith of 3000 Hz. while human voice has a
bandwith of 18000 Hz. Our voice does not sound the same over the phone
because an 18000Hz signal has to be fit into 3000Hz.

Now, when speaking of digital signals (Square signals) there is a theory
that says that any square signal can be generated by a sum of several sine
and cosine signals of diferent frequency. The more bandwidth you have the
more sine and cosine signals you can add and thus the square signal will
be better. Also, the more speed you need, you will also need more sine and
cosine waves, so you will need more bandwidth. This is why people
mistakenly asociate bandwith with speed. This theory comes from Furier's
series, which can be use to find that if we have a phone line with 3000 Hz
then the maximum speed of a well defined square signal will be 9600 bps.

How do we get 28800? Easy, computer signal are not transmited in square
form, they are converted (modulated) into an analog representation which
supports greater speed.

Now, knowing that it is not efficient to transmit data over the phone in
square form. I don't understand how can someone fit 56700 into a phone
line in square form.

On Tue, 26 Nov 1996, Michel Carleer wrote:

> Feel like I might share my knowledge about the 57600 bps modems. Here it
> is.
> Let's first describe the path followed by and the conversions endured by
> signals on a normal phone line, using normal modems:
> The PC (or the service provider) needs to transmit digital signals over an
> analogue phone line. It does this by performing an digital to analog
> conversion (DAC) in the circuits on the modem card. The modem then sends
> the analog signal through the phone line to the telephone company. The
> telphone company transmits the signal to the service provider SP (or the
> PC). It is received by another modem which performs an anlog to digital
> conversion (ADC) and presents the digital data to the CPU.
> Now, the catch is that in fact most telephone companies use digital lines
> to transmit the signal from one point to another. So, when the telephone
> company receives an analog signal coming from a modem (or from any phone
> set), the first thing it does is convert it to a digital signal, transmit
> it in digital form, and then at the other end of the connexion, reconvert
> it to an analog signal to send it to the receiving modem.
> The complete route of a signal from a PC to an SP is:
> PC-----DAC(modem)-------ADC(phone company entry
> point)------------------------------DAC(phone company exit
> point)-------ADC(modem)-----SP
> Of course the route from the SP to your PC is just the reverse from above.
> In other words, the only slow analog portions of a telephone connexion are
> the two short connexions between the telco and the end users. By removing
> these slow analog portions and the associated ADC/DAC conversions (which
> degrade the signal) at both ends, the transmit rate could be improved.
> But this is not practical on the user's side, because the user might well
> still want to transmit voice on the line, and there is no ADC in his phone
> set. On the other hand, service providers use dedicated lines and can
> without trouble ask to the telephone company for a digital line. One pair
> of ADC/DAC conversions is then eliminated. If the service provider asks for
> such lines, the transmit rate will be better, but only for the signals
> EMITTED by the SP.
> The route then becomes:
> PC-----DAC(modem)-------ADC(phone company entry
> point)------------------------------SP
> Using such a setup, it is possible to increase the baud rate to 57600, but
> only from SP to end user.
> This is why the so called 57600 bps modems will allow you to download at
> 57600, but only upload at 28800 (or 33600). And it will only be possible if
> the SP asks for special digital lines to the phone company.
> That's the best of my knowledge on the subject at the moment.
> Bye
> Michel R.C.E. Carleer
> Chimie Physique Moleculaire CPi 160/09
> Universite Libre de Bruxelles
> Ave. F.D. Roosevelt, 50
> 1050 Bruxelles
> Belgique
> Tel: +32-2-6502425
> Fax: +32-2-6504232
> e-mail:

* Jesus Arango *
* Ingeniero de Comunicaciones *
* Supernet - FSM LTDA. - Cablesistema *
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