Re: 57600 modems

Michel Carleer (mcarleer@ulb.ac.be)
Wed, 27 Nov 1996 18:33:52 +0100


Jesus,
I think you make a confusion between the possible bandwidth of a pair of
cables and the fact that telephone companies limit the bandwidth of the
transmitted voice signals to the region of 300-3000 Hz (or 4000 Hz
nowadays). And of course the main cables inside the phone company have a
much higher bandwidth and transmit many conversations simultaneously as a
multiplexed signal. Think for example at the phone satellite links (I mean,
inside or between phone co.). Also, special cables from the company to the
end user have a higher bandwith. One good example: ISDN.

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: mcarleer@ulb.ac.be

----------
> From: Jesus Maria Arango - Ing. Comunicaciones FSM LTDA.
<jarango@supernet.com.co>
> To: CU-SEEME-L@cornell.edu
> Subject: Re: 57600 modems
> Date: Wednesday, November 27, 1996 06:27
>
> 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: mcarleer@ulb.ac.be
> >
>
> *****************************************
> * Jesus Arango *
> * Ingeniero de Comunicaciones *
> * Supernet - FSM LTDA. - Cablesistema *
> * E-Mail: jarango@fsm.net *
> * Internic POC Handle: JA406 *
> * Tel: (574) 268-9000 *
> * Fax: (574) 268-1281 *
> ****************************************