57600 modems

Michel Carleer (mcarleer@ulb.ac.be)
Tue, 26 Nov 1996 19:54:06 +0100

Feel like I might share my knowledge about the 57600 bps modems. Here it
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
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
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.


Michel R.C.E. Carleer
Chimie Physique Moleculaire CPi 160/09
Universite Libre de Bruxelles
Ave. F.D. Roosevelt, 50
1050 Bruxelles
Tel: +32-2-6502425
Fax: +32-2-6504232
e-mail: mcarleer@ulb.ac.be