Newsgroups: comp.os.linux.networking
Subject: Re: Modem to Modem again
References:   <[email protected]>    <[email protected]>
Floyd Davidson  writes:

> Bob Hauck  wrote:
> >On 30 Jul 2003 16:42:34 -0700, John Culleton  wrote:
> >
> >> Now I have the two machines connected physically modem to modem via
> >> the usual telephone modular cord.
> >
> >Most inexpensive consumer modems won't supply talk battery or ring and
> 
> Niether are needed.

Neither of those are needed technically for the data communications. 
Because normal telephone line supplies the line current, many 
modern modems rely on the existence of those to operate correctly. 

> >so won't work in this configuration.  Hopefully you have the manuals for
> 
> Why wouldn't it work?

Practically all modern modems use line interfacing circuity that 
has active electronics on the both side of the line isolation transformer. 
The electronics on the line side of the isolation transformer is 
generally powered through the telephone line (it uses the normal 
telephone loop current as it's power). This kind of modem does 
not generally work without the telephone line loop current being present.

Some modems I have examined seem to have circuits in them to sense 
the presense of telephone loop current, os they can notify the 
user if the line is not connected to modem and user tried to 
go on-line.

A circuit like this can be used to supplu the needed line power 
to modems that need it.

Get two phone jacks and a 680 ohm, 1/2 W resistor. 
Connect the components to make the circuit below: 

reen              680 1/2W    +   12V    -          Red
--------------------/\/\/\------|'|'|'|'-----------------
To Phone                                          To Phone
_________________________________________________________
Red                                                  Green


Please note that with so called 56k modems you can't get 
the full line speed with this kind of circuit, because 
two consumer 56k modems just can't communicate with each other 
at 56k speed in any conditions. 56k modem can only communicate 
at the promise speed in ideal conditions with the digital modem 
(modem DSP card connected to digital telephone ciecuit) is 
dial-in server your ISP has. In practical life the speed is 
less than 56 kilobits per second.

I think you can expects  speed in order of 30 kilobits or so 
with this kind of modem-to-modem arrangement.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/