Re: routing VoIP through a regular phone line
From: Floyd L. Davidson (floyd_at_barrow.com)
Date: Thu, 09 Sep 2004 15:44:01 -0800
firstname.lastname@example.org (P Gentry) wrote:
>"C3" <_> wrote in message news:<email@example.com>...
>> > Connect to Linux box via internet and have that Linux box provide a
>> > dial tone so that the internet conncected host can then place a
>> > "standard" PSTN voice call?
>> That's correct.
>OK. Two problems:
>a) how to "pipe" the sound card's output to the modem
>b) how will the modem "transmit" this output to a _phone_
The sound card's output does not need to go to the modem,
just to the phone line. That is fairly easy to accomplish,
either using the modem, or not.
>I'll only look at the second as it makes the first pretty much moot.
>Modems are DTE devices -- _data_ terminal equipment.
>Modems (_mo_dulator/_dem_odulator) use the analog _circuit_ facilities
>of the POTS (ie., PSTN) to make a connection to another DTE at the far
>end -- phones are not DTE's.
Well, yeah, but that isn't significant.
>Two modems connected to each other via POTS use a terminal emulator
>for human use. Thus all the terminal emulation options available in
>modem software packages.
>Modems can also be used to dial into a network (connecting to another
>DTE device) and have that far end "plug" you into the network. This
>is normally done with PPP -- point-to-point protocol. PPP is a
>"generalized" protocol that can carry (encapsulate/frame) many link
>layer protocols. That's why you see things like PPPoE and PPPoATM.
>We won't go into the details of wire signaling, etc.
You've got that backwards. The little 'o' indicates that the
PPP is being carried (encapsulated) by the 'E' or the 'ATM', not
the other way around.
>Point is that a modem is a (digital) _data_ device, not an analog
>transmitter despite the fact that it uses the analog (voice) circuit
>facilities of the "phone company".
A modem is in fact *both* a digital device and an analog
>The voice _facilities_ of the phone companies would be available with
>an interface card that speaks FSO/FXO.
That is totally bogus too. FXS/FXO merely means that instead of
connecting a wire line loop to an interface designed for a
wireline loop, there is a non-wireline carrier system in
between. The FXS provides an interface to the real wireline
loop (e.g., a telephone set) and the FXO provides what looks
like a wireline loop to the interface (e.g., a line card in a
telephone switch). The carrier facility in between can be
almost anything (fiber, microwave, digital carrier, analog
> Acquiring something as mundane
>as a dial tone is not s easy as you might think. No need to go into
Oh, come on. It requires all of providing a DC path on the
loop. You can get a dialtone by shorting the cable pairs! You
just can't hear it when you do that... ;-)
>boring details (and their many variations) -- just know that modems
>cannot utilize analog _voice_ facilities.
Bullshit. That is exactly what they are designed to do. And
it's been a *long* time since the dialer (ACU) and the
modulator/demodulator portions of a modem were separate units.
>Faxes are another hardware interface that uses the _circuit_
>facilities of POTS. Faxes _pre-date_ computers -- sometime in the
>1930's IIRC and using the grandfarther of today's hi-res drum
>scanners. That's why the term "fax/modem" -- a modem is not a fax
>interface device without extra hardware.
A fax includes a modem. A modem isn't a "FAX" until you add
>"Voice" modems have minimal ability to process a voice signal --
>namely record or playback a "voice stream". With software they can be
>used to build suprisingly sophisticated (and large) voice mail
>systems. In both cases this is a dial-in capability -- not dial-out.
>Well, not entirely true, as these modems could be used for messaging
>services, like _sending_ pager text.
I'm not really familiar with voice modems, as I've never owned
or used one. But what you just said is in effect that a voice
modem will do *exactly* what the OP wants!
>For all this, modems still cannot establish and carry on a full duplex
>_voice_ connection like a simple handset! But then they weren't
>designed to do so.
I don't see why not! As I said, I've never used a voice modem,
so I'm not sure they actually have the ability to do what you
described above. But if they do, then clearly they can carry on
a full duplex voice connection. (Incidentally, a "simple
handset" cannot! It requires a slightly more complex "telset",
which means there has to be a hybrid network in order to have
>If what you propose were possible, you would be able to sit at the
>Linux box, strap on a mic'ed headset, and dial your neighbor who picks
>up their phone handset, and chatter away. Ever picked up your handset
>when a modem is at the other end "calling"?
I see no reason at all that you cannot arrange to dial your
neighbor and chatter away. The modem doesn't necessarily *have*
to produce carrier tones! In fact, you might notice that it
dials up a connection and if you tell the modem to dial your
neighbor and pick up your an extension line, you can talk to the
neighbor just fine and the modem will not make a peep.
>I've taken liberties and shortcuts galore, but hopefully you have some
>sense of _why_ your setup will not work -- the idea is OK but the
>hardware is against you :-(
If the "voice modem" has the ability to send and receive audio
over the phone line (which means the modem must have both an
input and an output jack for audio), then it is definitely
possible to control the modem via the rs-232 connection and use
the computer's sound card(s) for the audio interface. Note too
that if the modem cannot do that, it is technically blindingly
simple to provide that functionality (it is basically the same
thing as a "phone patch" used by CB or Ham operators). The
modem can be used for dialing and for hook-switch control.
A sound card that will work full duplex is also necessary, and
of course there is the little matter of software to control all
Of course, when all of this is put together what it amounts to
is an expensive, hard to maintain, piece of junk. And it is
going to _sound_ exactly like what it is...
-- FloydL. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) firstname.lastname@example.org