routing VoIP through a regular phone line

One must first have the "IP" established before one can accomplish the "VO".

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Hello. I would like to set up a linux box connected to a phone line so that I can connect to it over the internet and receive a dial-tone as though i had picked up the phone where the linux machine is.

In theory, this seems very simple to do. Once authenticated, the linux box issues a couple of commands to the modem, starts recording from the modem and sending that data, and also passing the incoming voice data to the phone line.

I don't want to have to run some huge VoIP software package. Ideally, this sort of thing can be achieved in a very lightweight fashion.

Comments please.

Reply to

By "some huge VoIP software package" do you mean Asterisk? I agree that it's not tiny but it's so easy to configure to do such things that it's easy for me to justify using it on any but the smallest systems.


Reply to
Kyler Laird

It still doesn't make sense. Why not explain what you want to accomplish?

Your diagram indicates a digital connection from end to end, so the VOIP would appear to be something required not in the diagram, but at the two distant ends.

Reply to
Floyd L. Davidson

Even with the picture it's not real clear to me just what you want.

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?

Via VoIP across the internet to Linux box, which then would have to decode (in real time) the digital signal, construct an analog signal, and "relay" the voice signal -- after placing the call and establishing the connection of course. And it must do the same in reverse to the signal at the far end to transmit the VoIP packets back to the internet connected host that "originated" the call.

All this at modem speeds that top out at 33kbps upload? That requires a (voice)modem at the far end to connect to?

If you think that's easy, better do some serious googling for a week or two ;-)

If you stopped to think (boy, this should be easy) then you figure everyone would be doing it and that every Linux distro would provide a "ready-made" setup/config tool.

So, I'm thinking, I don't understand what you have in mind ...

If this is close to what you have in mind, then something like Asterisk and an interface card capable of interacting properly with the PSTN voice system will be the easiest way to go.

Or for something "easier" and some $:

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If you have something else in mind ...

regards, prg email above disabled

Reply to
P Gentry

Here is a diagram:

Remote host Linux box modem phone line

Surely this isn't that hard to set up.

Reply to

That depends on what kind of connection the "internet" in your diagram is. If it's the same phone line as is connected to the modem then I'd say forget it.

A single 56K phone modem will not have the bandwidth to achieve this reliably (quite apart from a phone modem not being able to offer this reliability in the first place).

Reply to
Jeroen Geilman

Well, that's obvious. It's my fault for not specifying all the details in the first place. The internet connection is a cable connection with a fast download speed, and a 128kbps upload speed.

How do I set it up?

Reply to

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.

Well, yeah, but that isn't significant.



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.

A modem is in fact *both* a digital device and an analog "transmitter".

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 carrier, whatever).

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... ;-)

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.

A fax includes a modem. A modem isn't a "FAX" until you add a printer...

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!

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 full duplex.)

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.

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 this.

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...

Reply to
Floyd L. Davidson


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_

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.

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.

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".

The voice _facilities_ of the phone companies would be available with an interface card that speaks FSO/FXO. Acquiring something as mundane as a dial tone is not s easy as you might think. No need to go into boring details (and their many variations) -- just know that modems cannot utilize analog _voice_ facilities.

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.

"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.

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.

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'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 :-(

hth, prg email above disabled

Reply to
P Gentry

That's correct.

I have a voice modem. There really isn't any decoding going on. The Linux box is receiving about 8kB/s from a remote host and then putting that data through the sound card's DAC, and then passing that onto the voice modem.


Ignore the data capabilities of the modem. The modem's only purpose is to dial a PSTN line and receive (and send) data from (and to) the sound card.


Reply to

The only news to me is that voice modems are not capable of performing the very limited task needed of them.

I always believed that voice modems are simply modems that can receive a signal through a 3.5" minijack and pass it onto the phone line, and also to send out the sound from the phoneline onto another 3.5" minijack plug. These two can be connected to the input and output of a soundcard.

The only thing left is to dial the number, that's just an ATDT away, isn't it?

Aren't voice modems capable of this?


Reply to

Arghhhh -- Floyd D. saw me blathering in public. Was afraid of that ;-)

I'll give you my phone # if you think it's that easy :-)

Just what, pray tell, will the modem "talk" to at the other end then? A modem modualtes/demodulates the "audio" signal into a byte stream. That's why they require settings like 8NP or 7NP or 7+P (# of bits/parity)

Surely you're kidding. Dial-in BBSes, VT100 and ANSI terminal menus, command sets, and keyboard layouts -- mostly of historical interest perhaps? Modems are a _byte_ stream _digital_ device -- and asynchronous at that. In this context, think telnet.


Yeah, noticed that 5 mins _afterwards_ -- got interupted and the the old brain cells were slow to kick in with, "Duh!". Figured someone might catch it.

Yes, that's why they are called mo-dems, but they have no ability to transmit an _un_modulated voice signal. The mo and the dem is how they "encode" and "decode" the digital bits.

Was only included to indicate the "reality" that a FXS/FXO capable card is the only reasonable ($) hope of carrying on a "relayed" connection. Plus these are the kind of cards OP will most likely see referenced. Have not kept up over the past 5 years, so if you know of something better I would be interested to hear about it. A link?

"dial tone" in the sense that we think of a handset dial tone -- but you're _right_, I should not have injected this as it doesn't further anything.

Splitting tech (short)hairs -- and who said _anything_ about any kind of separate units. (Though I did toy with the idea of "remembering" the first modems that were cradles _for_ a handset!)

If you can come up with a better phrase to indicate what the average joe thinks of when when speaking about "voice" and "phones" I'm more than willing to accept -- never cared for this (my) phrase used here, but wasn't going to spend more than ten mins thinking of a better expression. Just call me slow witted ;-) BTW, don't try your method with a trunked PBX -- they acquire dial tones in a differnet manner.

Well, here I'll say quite definitively that you are simply too "wired into" current technology packages and that about faxes and fax machines (old and current) you have this "backwards" -- have installed

100's of fax machines _and_ cards that were modem-less. They are different technologies both using the "phone lines". Neither one _requires_ the other, despite their "similar" use of the phone lines. They _can_ share certain abilities/requirements, like acquiring a dial tone ;-) For establishing a session and transmitting data they are completely different.

One would think so. The dial-in capability (voice mail) requires _both_ hardware _and_ software additions to a basic modem -- and if "extended" you might think this would allow a modem to act as a handset. (Not sure why this was never carried out -- technical, $, regulations, opposition, public acceptance?) The dial-out capability was a means of "plugging into" services provided by the phone company. Without the CO facilities it would not work -- nothing to work with.

You misunderstand "handset" -- I'm speaking of the RJ11 connected phone most "hard wired" phone users pick up to dial out or answer a call.

You _have_ to pick up your "handset" to chatter away _because_ a modem can't provide that capability. In fact, early modems wouldn't even allow you to "interrupt" a modem session (they wouldn't "release") this way. Last time I actually used this "method" of dialing out, you also had to pick up the handset within a limited amount of time. And it won't work with most (any?) modems without a pass-through jack.

Nope, it "looks" like just a regular plug-in or serial modem -- no separate "audio" cords -- though some provide external jacks and some require/use your sound card. When someone calls in to _leave_ a voice mail, the handset they are using is effectivey the "mic" -- in fact, many "voice modems" require you to use the pass-through handset as a mic to record "intro" or "direction" messages for the voice mail system. (All the ones that I used on-site were this way.)

This was (is) done sometimes to a limited extent but due to the proprietary nature of most (all?) sound cards at the time (circa '93-95) it was "less that reliable".

And as above, I'm not sure why it was never followed up.

Unfortunately, that's about all a modem can be used for at this level. Beyond this and the byte stream encoding/decoding (mo - dem) starts up.

Which is why I suggested that OP look at providers that offer internet VoIP services or internet phone services -- whichever he is most interested in.

BTW, I hope the folks around here that need some RS232 advice (among other things) know what a resource you provide. I did _one_ measly and "easy" project collecting data from a bar code scanner that downloaded library spine codes. On top of that, you regularly watch the posts and provide accurate help.

regards, prg email above disabled

Reply to
P Gentry

Really, it is. Every sound card has a "line in" and a "line out" jack. You want that to go to a telephone line? Put any standard telephone hybrid network between the sound card and the telephone line! Every phone patch, telephone set, modem, etc. etc. has such a network in it. (The network from a telephone set won't work though, only because it has a "sidetone" path that is great for a telset, but bad for this application.)

The modem need not talk to anything. It will quietly sit there doing nothing if you don't enable the transmitter. The usual configuration is that one modem is in "originate mode", which does *not* send tones until triggered by reception of tones. The other modem is in "answer mode", which immediately does send tones. As long as the modem being used is in originate mode there will be no tones unless you dial up a modem that is in answer mode.

The first sentence of the original statement is wrong. It flatly says that *all* modems connected via POTs for human use necessarily use a terminal emulator. That is simply not true. I've written dozens of small serial port programs that connect two modems via POTS and talk to some remote device (everything from telco Channel Banks to remote temperature sensing devices). Most of those uses do *not* involve a terminal emulator.

Your followup isn't too great either! Modems are not "byte stream" devices, nor are they asynchronous. The interface to the computer is both of those; but the communications between modems is almost always (as in, I don't know of a single modem that is not) a bit oriented data stream. With v.32 and v.90 it is synchronous too. (Actually, even Bell 103 modems are synchronous at the modem to modem level, but they are not packetized so that distinction is meaningless.)

You should say "often", not "normally". Perhaps the use you are familiar with is normally just that. Others may normally see something very different...


A modem today is not solely a modulator and a demodulator. Most modems include a pretty serious bit of computational capability, not to mention signal processing. Some of them even seem to have the ability to transmit "an _un_modulated voice signal"!

The simple "modulator/demodulator" was actually called a "terminal unit", and the telephone industry used them to multiplex Teletype channels onto a carrier voice channel. (A

43A1 TU used about 5 tubes if I remember right, would run at speeds up to 75 baud, and could be stacked with as many as 16 to a single voice channel because it used 180 Hz FSK!)

(Speaking of making modems do things the manufacture never meant them to... I used to use 43A1 Terminal Units as a tube tester! They used WECO 408 tubes, as did many other pieces of equipment, X type SF signaling units being one example. But the SF Unit didn't need a particularly high gain 408 tube, and the 43A1 did. So I set up a row of about 48 sockets with the only filaments wired up, and would plug 4 dozen tubes in and let them burn in for a few days to stablize them. Then I'd take them one at a time an plug them into a spare 43A1, with an AC VTVM probe on the output pin jack. Tube went back into boxes, sorted by which would be used for SF Units, other equipment, or 43A1's. Low gain, medium gain, and high gain... Eventually that was no longer necessary because WECO started doing essentially the same thing, and sold a 408A/B tube specifically for the 43A1 units.)

Whatever, when "modem" concept first moved to the customer location the speed and bandwidth were slightly increased (135 baud!), and an external dialing unit was required. But shortly the two were combined, and a "modem" as we know them was born... and has *never* since then been restricted to just a modulator and a demodulator.

A modern modem can probably handle voice. It can also do a significant bit of digital signal processing (of the analog signal) because every v.32, v.34, v.90 or v.92 modem contains a digital echo cancellor. A pretty fancy little device, actually.

That just is not true. An FXS/FXO setup is merely one particular type of "relayed" connection. It doesn't specifically apply to the OP's scenario because the OP has no need to convert a wireline loop interface into something that will emulate one for the distant end.

There is some similarity though, and if you remove the "only reasonable" bit, it does make a good example of what is needed:

2-wire 4-wire 2-wire +-----+ wireline +-----+ carrier +-----+ wireline +--------+ | Tel | loop | |--->>>---| | loop | telco | | Set |-----//---| FXS | | FXO |----//----| switch | | | | |---> >boring details (and their many variations) -- just know that modems

See the above discussion.

Perhaps, but that is not the point. The point is that the modem has both mod/demod capability *and* supervision capability. Using only the supervision capability isn't exactly unheard of.

Oh, whoop dee do. Some (but not all) PBX's use "ground start" rather than "loop start" on wireline trunks. Big deal. We aren't talking about ground start lines (and there *are* both telephones and modems that can use ground start too). Moreover, in addition to those two interfaces which are commonly used for lines, there is the myriad list of trunk interfaces too, which many PBX's also use. Who cares, since that was not what the OP specified, nor is it necessary for the OP to even consider anything other than interfacing to a standard POTS line (unless of course he arbitrarily wants to).

You have *never* installed a fax machine that did not have a modem in it!

That is absolutely wrong. *Every* FAX by definition has a modem as an essential part, by definition. That modem may not use v.32, v.34, v.90, or v.92 modem protocols, but then again it might not use v.26, v.27 v.29 or v.33 modem protocols either (and neither would the modem *you* use). Regardless, those are all "modem protocols", and FAX modems, for example, use v.29 modem protocols for 4800, 7200, and 9600 bps transmissions. FAX modems might also use v.17, v.27 or v.34 modem protocols.

Bullshit. FAX machines use a modem to 1) acquire a dial tone,

2) establish a session, and 3) transmit data. That might even be done with *exactly* the same hardware and modem protocol (v.34) that is used for computer data. In fact, any high speed FAX machine that works at a speed greater than 14.4Kbps is going to use v.34 protocols. The slower protocols are still more common, but they are different only in detail, not in functionality or overall purpose from the various modem protocols used for computer data.

Well, gee... a POTS line that won't work without a CO is not exactly a revelation! ;-) Your regular telephone set requires that too!

I'm not sure what the significance is that it requires more software and hardware than just a modem. So what? Computer data transfer requires more software too. It also requires significant hardware, and the only difference is that the hardware required is so universally needed that it is provided by default. But a serial port is *not* necessary for a computer to function otherwise...

A handset plugs into a telset. The telset (not a handset) plugs into the telephone line.

Typical residential users rarely see either a handset or a telset without seeing the other, and since the introduction of the "Princess" telephone way back when it is not uncommon (e.g., cell phones) to see the two devices combined as a single unit.

However, they are in fact two very separate items. Commonly the handset on many telsets has a cord that can be replaced (with a longer one, for example). But in some places you'll find the two are *very* distinct. Operator switchboards, for example. Each operator has a *personal* handset, that nobody else ever uses. They sit down and plug it into a telset, which is perhaps one of several, but all of them would commonly be used by whoever happens to be physically where that telset is located.

Your misuse of the terminology is not a reflection on my understanding of it... ;-)

That is simply not true. Voice modems provide exactly that functionality.

But *clearly* you are being limited by your ability to configure the software. The hardware is quite capable of doing exactly as described, and there is a myriad of software available to use that functionality in a variety of ways.

You are contradicting yourself. A non-voice modem does not connect to the sound card. A "voice modem" does, and uses the "some provide external jacks and some require/use your sound card" functionality. If someone calls to leave a message, the audio is passed from the telephone line to the sound card input, and the sound card converts it to PCM or whatever type of digital encoding and stores it on disk. The outgoing message to the calling party was previously stored on disk, and the sound card converts it from PCM, or whatever, and outputs that on the "line out" jack that connects to the "line in" on the modem and that is sent to the telephone line.

Precisely what the OP wants to do, except he wants real time digital i/o rather than using store and forward audio from a disk file.

Eh? You've been saying it can't be done, and now point out that it has been being done for a decade. And I won't agree at all with the idea that is was seriously limited by the '93-95 time frame, though I'd say that was indeed true a decade before that. Of course, we are talking about a technology that has been commercially available for 20 years now! It may not be fully mature, but this isn't exactly bleeding edge stuff any more!

Lots of call centers use exactly that functionality, so it really isn't correct to say it was never followed up on. It just doesn't have much use in the consumer modem market, that's all.

No it doesn't. There is no requirement that the modulator be enabled at all.

And supervision is not the only potential use either. Virtually every modem contains a very significant capability for digital signal processing, and every one of them implements an echo cancellor. That has benefits for voice calls too. Likewise it would take very little additional programming to allow the modem to do just about anything you'd like to the telephone line (from test signals to voice recognition). All that's needed is a

*reason* for someone to add extra functionality to the modem. In many cases it would require no extra hardware, and could be added by merely flashing the modem's memory.

I don't see much difference... VoIP hasn't quite gotten to the point where it is much more than "an expensive, hard to maintain, piece of junk" itself! However, it is right on the verge... and when it really does arrive it *will* cause an entire paradigm shift that will forever change the telecom voice message business.

You sure make it hard to pick on you. If you'd just be a "normal" Usenet guy, and snarl a little, we could call each other names and all kinds of things, eh?

(And I really do appreciate that you are NOT a "normal Usenet guy"! And while you may be saying a few things that are quite technically correct, that is of little significance compared to the interesting information that the resulting discussion produces. I may know a bunch of technical details, but you are the one with enough imagination to make this all interesting!)

Reply to
Floyd L. Davidson


I was wondering with this discussion how a "Winmodem"/ AMR Card might work as I have heard them disparagingly called a limited soundcard.

Iff you have access to an ADC/DAC pair in a soundcard type configuration there should not be a problem with voice.

(If not don't the whole point is moot as well as if the ADC/DAC doesn't have enough bits..)

The advantage of one of these cards should offer all the needs to connect to a line. (Voltage isolation, line pickup etc. and although a permitted attachment they probably don't have permission as a phone..)

The only disadvantage is that there is no software written for using this type of card as a telephone interface.

Reply to
David M

Here is some hardware to facilitate the switch between the voip router and the plain old telephone service!

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or wireless wi fi links

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Reply to
Marc H.Popek

Depends on what you mean by "limited". The "voice" in voice modems refers to the dsp and codec firmware that _digitizes_ the analog voice stream (and vice versa). It's the software that actually provides what we, as users, think of as the functionality. Ie., voice mail, answering machine, video phone (haven't seen one of those in a while), etc. Voice mail and fax are probobaly the two most used "extras" beyond simple data linking for dial-up networking.

"Everyone" (me too) has tried this at one time or other -- these lines (sound card to modem) won't "match" for one thing and the other problem is that the input/mic jack on the modem is just to _record_ a message. Obviates the need for a separate sound card for input _or_ a "speakerphone" (mic and speackers). A person-in-the-flesh doesn't really need a modem to "speak" on the wire -- that's what POTS and handsets are for.

All modems _can_ dial out. The problem is that without human intervention (lifting up the handset, eg.) or some software app the modem will try to contact another _data_ modem. If you lift the handset before the modem/modem handshake begins, the passthrough circuit for the handset "shorts out" the modem (ie., modulator and other data circuits) and you can carry on a normal voice conversation. In this case you're just using the modem as a phone dialer.

Googled about the past two days to catch up on modem developments and not much has changed regarding "garden variety" modems for home/office use. The "voice" capabilities (beyond the raw signal processing) are mostly found in software and all I've looked at are still file based

-- ie., record/playback a compressed audio file. Check here for some software examples:

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FWIW, you _can_ sometimes hear some innocent person speaking at the other end of a data dial-out -- usually cursing about the strange noise ;-)

I think you might be able to build an app that would accept a voice input stream from another app and send it out a "voice connection" on the modem. That is I _think_ the "on chip" hook for voice apps is generic enough to allow this, but for the life of me I wouldn't want to wrestle with the timing, dialing, codec, and data transfer issues involved -- I would just pick up a phone ;-)

Doing this with an app which handles incoming VoIP (which is h.323 based -- like netmeeting) and translating(?)/transfering that voice stream to a modem app that decodes/relays the analog voice content onto a POTS line (which is already dialed out and connected) seems like a lot of work. Maybe that's why there are commercial companies that offer this sort of VoIP-POTS interconnection. High quality digital PBX networks can too. Eventually, the phone lines will likely be 100% digital, but we're not there yet -- maybe not for many years as no one sees enough $ in such an expensive upgrade.

hth, prg email above disabled

Reply to
P Gentry

That doesn't seem to be the case. There are variations, and some modems are indeed limited as you say (which hardly seems to be a limit!), but others are not. (I hadn't realized the extent of what these modems are capable of doing. But a quick look at the web site you provided below, plus a little searching with google provided all kinds of interesting information.)

Typical consumer modems might be classified as:

1) Data 2) Data and FAX 3) Data, FAX and voice A) full duplex able B) half duplex only 4) Data, FAX, voice and speakerphone

I was assuming the speakerphone capability was what a "voice modem" had, but it seems that is not the case. However, the capability of a voice modem without the speakerphone part is

*much* more useful than one with just the speakerphone capability and not the DSP functionality available with the less able "voice modem".


But if you use the right modem (a voice modem with speakerphone capability) there *is* that capability. And if you don't use a speakerphone modem, the you merely need to use a sound card to interface to a handset and pass the digitized audio to and from the modem via the serial port. And obviously that is much nicer with a modem capable of full duplex operation than it would be with one only capable of half duplex, so the right modem makes a big difference in how useful this is.

(As to the idea that the modem lines don't match on the sound card, that isn't true. Most sound cards have two input lines and one output. The two inputs are a high impedance and low impedance. The modem might well have the same thing. Or either of them may be limited to a low impedance output and a single high impedance input. Matching to a low impedance input if you have only a high impedance output device is difficult, but matching a high impedance input when you have a low impedance output is just a matter of one or two resistors. That's why if they go cheap, the only input will be high impedance.)

Not true. The modem might well be configured to *respond* if another data modem answers, but in fact it doesn't necessarily care if that happens or not. Of course the most common use is to set a timer telling the modem to hang up if no data connection is made, but that is merely a matter of modem configuration. It does not require either human or application software intervention with each call.

There is no "passthrough circuit" that shorts out the modem on most modems. Some do have manual switches on the front panel to connect a telset to the line instead of the modem, but that isn't what is being used.

Most modems have a "passthrough circuit" which may or may not (it depends on the modem) disconnect the telset line as soon as the modem goes off hook. That is why there are two RJ-11 jacks on the back of the modem. Some modems don't cut the telset line when the modem is active though. With those modems it is very easy to use that jack for a telset and have the computer use the modem as a dialer.

A useful URL. It does *not* support what you are saying though! It

*clearly* indicates that the modem's hardware facilities are a major part of the voice capabilities, and that when coupled with a computer can do *exactly* what the OP wants to do. Moreover, it can be done *much* easier than I was thinking! I didn't realize the modem would do the digital/analog conversion, and I thought the sound card was necessary for that. Obviously that is not the case, which makes it all *much* nicer!

Instead of requiring software that utilizes the sound card's DSP, all the OP needs is software that converts incoming IP data to whatever format the modem will accept, and feed it directly to the modem over the RS-232 link. No external audio connections are required at all.

Given the descriptions provided on the URL you provide above, that appears to work exceedingly well (given the right modem at least).

BULLSHIT. Why do you keep saying things like that? Have you

*ever* answered a ringing phone and had a calling data modem give you a blast of data tones? No! And the reason is because the *originate* modem is absolutely silent until the *answering* modem sends tones.

In FAX mode a modem is slightly different, and sends a very short bleep of start tone at regular intervals to activate the silent answering modem.

In *neither* case is there any "strange noise" that somebody is left cussing at. (Unless someone is *purposely* dialing out and forcing the modem into answer mode just to annoy people. That has no useful purpose, as no data modem will respond to it.)

Yes indeed they are!

Look at the web page you cited! That is *exactly* the kind of applications they are discussing!

No more work than any of the answering machine, voice mail, or whatever applications. Geeze, I don't know about you but I'm all too often answering the phone to be presented some short computer generated message asking me to hold the line until some real asshole answers. They've got about 1.37 seconds to perk my interest before I hang up on 'em...

This is just *not* something that is difficult or magic in any way. The programming necessary to do it has been being done in telephone switching systems for 30 years now! The fact that consumer products have now become available at prices making the same programs useful in products a customer might like to use is the only thing that is "new", and frankly that isn't new either given the 10 years that it's been available. The only thing perhaps "new" is an almost totally ubiquitous Internet that we have today, whereas ten years ago few people had even heard of it.

That is indeed a pretty good summary!

For those of us who've been taughting digital services for decades now (ISDN: It Still Does Nothing), the advent of VoIP is something of a long sought for magic key that will open the pandora's box.

Consider a little history with a different perspective:

In the mid-1980's the Internet existed, and almost nobody in the telephone industry new what it was. Those who did saw a need for digital data services, but could not convince anyone to offer them. Hence the R&D people, who understood computer networks, developed ISDN, and the operations folks, who didn't understand computers or networks, said "What for?" The R&D folks said "It's a needed service.", and the Ops folks said "Who will pay for the investment required?". So ISDN was not installed, because the Ops folks did not see a market!

Instead, modem companies spent millions on R&D to develop first v.34 and then v.90 modems, and made *billions* of bucks. That is what the telephone industry missed out on because of the Pointy Haired Bosses that inspired Scott Adams to create the Dilbert cartoons. Adams, of course, worked in the telecom industry as an ISDN applications development engineer!

By 1995 things started to change, and common people began to hear about The Internet. But even then telecom industry management just simply could not grasp what it meant. That led to massive upheavals! (You might remember that AT&T went through about 3 or 4 CEO's in a two year period trying to find one that could force the in-place upper and middle management of AT&T out of the old concepts of what a telephone company is.)

It has been an uphill battle getting telecom management to make changes in the traditional concepts of what the industry is. And VoIP is the death warrant for traditional analog, circuit switched, message traffic!

Just as 10 years ago virtually every company that did not figure out how to adjust their business methods to make use of the Internet found themselves out of business or bought out by new management, the same thing is about to happen with digital voice services.

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Floyd L. Davidson

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