routing VoIP through a regular phone line

Discussion in 'VOIP' started by C3, Sep 7, 2004.

  1. C3

    C3 Guest

    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

    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.
    C3, Sep 7, 2004
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  2. C3

    ynotssor Guest

    One must first have the "IP" established before one can accomplish the "VO".
    ynotssor, Sep 7, 2004
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  3. C3

    Kyler Laird Guest

    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.

    Kyler Laird, Sep 7, 2004
  4. C3

    C3 Guest

    Here is a diagram:

    Remote host <---- internet --------> Linux box <---------> modem <------->
    phone line

    Surely this isn't that hard to set up.
    C3, Sep 8, 2004
  5. 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).
    Jeroen Geilman, Sep 8, 2004
  6. C3

    C3 Guest

    If it's the same phone line as is connected to the modem then I'd say
    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?
    C3, Sep 8, 2004
  7. C3

    P Gentry Guest

    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 $: phone call&ie=UTF-8&oe=UTF-8

    If you have something else in mind ...

    email above disabled
    P Gentry, Sep 8, 2004
  8. 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.
    Floyd L. Davidson, Sep 8, 2004
  9. C3

    Grahame Guest

    The easiest solution is to get an VoIP phone that simply has then to be
    connected to your ADSL modem or HUB - They used DHCP to get the necessary
    IP, routing and gateway details, but after that your computer doesn't have
    to be on. I got a GrandStream BudgeTone-100 Open Standards based VoIP
    complete phone (look like normal phone) that you just plug into your hub.
    Uses UDP ports 5004 & 5060, which I have set up in my firewall. Everything
    after that is just a phone call as usual. For more info see:

    Cheers. Grahame.
    Grahame, Sep 9, 2004
  10. C3

    C3 Guest

    Connect to Linux box via internet and have that Linux box provide a
    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.

    C3, Sep 9, 2004
  11. C3

    P Gentry Guest


    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 :-(

    email above disabled
    P Gentry, Sep 9, 2004
  12. C3

    C3 Guest

    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

    Aren't voice modems capable of this?

    C3, Sep 10, 2004
  13. 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
    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...
    Floyd L. Davidson, Sep 10, 2004
  14. C3

    P Gentry Guest

    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
    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.
    RADIUS? AOL? MSN? Huh?
    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
    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
    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
    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.

    email above disabled
    P Gentry, Sep 10, 2004
  15. C3

    David M Guest


    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.
    David M, Sep 12, 2004
  16. C3

    Marc H.Popek Guest

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

    or wireless wi fi links

    Marc H.Popek, Sep 12, 2004
  17. 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
    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

    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 |
    | | | |---<<<---| | | |
    +-----+ +-----+ +-----+ +--------+

    The reason for this is because the "4-wire carrier" cannot
    transmit DC pulses or loop current. It might be a microwave or
    a fiber optic system, for example, and the telset may be located
    hundreds of miles from the telco switch. So the DC pulses and
    loop current are converted to some other form of signaling.
    With analog systems it was almost always SF (2600 Hz "Single
    Frequency"). With digital systems it almost always uses Robbed
    Bit Signaling (the AB bits in a the SuperFrame of a DS1 rate
    digital line).

    The similarity with the OP's scenario is that if we chop off the
    entire left hand side, and provide something like this,

    4-wire 2-wire
    +------------+ +-----+ wireline +--------+
    | computer |---->>>----| | loop | telco |
    | voice card | | FXO |----//----| switch |
    | |----<<<----| | | |
    +------------+ +-----+ +--------+

    We have the same thing, except the computer has to synthesize
    whatever it is that the FXO expects to see on it's 4-wire voice
    channel side for signaling and supervision. That isn't
    technically difficult, and there no doubt are off the self units
    where the FXO card plugs right into a PCI slot in the computer.

    However, another way to to it is to use out of band signaling,
    and a voice modem,

    4-wire 2-wire
    +------------+ +-----+ wireline +--------+
    | computer |---->>>----| | loop | telco |
    | voice card | |modem|----//----| switch |
    | |----<<<----| | | |
    +------------+ +-----+ +--------+
    | |
    | RS-232 supervision |

    That might seem odd to some, but consider this very common
    telco equipment configuration for a FXS/FXO,

    2-wire 4-wire 4-wire
    +-----+ wireline +-----+ carrier +-----+ trunk +--------+
    | Tel | loop | |--->>>---| |----->>>----| telco |
    | Set |-----//---| FXS | | FXO | | switch |
    | | | |---<<<---| |-----<<<----| |
    +-----+ +-----+ +-----+ +--------+
    | |
    | E&M DC Supervision |

    The difference is that the telco interfaces to a 4-wire _trunk_
    card rather than to a 2-wire _line_ card.

    Clearly the OP can make a choice between in-band and out-of-band
    signaling, and that amounts to choosing which hardware to buy.
    I'm sure a search on google would turn up a list of interesting
    links. It is also possible that posting a query to
    comp.dcom.modems and crossposting it to alt.dcom.telecom and might be productive.

    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
    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
    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!)
    Floyd L. Davidson, Sep 12, 2004
  18. C3

    P Gentry Guest

    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:

    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.

    email above disabled
    P Gentry, Sep 13, 2004
  19. 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
    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
    Floyd L. Davidson, Sep 13, 2004
  20. C3

    P Gentry Guest

    Thought you might be interested in this:

    email above disabled
    P Gentry, Sep 16, 2004
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