All the technology in my house is modern, but I want to build a dialup network that I can use to produce sounds from Bell 101/103 (if I can find the proper softmodem) to V.92bis. I honestly don't know where to start here, but I do have multiple computers. Would I just purchase two USB modems and put them together? How could I record the sound from them, and command them to use specific protocols? Could I shift between using them for internet passthrough and using them as very literal modems for terminals?

EDIT: Also, is there any software I can use to generate dial-up tones from V.8(bis) to V.92 without need for a USB modem? It seems to me it should be obviously available somewhere, but the best I've found is minimodem, which - although it's great - is nowhere near enough.

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    Does this answer your question? Using “vintage proxy” with dial up simulator – snips-n-snails Sep 2 at 4:32
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    I read the last bit at first as 'purely for the kids' and thought, wow, the poor kids trying to do Zoom classes over dial-up... – Jon Custer Sep 2 at 20:13
  • This is what I came here for – dashnick Sep 2 at 22:57
  • @snips-n-snails, that question and answer assume that the relevant hardware is already set up, while this one is about setting up the hardware in the first place. – Mark Sep 2 at 23:42
  • A hilarious question! My own experience with dial-up modems (supposedly) at 56K, with speed dropping to a couple dozen bits/bytes per second... to download the latest 20M update to my partner's "Microsoft whatever"... with projected time-to-completion of 100+ days... was traumatic. – paul garrett Sep 3 at 23:05

A/the correct solution would be to set up some sort of small local PBX. In the spirit of retro you can probably score an analogue PBX for not very much money and the mechanical ones are arguably even more fun to play with than retrocomputers because you can watch them working, but if you would like that bit to be somewhat smaller and more reliable than a clattering Strowger exchange and aren't intimidated by Linux, a couple of VoIP ATAs ("Analog(ue) Telephone Adaptor") and a machine running something like bare Asterisk or a full FreePBX system would be how I'd do it. Or rather, how I did do it back in ~2005 for a substantially similar project to yours.

For testing the PBX you may want to start with a pair of phones until you're satisfied that calls work. Then it's a case of plugging in the modems and calling from one to the other using a pair of terminals or terminal emulators. This may need some further tweaking of VoIP settings since high-speed modems are more picky than humans about sound quality, but for a local-only configuration it should work so long as you're using the G.711 codec.

With VoIP, you should be able to easily configure a software "tap" and get a call recording in perfect digital quality. A more advanced analogue PBX should offer a tap feature, otherwise you're going to just have to dig out the croc clips and look for somewhere plausible to attach a tape recorder.

If you're OK with something a bit jankier than going the full PBX route, connecting two phone sockets back-to-back and using a PP3 battery to provide (barely) line voltage is apparently sufficient to convince modems that they're connected to a phone line. You'll need to tell the caller to ignore the lack of dial tone and force the receiver to send "ATA" (in this context, that's a Hayes modem command) because there will be no ringing voltage to tell it there's an incoming call, but this may be enough. Again, some ingenuity may be required to get a recording.

From here, if you want to use it rather than just record the tones, it should be simple enough to install a BBS application on one machine and dial into it from the other. This is perhaps as far as a non-expert may wish to go.

For the full late-90s dialup Internet experience, you get the fun(?) job of setting up a mini-ISP, probably on a Linux machine—doing this on the same machine that's running Asterisk should be fine—with at least some sort of (faked?) RADIUS login and PPP. Apart from the details regarding physical serial ports, this now falls outwith the scope of retrocomputing since modern broadband still involves PPP, RADIUS, etc, and in some devices you even get the pleasure(?) of using obscure, possibly even undocumented, Hayes modem commands to configure the connection.

Note that you'll only be able to get up to V.34(bis) with a pair of analogue modems. V.90 and other "56K" technologies requires a digital connection at the ISP end. VoIP is that digital connection, but when I last looked into this a couple of years back, I could not find a suitable softmodem which could answer V.90 calls. (I briefly considered writing one, but it's a lot of work for little reward.) Previous work to record V.90 involved finding somebody with a decades-old ISDN modem bank and a suitable PSTN connection. I think they had to make international calls to Australia in the end, and the extra latency of going half way round the world meant that the tones sounded a bit off compared to memories of local dialup calls back in the day.

One advantage of setting this up using VoIP rather than analogue is that it's much easier to connect to the wider world by signing up for a VoIP service and bridging the networks. Other interested people can then dial in to your system, or you can dial into theirs.

If you are really into the idea of using an analogue PBX instead, @gewt on Twitter (profile tagline "cat lady, but the cats are phones") is a fellow fan of them and should be able to offer advice or point you at suitable units.

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    You can get PBX emulators; they have two phone ports. When the phone on one dials any number the other rings. On answer, the two phones are connected. However the cited PBXs might be easier to get – CSM Sep 2 at 16:51
  • Thanks a ton for your answer! I'm honestly totally uninitiated, so I'm still curious as to a lot of the things said here. What's a VoIP ATA? Or a PP3 battery? I hate to be so pushy, but how did you do your original project? I really don't know where to begin, to be frank. And finally, would this also work with VERY old standards, like Bell 103? How could I perhaps go about setting that up? – jediKatana Sep 2 at 19:23
  • An "ATA" is (essentially) an analog telephone adapter (and it may actually stand for that). It is a box that speaks POTS (Plain Old Telephone System/Signalling) on one end and VoIP on the other, but the exact VoIP signalling protocol depends on kit and flashed firmware. Most, if not all work for fax modems, most of the ones that work for fax work for higher-speed modems. Some of the fax-capable ones will actually terminate the fax modem signalling directly and ship the fax data using a "fax over IP" carrier protocol (that I honestly can't remember the name of). – Vatine Sep 2 at 21:10
  • And I was expecting the answer to be skip the modems and use a null modem cable, but ok then. – Joshua Sep 3 at 1:52
  • For Asterisk, there is a plugin that uses spandsp as a modem emulation. – Simon Richter Sep 3 at 9:23

Another approach would be to use a specialized piece of test equipment known as a telephone line emulator or telephone line simulator, which is a device with multiple phone jacks that simulates the entire mechanics of a telephone exchange. Plug in two telephones and you can dial from one to the other.

These devices are available relatively cheaply now on eBay and other places. Teltone was one manufacturer of such equipment and I have used their TLE series of emulators for telephony testing.

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Some modems can drive a reasonably short dead line and talk to another modem with a direct connection. In that case you just connect them together. On the software side, when configured correctly it'd behave much like a direct serial connection. Both modems would have to be configurable to pick up and switch directly into the appropriate mode, since there's no phone service. This was usually done with the appropriate AT commands, which would be documented in the manual. Such modems did, and maybe still do exist, though I can't advise you on models.

As to recording their sound, the only obvious way to me is to use a line splitter and a phone adapter to microphone input adapter. But this approach may affect their ability to operate at higher speeds (or at all, in this configuration).

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    Does "some modems can drive a reasonably short dead line" apply to US modems? To my knowledge, that doesn't work for modems used in my country (Germany) - they need the voltage levels provided by the telecom installation. – dirkt Sep 2 at 8:42
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    Sorry, but that's somewhat vague. Do you have experience with concrete models? – dirkt Sep 2 at 18:03
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    This is the first time I hear about a modem supplying a voltage. That would be against all circuit description I know and even more so any regulation I ever learned. An analogue terminal (telephon, modem, etc) modulates the current supplied by the exchange. It should and does not supply anything in terms of voltage/current. Of course, above coupling can be made by simply adding a fitting power supply. – Raffzahn Sep 2 at 21:33
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    Yes, I did this a few times with USRobotics external modems from the USA in the 1990s. It does work. I would not doubt that it's device-dependent though. – Michael Hampton Sep 2 at 22:29
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    "the only obvious way to me is to use a line splitter and a phone adapter to microphone input adapter" -- most, if not all, modems that would be used in such a project can activate an external speaker that reproduces the audio signal being transmitted and received over the phone line. If fidelity is not of great concern, it would be simple enough to simply leave the speaker on (by default, it typically turns on only until the handshaking is done) and record with a conventional microphone. – Peter Duniho Sep 3 at 0:43

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