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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.

user3840170
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jediKatana
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6 Answers6

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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.

pndc
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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 02 '20 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 02 '20 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 02 '20 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 03 '20 at 01:52
  • For Asterisk, there is a plugin that uses spandsp as a modem emulation. – Simon Richter Sep 03 '20 at 09:23
  • @jediKatana A PP3 is that small cuboid nine-volt battery found in larger devices such as smoke alarms. You don't specifically need a PP3, just that they tend to be the most conveniently-available voltage source for this task. Cutting the plug off an old wall-wart would also do the job, although if it's a poor-quality one (most are) then it may well put a 50Hz/60Hz buzz on the line which is why a battery is preferable, but a regulated PSU (perhaps from a laptop?) should work. But this is a hack and you shouldn't spend any money on PSUs for it lest it not work sufficiently well for you. – pndc Sep 03 '20 at 10:37
  • @SimonRichter The spandsp website appears to have fallen off the Internet, and it only supports up to 14,400bps anyway rather than the desired V.92bis. It also seems to be mainly intended for fax rather than data. – pndc Sep 03 '20 at 10:40
  • ATA is the answer command sent to the modem. It is part of the Hayes command set. All commands started with AT. ATD was dial and was followed by the number to dial. While Hayes developed the command language for thier modems, many other manufacturers followed the same set of commands and it became a defacto standard. – jmarkmurphy Sep 10 '20 at 23:46
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    @jmarkmurphy I see how some of what I wrote was confusing. "ATA" has multiple meanings, and I use two of them in my answer. I shall edit it to clarify. – pndc Sep 11 '20 at 09:26
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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.

GrumpyYoungMan
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  • I was going to mention the same idea. A line emulator is a really fun way to connect computer/modem A to computer/modem B locally. I personally own a Teltone TLS-3 like the one pictured here https://www.valuetronics.com/media/catalog/product/cache/14e8e6567e2058d87acfc1b341cc0a78/t/e/telton_tls3.jpg – Geo... Mar 23 '24 at 20:56
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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).

RETRAC
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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 02 '20 at 08:42
  • I am in neither the US nor Germany so I can't really answer that. Many, perhaps most, modems /do/ expect the telecom's standard DC potential on the line. But some will run with the small voltage they will apply themselves. Some will run with a small DC voltage added with a battery or other supply. Some need a full PBX-quality circuit emulator with telecom level voltage. It is model specific, as I understand it. – RETRAC Sep 02 '20 at 14:43
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    Sorry, but that's somewhat vague. Do you have experience with concrete models? – dirkt Sep 02 '20 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 02 '20 at 21:33
  • https://www.youtube.com/watch?v=vnI6jGs5uKU here's a video of such a thing in action. It must obviously be sending something down the line to work. I remember doing this in the late 90s and it working then as well. I really can't provide more info than that. – RETRAC Sep 02 '20 at 22:10
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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 02 '20 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 03 '20 at 00:43
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To connect two telephones together, you need a battery. The battery supplies line current to the telephones.

A modem typically listens for a dial tone, and a ring tone. You can buy dial tone and ring tone emulators, but you can also configure a modem to not listen: many modems are adjustable to different tones, (used in different countries), and many can be set to dial-out even when the dial tone is absent. Same for ring tones: you can set the modem for different ring tones, or just tell it to pick up anyway.

If you connect two modems with a battery (to get the line current), and tell them both to pick up, they can talk to each other. Nothing else is required. I have done this. BUT you have to have modems that will do this, and have the documentation for the commands, and have modems that can negotiate connection in this situation.

The "negotiation" phase is more complex for high-speed modems, and less likely to work. On the other hand, high-speed modems are more likely to be international and flexible in their dial-ring requirements. A 300-baud modem with an acoustic coupler is guaranteed to work (if a 50 year old modem is still working and not broken)

Battery should be 3-9 Volts, Typical line current is 20mA. A 9V battery will do it.

david
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Cathode ray dude did a video about this using a SPA122 ATA

https://www.youtube.com/watch?v=EGFIEF6siIE

And details about how to set it up here: https://gekk.info/articles/ata-config.html

horatio
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    Please post relevant information from those links here. Posts consisting only of links are not very helpful and are likely to be deleted. – Chenmunka Sep 06 '21 at 07:55
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Silicon Chip magazine had a Telephone Exchange Simulator for Testing Modems project. The first page of the construction project is available online. Unfortunately I don't have that particular months magazine issue for 1998. Best guess the project is a maximum of 9 pages. This will be relatively more expensive than any of the other solutions.

PDP11
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