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[dupe] Build your own dial up ISP in 2019 (dogemicrosystems.ca)
154 points by rahuldottech on Oct 21, 2019 | hide | past | favorite | 39 comments

I thought before the idea of modem-over-VoIP would be cool to have a BBS which was more retro than just a telnet BBS could be.

(At my house, my only options are VoIP and cellular. POTS service has been shut down.)

I wonder how well modems work over VoIP though? My understanding is that the voice-oriented compression causes problems (although you can turn that off by using G.711), but the bigger issue is the unreliable timing of most IP connections upsets the modem and it keeps on disconnecting.

ITU V.150.1 (aka V.MOIP) is supposed to fix that but I don't know think many VoIP providers implement it.

I wonder what the bps limits would be. Can it do 33.6k?

Didn't V.90 (56k) require a special setup with ISPs where you had fewer that one analog-to-digital hop? And IIRC, you could go from a 56k modem to another 56k modem. ISPs had different hardware.

> And IIRC, you could go from a 56k modem to another 56k modem.

Nope, the theoretical max between two 56k modems via a normal CO connection is 38k, in reality it usually meant 33.6k at best.

As you sorta alluded to, a 56k modem increases that due to the removal of the ADC on the ISP side, with the ISP having a digital connection to the CO.

ah sorry, I meant to say "couldn't go from 56k to 56k." BBS sysops were maxed out at 33.6k.

If you were looking to upload anything you limited your modem to 36k6 anyway as the higher options were asymmetric with upload rates half that of download. When I connected to my ISP at the time with no limits set I'd get between 38K and 41K which gave 19K-to-20.5K up, but I could reliably get a 36k6 symmetric link both ways if I told the modem not to bother trying for faster. Of course for most uses this is fine as it is with asymmetric connections today: most users download a lot more than they upload. There was also extra latency with the faster connections IIRC, so gamers would stick to 36k6 too.

I think you mean 33.6k (or 33k6). V.34 tops out at 33.6k so unless you were using a non-standard protocol on an extraordinarily clean copper connection, you wouldn’t have gotten that.

As to the asymmetrical part of your comment, definitely true in my experience there. I could go into detail on the technical reasons why that was the case, but don’t see the point unless someone wants me too.

Yep, I meant 33k6. Same typo twice.

That takes me back to the HS\Link protocol I used on BBSes back in the day, which let you upload and download at the same time:



I've done 2400 baud and lower over VOIP. 9600+ gets bit unstable.

I used 4800 baud over VOIP for many years. Make sure you turn on error correction. 2400 baud modems didn't have error correction, but 56K modems running at 2400 baud do. :) Also compression, which helped a lot.

Naive question, I've not used VOIP before, is the modem, implemented entirely in software to achieve this?

I use a hardware modem (USB modem actually) attached to a hardware VOIP analog telephone adapter. I am not familiar with software VOIP modems but hopefully someone else can chime in if it's possible.

I would like to figure this out.

I visit a cabin that has a POTS line that is active but rarely used, and would love basic connectivity.

LTE is not available in that area.

Amazing. I also plan to build that.

I wonder if telephone providers have some sort of recognition to detect people using (abusing?) a free call plan for transfering data. Certainly, if your "call" takes several days, that may violate some TOS.

(The use case I have in mind is to save money by using cheap prepaid SIM cards without an internet plan. It is purely for interest/proof of concept, not for scamming...)

i know in Ireland, most of our Unlimited providers have a limit of 45000 min a month... which is grand, given there are only 44640 min in a normal 31 day month... but GSM is a lot more limited in bandwidth than standard POTS: old GSM Modems would top out at 9.6kbps, with some having a 14.4kbps option... it wasnt till GPRS that you got faster speeds (packet switched, not Modem).

> old GSM Modems would top out at 9.6kbps, with some having a 14.4kbps option

Uhm, you are off on your numbers. GSM channel size is 200khz, but TDMA is used on top of that only leaving ~20khz for voice traffic (remember analog modems are “voice” traffic). So naively one might assume since 14,400 is under 20,000, it would be theoretically possible to achieve, but that’s not true in reality due to losses in the digital to analog to digital conversion due to noise. In the case of POTS, which has a max channel size of 64k, the theoretical maximum is roughly 38k (the reason 56k modems worked is due to the removal of the ADC conversion on the ISP side). Using a similar conversion for GSM, that gives you a theoretical maximum of 12k. If nothing else was a factor, then 9600 would be doable, but due to other issues with GSM, the real world experience usually was closer to 2400, sometimes getting 9.6k when everything was working great.

Source: I owned a dialup ISP in the 90s and routinely logged in from remote to manage user accounts from my 386 Thinkpad connected to a GSM phone. After selling the ISP, I worked for multiple years in the telco field, on the RBOC side (Southwestern Bell and later SBC), the cell carrier side (Voicestream and later T-mobile), and LDDS side (MCI and later MCI Worldcom).

> it wasnt till GPRS that you got faster speeds

Forgot to mention, GPRS uses the same time slots as voice calls do over GSM. Each time slot is still limited to about 9.6k useable data rate as with using a modem, but GPRS allows bonding to use more than one time slot. As such, using three time slots would get you 28.8k, using six gets you 57.6k, etc.

both comments are very handy to know! thanks. i remember trying to get internet access on my Nokia 7650 back in the day, and been given HSDCA and GPRS as options... larger downloads usually worked out cheaper on HSDCA (it was charged per min, no bandwidth limit, bar the standard limits) but GPRS worked out handier for "browsing" which, back then, was not standard either... big difference between now and then...

Back when I had an ISP in the mid 90s, I felt like I was king of the world as I had a T1 going to my place (as I only had to pay for the local loop, bandwidth was “free” by owning the ISP). My dream back then was to have those speeds everywhere I went. Fast forward 25 years and my watch can get 20x that speed even in a random rural area.

Unlimited call plans in the UK often commence charges at a time limit, often one hour. You can hang up and redial to continue the conversation for free, but presumably it's to make doing stuff like this a little bit harder.

Yeah, use an old GSM phone that can act as a modem and go nuts -- at 9600 baud. :)

I suspect it won't be a very user-friendly experience, but as a POC? Sure!

No TOS but speed limitations. Its back to 1990

In 1990 in the uk we paid

1) per minute charge on the phone (typically 1-10p)

2) per monthly charge to phone company for line rental

3) per monthly charge to isp for access

4) per minute charge to isp while we were dialed in

Its a BBS not the internet. I did run a 96 line system for fun and business, regular dialup.

Sure, as were things like CIX, and indeed compuserve in the early days

the transmission medium is a voice call

Even worse, if you're going IP-over-VoIP, the "physical" layer is pessimized for data transfer. I would assume worse performance than through PSTN.

indeed, but it serves as proof of concept / experimental, rather than practical application

Unrelated but can some one tell me what is the "portable" computer in the picture? https://dogemicrosystems.ca/mywiki/images/3/3c/DIY_dial_up_I...

That looks to be an INET Spectra of some kind. Possibly an 8243-1 based on a few searches.

Thank you!

It seems like a Dolch PAC 64 computer (real maker), re-branded due to some OEM contract.


Oh haha, I remember finding this a couple months back when I was discussing with a colleague, an old serial modem I have. I was wondering how difficult it would be to simulate a modem connection between the terminal and a PC as opposed to a direct serial line between the two. Now that I found this again, I might have to try it out.

Many "Winmodems" in the 90's and beyond were little more than soundcards connected to an RJ-11. Modulation/demodualtion was done in software, and this was in the sub-1GHz days of PCs. So this was actually not uncommon, but of course binary only.

There were some Linux projects to do this in a generic way - basically generate the required tones over the audio output and interpret tones over an audio input - and implement the various ITU V-series standards (though some of them were patented and could not be implemented, like V.92 for 56k).

There's another one other than iaxmodem (https://stackoverflow.com/questions/3069945/open-source-soft...) but I can't remember what it is.

"First, catch your rabbit." - Old recipe for rabbit stew.

This lists a real serial modem as a prerequisite, and those are kinda getting to be collectors' items. Is it feasible to build a modem out of a Raspberry Pi Zero or an Arduino? You'd need to hook the phone line to the GPIO pins, do the ADC and DAC stuff in software, implement the V.22 protocol in software, and also implement the Hayes Smartmodem stuff in software, meaning this is one of the few times a Hacker News reader would have occasion to implement the Hayes Code. Is there any show-stopping problem I'm not seeing? Massive electrical incompatibility?

Quite cool :)

I have my trusty old USR Courier and USR Sportster 14400 in a box. But I'm a bit more tempted to try and get pcboard running again using some of this setup. I don't have a landline in my house, only fiber so the asterisk option would be the way to go.

I also have disk images of my ms dos 6.22 floppies and PCBoard 15.22. I also have desqview for multinodes but I remember it was a PITA to get it running.

Now I just need an old 486 that is still running, my oldest running machine has dual pentium pro cpus and it doesn't behave all that nice with ms dos.

Interesting. Still thinking back local setup a cell during protest to avoid interception ... guess phone call affected.

Dial disks have gone extinct, so no more dial-up in the 21st century.

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