[PLUG-TALK] Modem fun

Sun May 25 15:55:00 UTC 2025

It's not the compression during digitization it's the D/A conversion itself.

In general voice is NOT compressed in PBXes because with compressed voice you lose 1 packet and it trashes 500ms to 1s of voice and the user definitely notices.  Telcos DO compress voice but ONLY over lossless circuits like T1s and DS3s and OC3s and such where they can guarantee no packet loss.  People don't understand that the PSTN is a mass of circuits but they are VERY different than circuits on the Internet.

The industry ran into this during the 56k dialup modem period.  I admined Internet Partners during this period and learned much about it.

In general there are 3 different types of PBXes.  There's the old ones that ran phones with the multiple blinking light pushbuttons you see in the movies, those were 100% analog like your ShadyTel PBX.

There's the "digital PBX"es those use ISDN circuits internally and convert everything to analog at the phonesets and trunk ports, these were commonly sold to businesses with "digital telephones" that were proprietary to the phone system, the Panasonic KX system is an example.

There's the "VoIP PBX"es those are nothing more than a management program telling every VoIP device out on the network where to go.  For a modem signal to work over a VoIP network, the 2 ATA's have to support modem signals, and you must have a network with packet prioritization that's completely lossless in between the ATAs, which isn't really achievable on anything other than a very controlled test network.

Modem signals get messed up with the digital PBXes because it's the A/D section during digitization.  With 56k modems the "server side"modem is connected to the switch using ISDN, the modem is 100% digital on that side.  The client-side modem is an analog circuit till it gets to the switch whereupon it is converted to digital and remains digital until it gets to the modem.

This means the "server side" digital modem has complete control over the A/D converter that the circuit passes through to the actual modem, it can put whatever it wants to the A/D converter.  ISDN is NOT talking packets to the A/D converter, by the way.  The digital modem on the "server side controls the actual analog waveform that the analog modem sees.

What we constantly had problems with is multiple D/A conversions of the circuit, that's where you run into the trouble.  Typical scenario was a "pair splitter" the phone companies used a lot of these when they ran out of pairs in a line in the street.  Multiple D/A conversions means the "server side" modem no longer has control of the waveform the modem sees.

The reason the "digital" PBXes mess with the modems is internally, those are ISDN channels inside of the PBX however they are NOT in general brought out to BRI ports because very few PBXes sold to corporate types provided ISDN BRI FXO (Foreign Exchange Office) ports.  Many DID have ISDN BRI or ISDN PRI FXS (Foreign Exchange Station) ports but those were for connecting to the PSTN.  So an analog-to-analog circuit created by one of those PBXes, is analog to digital then back into analog, violating the 1 A/D conversion rule.

If Shady Tell used an old DMS100 this is an actual telephone company exchange switch, they could present BRI ports which could be plugged into something like an Ascend MAX which is a "server side" 56k digital modem device, and analog ports that WOULD allow modems to establish 56k connections to the Ascend, or 28.8k connections to each other.  The DMS100 like a PBX does violate the 1 D/A conversion rule for analog-to-analog connections, but because it's A/D converters are far better than the crude ones in a "digital PBX" it will allow 28.8k connections from analog-to-analog modems.  The A/D converters in a typical "digital PBX" such as a Panasonic KX series commonly sold to businesses, are just not good enough to provide the identical output waveform as input waveform on an analog to analog connection so they mess up modem signals badly.

Exchange switches had to support FAX lines and FAX is just as sensitive to this sort of thing.

Understanding how Very Low Speed circuits operate (9600baud and below) is still important as it has application for specialized setups such as remote ticket printing over low-quality, shoddy cable, and unreliable radio & satellite connections.

Up until a few years ago I had a rock quarry as a customer that had load out stations in various parts of the quarry connected together by antique 50 year old cables, non category rated, some of them water-flooded.  When I got them they had a lot of problems with remote ticket printing and I looked it over and basically said no, no, no you are doing this completely wrong.  Either re-pull all the underground cables with Cat 6 ethernet or let me deal with it.  Following that I used a combination of 50Ma current loop, old antique 10BaseT ethernet hubs, switching scales from Ethernet to serial RS232, and other tricks of the trade to stabilize things.  No other technician that ever came in to work on anything ever understood what I was doing (hey, this is Oregon after all, heavy industry is just not our "thing" here) but it was reliable.   I did keep on them about replacing cables and very slowly over the following decade I did manage to get them prodded into doing this case by case, but some of those cable pulls were super nasty and I went through a series of wire jockies even though the quarry paid handsomely for doing it.  It's really hard to get a wire jocky to come back to your site and pull a drop after he's spend half the day in mud up to his knees, feeling around for a fiberglass fish rod poking out of a pipe submerged in the mud, a meter or so away from high voltage cables and junction boxes marked "warning" junctioning cable going through similar pipes LOL.

Ted

-----Original Message-----
From: PLUG-talk <plug-talk-bounces at lists.pdxlinux.org> On Behalf Of Russell Senior
Sent: Saturday, May 24, 2025 2:40 PM
To: plug-talk at lists.pdxlinux.org
Subject: [PLUG-TALK] Modem fun

You might remember our speaker from last May, Adrian Black. His youtube channel today had a program where he used an analog PBX from the late 1990s to play with his old dialup modems:

   https://www.youtube.com/watch?v=E7LYCERDnX4

Just for your weekend nostalgia fix.

Back pre-pandemic I played around with my modems to test them out before going to a ToorCamp (where an analog phone system is set up by the fun people at ShadyTel), and I very quickly discovered that analog modems no longer function over modern digital telephone systems, because of compression during digitization or something.

So, I guess if you really want to play, you need one of these old PBXs.

Hopefully, watching the show will allow me not to do it myself.

--
Russell Senior
russell at pdxlinux.org
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