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56k modems relied on digital trunk lines

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Re: 56k modems relied on digital trunk lines

#91

Post by iramiller »

linsomniac wrote: post #85
rasz wrote: post #47

Earlier quoted context omitted.

> DS1 (Digital Signal 1) out one end and a rack full of Zyxel modems on the other side Why use real physical modems when you already have subscribers signals converted to convenient digital form in DS1 bundle? Wouldnt it make more sense to put a box with one fat DSP doing 24 modems all in bulk inside a box with DS1 and Ethernet sockets at the ISP location instead?

You're thinking about the Livingston/Lucent Portmaster 3. https://osmocom.org/projects/retronetworking/wiki/Livingston... These were great boxes, and the only way you could get 56K was to call into an ISP with one of these or similar on their end -- the trickery that allowed 56K relied on one end being fully digital. I was working for an ISP around that time and we had a bunch of Portmaster 2s connected via RS-232 ca…

Similar story with Portmaster 2s and a wall of modems layed out. The resulting blinking lights acted like a load monitor of sorts as the activity would spread across the wall as customers dialed in after work and signed off at night. Not mention a wall of flashing red lights made a pretty good picture of ‘the internet’ for those just starting out on this adventure in 1997.

Re: 56k modems relied on digital trunk lines

#92

Post by kstrauser »

linsomniac wrote: post #85
rasz wrote: post #47

Earlier quoted context omitted.

> DS1 (Digital Signal 1) out one end and a rack full of Zyxel modems on the other side Why use real physical modems when you already have subscribers signals converted to convenient digital form in DS1 bundle? Wouldnt it make more sense to put a box with one fat DSP doing 24 modems all in bulk inside a box with DS1 and Ethernet sockets at the ISP location instead?

You're thinking about the Livingston/Lucent Portmaster 3. https://osmocom.org/projects/retronetworking/wiki/Livingston... These were great boxes, and the only way you could get 56K was to call into an ISP with one of these or similar on their end -- the trickery that allowed 56K relied on one end being fully digital. I was working for an ISP around that time and we had a bunch of Portmaster 2s connected via RS-232 ca…

Oh man. I was talking to my wife’s acquaintance and he was excited to talk shop when he found out I was technical. He worked at “this small company you’ve never heard of, Livingston.” “As in, Portmaster?” “You know about that?!”

Yeah, friend. I’m very familiar, and it was amazing tech. It sure kept the data center cooling system busy, though.

Re: 56k modems relied on digital trunk lines

#93

Post by kstrauser »

bigbuppo wrote: post #29
p1mrx wrote: post #13

Earlier quoted context omitted.

We were stuck at 26400 until 2001, when DOCSIS reached the area. I recall hearing speculation that a https://en.wikipedia.org/wiki/Pair_gain was the cause. Around 2000, I saw a crew pulling new phone lines through the neighborhood because everybody was getting a second line and they were running out, but even after switching to the new copper, we were still stuck at 26400. 20+ years later, it looks like 25/5 ADSL is…

More than one A-to-D conversion would knock it down to no more than 33.6. Pair gain units would cause problems. Bridge taps and load coils could also be problematic, but that was much more of a concern on DSL. Older amplifiers had very narrow filters and would also cause slow speeds. Echo cancellation hardware would ignore the in-band signal to get out of the way and cause problems. As there was no legal compulsion t…

I worked tech support at an ISP. Occasionally customers would call in from their brand new subdivision to complain about our crummy service. “This house is brand new and we only get 26.4. What’s wrong with your system?”

And then I had the satisfaction of explaining that Ma Bell cheaper out and used pair gains to build out their block cheaply. Sorry, call them to complain, or move.

Re: 56k modems relied on digital trunk lines

#94

Post by pjdesno »

hackthemack wrote: post #35

I have to wonder if the cap (theoretical) on the copper wires was more because of the technology standards in play at the time. Surely the copper wires could have handled more if they did not have to carry voice communication (with the old tech specs of the time) any longer? Ok. Searched around. Here is an article that states old copper could have carried 1 gigabit. https://www.newscientist.com/article/2317040-ordina…

Those copper wires ran from your house to the local central office, the "last mile" of the connection. (which was sometimes 2-3 miles long)

A quick read of the linked article seems to indicate that it's BS, as it doesn't account for the real topology of the local loop. In particular, in older neighborhoods you had a bundle of pairs going down the street, and a new connection was made by patching in to a free pair, creating a "T" shaped circuit. When a house was disconnected, part of this "stub" might have been left attached; over time a single pair might accumulate multiple disconnected stubs. The capacity of that copper circuit is far lower than a straight run.

In addition in many cases corrosion and water cause noise, further reducing bandwidth - I can remember having noise so bad on rainy days that I had to call and get them to fix it. (I assume they patched us onto a free pair and abandoned the noisy one)

Of course none of this is related to the end-to-end bandwidth of the old telephone system. Starting in the 50s a longer-distance phone call would get a single-side-band channel on a microwave link, with about 3KHz allocated. Later on calls got sampled at 8KHz with 8-bit mu-law (logarithmic) encoding, or A-law in Europe, and transmitted digitally.

Re: 56k modems relied on digital trunk lines

#96

Post by giantrobot »

numpad0 wrote: post #16
xnx wrote: post #7

Earlier quoted context omitted.

I'm still mad that they high-quality voice audio possible through ISDN didn't become ubiquitous. It's ridiculous to hear the clipped frequency range of a plain old telephone line during radio interviews in 2025.

24 [bit] * 192 [kHz] = 4.608 [Mbps]. Maybe not sensible to do so, but many people could have Discord calls in uncompressed "high-res" WAV. It's crazy that there aren't even 16bit/44.1k modes in most voice call apps.

That's an absurd bit rate for humans, especially talking about voice.

24-bit samples is ridiculous overkill. That's a huge dynamic range that's completely unnecessary.

At 192KHz you'd be able to capture 96KHz signals, far, FAR outside the range of human hearing. Human hearing peaks at 22KHz so you only need a sample rate of 44KHz to capture the total range of human hearing.

For human voice you don't really need better than 16-bit samples at 12KHz or so. That's for great quality voice.

The only reason audio mastering is done at huge sample sizes and sampling frequencies is to prevent aliasing during mixing and to preserve higher frequency harmonics. There's absolutely no need for such rates delivering to human beings.

Also higher fidelity audio sampling is available for phone calls. The issue is more political than technical. Cellular carriers don't like to negotiate higher quality calls between one another so inter-carrier calls tend to fall back on the lowest common denominator AMR-NB codec. Intra-carrier calls don't even reliably pick AMR-WB let alone EVS available with VoLTE.

Re: 56k modems relied on digital trunk lines

#97

Post by bryanlarsen »

Synaesthesia wrote: post #73
bryanlarsen wrote: post #54

Earlier quoted context omitted.

800 baud? Do you mean 300 baud? 800 baud is not a standard speed so if they did exist you'd have to supply both ends of the connection..

Oh yeah it was 300!

Darn, I was hoping it was some sort of oddball thing. Those are always interesting.

Re: 56k modems relied on digital trunk lines

#98

Post by piperswe »

numpad0 wrote: post #58
Synaesthesia wrote: post #42

Earlier quoted context omitted.

Speech compresses really well, with modern techniques you can get it down a lot. The big issue with analogue landline phone calls is the audio bandwidth is so limited. It's not the full frequency spectrum, most of it it cut off.

I have no problems with Opus or mp3 at low bitrate, in the same way I have no problems with microwaved food. I just think it's crazy that no one is doing 4Mbps audio while we're routinely streaming 20Mbps video as cheap distractions.

4Mbps audio doesn't really make sense when you can just about perfectly replicate any human-audible sound in ~1.4Mbps

EDIT: I do agree that lossless (or at least high bitrate modern lossy, like 256k Opus which is basically transparent) should be available in many more situations though.

Re: 56k modems relied on digital trunk lines

#99

Post by twic »

marcus0x62 wrote: post #83
hackthemack wrote: post #35

I have to wonder if the cap (theoretical) on the copper wires was more because of the technology standards in play at the time. Surely the copper wires could have handled more if they did not have to carry voice communication (with the old tech specs of the time) any longer? Ok. Searched around. Here is an article that states old copper could have carried 1 gigabit. https://www.newscientist.com/article/2317040-ordina…

The practical implementation of this (at the time) was ADSL, HDSL, IDSL, and SDSL. Those technologies all took one or two copper pairs, and terminated them on a DSLAM (or similar device in the case of HDSL) instead of on a telephone switch. For physical pairs connected to an analog voice port on a telephone switch, between the band pass filtering and DSO coding, you were never going to get more than 56kbps. The xDSLs…

If the copper isn't in good enough condition, you can always try with wet string instead:

https://www.revk.uk/2017/12/its-official-adsl-works-over-wet...

Re: 56k modems relied on digital trunk lines

#100

Post by pantulis »

timthorn wrote: post #66
plorg wrote: post #39

Earlier quoted context omitted.

Revealing my ignorance here, but was (is?) there a telephone equivalent of anycast such that, say, the 1-800-... Or 1-900... numbers would be routed differently based on location? My basic knowledge of phone systems suggests it would at least be possible.

Yes, the Intelligent Network was the big thing in the 1990s. It allowed for routing as you describe as well as calling cards and many other features: https://en.m.wikipedia.org/wiki/Intelligent_Network

Oh the joys of watching terminal logs with SS7 data frames!
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